feat(auth): 添加 Linux DO Connect OAuth 登录支持

- 新增 Linux DO OAuth 配置项和环境变量支持
- 实现 OAuth 授权流程和回调处理
- 前端添加 Linux DO 登录按钮和回调页面
- 支持通过 Linux DO 账号注册/登录
- 添加相关国际化文本

🤖 Generated with [Claude Code](https://claude.com/claude-code

)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Linux DO Connect.md

+# Linux DO Connect
+
+OAuth（Open Authorization）是一个开放的网络授权标准，目前最新版本为 OAuth 2.0。我们日常使用的第三方登录（如 Google 账号登录）就采用了该标准。OAuth 允许用户授权第三方应用访问存储在其他服务提供商（如 Google）上的信息，无需在不同平台上重复填写注册信息。用户授权后，平台可以直接访问用户的账户信息进行身份验证，而用户无需向第三方应用提供密码。
+
+目前系统已实现完整的 OAuth2 授权码（code）方式鉴权，但界面等配套功能还在持续完善中。让我们一起打造一个更完善的共享方案。
+
+## 基本介绍
+
+这是一套标准的 OAuth2 鉴权系统，可以让开发者共享论坛的用户基本信息。
+
+- 可获取字段：
+
+| 参数              | 说明                            |
+| ----------------- | ------------------------------- |
+| `id`              | 用户唯一标识（不可变）          |
+| `username`        | 论坛用户名                      |
+| `name`            | 论坛用户昵称（可变）            |
+| `avatar_template` | 用户头像模板URL（支持多种尺寸） |
+| `active`          | 账号活跃状态                    |
+| `trust_level`     | 信任等级（0-4）                 |
+| `silenced`        | 禁言状态                        |
+| `external_ids`    | 外部ID关联信息                  |
+| `api_key`         | API访问密钥                     |
+
+通过这些信息，公益网站/接口可以实现：
+
+1. 基于 `id` 的服务频率限制
+2. 基于 `trust_level` 的服务额度分配
+3. 基于用户信息的滥用举报机制
+
+## 相关端点
+
+- Authorize 端点： `https://connect.linux.do/oauth2/authorize`
+- Token 端点：`https://connect.linux.do/oauth2/token`
+- 用户信息 端点：`https://connect.linux.do/api/user`
+
+## 申请使用
+
+- 访问 [Connect.Linux.Do](https://connect.linux.do/) 申请接入你的应用。
+
+![linuxdoconnect_1](https://wiki.linux.do/_next/image?url=%2Flinuxdoconnect_1.png&w=1080&q=75)
+
+- 点击 **`我的应用接入`** - **`申请新接入`**，填写相关信息。其中 **`回调地址`** 是你的应用接收用户信息的地址。
+
+![linuxdoconnect_2](https://wiki.linux.do/_next/image?url=%2Flinuxdoconnect_2.png&w=1080&q=75)
+
+- 申请成功后，你将获得 **`Client Id`** 和 **`Client Secret`**，这是你应用的唯一身份凭证。
+
+![linuxdoconnect_3](https://wiki.linux.do/_next/image?url=%2Flinuxdoconnect_3.png&w=1080&q=75)
+
+## 接入 Linux Do
+
+JavaScript
+```JavaScript
+// 安装第三方请求库（或使用原生的 Fetch API），本例中使用 axios
+// npm install axios
+
+// 通过 OAuth2 获取 Linux Do 用户信息的参考流程
+const axios = require('axios');
+const readline = require('readline');
+
+// 配置信息（建议通过环境变量配置，避免使用硬编码）
+const CLIENT_ID = '你的 Client ID';
+const CLIENT_SECRET = '你的 Client Secret';
+const REDIRECT_URI = '你的回调地址';
+const AUTH_URL = 'https://connect.linux.do/oauth2/authorize';
+const TOKEN_URL = 'https://connect.linux.do/oauth2/token';
+const USER_INFO_URL = 'https://connect.linux.do/api/user';
+
+// 第一步：生成授权 URL
+function getAuthUrl() {
+  const params = new URLSearchParams({
+    client_id: CLIENT_ID,
+    redirect_uri: REDIRECT_URI,
+    response_type: 'code',
+    scope: 'user'
+  });
+
+  return `${AUTH_URL}?${params.toString()}`;
+}
+
+// 第二步：获取 code 参数
+function getCode() {
+  return new Promise((resolve) => {
+    // 本例中使用终端输入来模拟流程，仅供本地测试
+    // 请在实际应用中替换为真实的处理逻辑
+    const rl = readline.createInterface({ input: process.stdin, output: process.stdout });
+    rl.question('从回调 URL 中提取出 code，粘贴到此处并按回车：', (answer) => {
+      rl.close();
+      resolve(answer.trim());
+    });
+  });
+}
+
+// 第三步：使用 code 参数获取访问令牌
+async function getAccessToken(code) {
+  try {
+    const form = new URLSearchParams({
+      client_id: CLIENT_ID,
+      client_secret: CLIENT_SECRET,
+      code: code,
+      redirect_uri: REDIRECT_URI,
+      grant_type: 'authorization_code'
+    }).toString();
+
+    const response = await axios.post(TOKEN_URL, form, {
+      // 提醒：需正确配置请求头，否则无法正常获取访问令牌
+      headers: {
+        'Content-Type': 'application/x-www-form-urlencoded',
+        'Accept': 'application/json'
+      }
+    });
+
+    return response.data;
+  } catch (error) {
+    console.error(`获取访问令牌失败：${error.response ? JSON.stringify(error.response.data) : error.message}`);
+    throw error;
+  }
+}
+
+// 第四步：使用访问令牌获取用户信息
+async function getUserInfo(accessToken) {
+  try {
+    const response = await axios.get(USER_INFO_URL, {
+      headers: {
+        Authorization: `Bearer ${accessToken}`
+      }
+    });
+
+    return response.data;
+  } catch (error) {
+    console.error(`获取用户信息失败：${error.response ? JSON.stringify(error.response.data) : error.message}`);
+    throw error;
+  }
+}
+
+// 主流程
+async function main() {
+  // 1. 生成授权 URL，前端引导用户访问授权页
+  const authUrl = getAuthUrl();
+  console.log(`请访问此 URL 授权：${authUrl}
+`);
+
+  // 2. 用户授权后，从回调 URL 获取 code 参数
+  const code = await getCode();
+
+  try {
+    // 3. 使用 code 参数获取访问令牌
+    const tokenData = await getAccessToken(code);
+    const accessToken = tokenData.access_token;
+
+    // 4. 使用访问令牌获取用户信息
+    if (accessToken) {
+      const userInfo = await getUserInfo(accessToken);
+      console.log(`
+获取用户信息成功：${JSON.stringify(userInfo, null, 2)}`);
+    } else {
+      console.log(`
+获取访问令牌失败：${JSON.stringify(tokenData)}`);
+    }
+  } catch (error) {
+    console.error('发生错误：', error);
+  }
+}
+```
+Python
+```python
+# 安装第三方请求库，本例中使用 requests
+# pip install requests
+
+# 通过 OAuth2 获取 Linux Do 用户信息的参考流程
+import requests
+import json
+
+# 配置信息（建议通过环境变量配置，避免使用硬编码）
+CLIENT_ID = '你的 Client ID'
+CLIENT_SECRET = '你的 Client Secret'
+REDIRECT_URI = '你的回调地址'
+AUTH_URL = 'https://connect.linux.do/oauth2/authorize'
+TOKEN_URL = 'https://connect.linux.do/oauth2/token'
+USER_INFO_URL = 'https://connect.linux.do/api/user'
+
+# 第一步：生成授权 URL
+def get_auth_url():
+    params = {
+        'client_id': CLIENT_ID,
+        'redirect_uri': REDIRECT_URI,
+        'response_type': 'code',
+        'scope': 'user'
+    }
+    auth_url = f"{AUTH_URL}?{'&'.join(f'{k}={v}' for k, v in params.items())}"
+    return auth_url
+
+# 第二步：获取 code 参数
+def get_code():
+    # 本例中使用终端输入来模拟流程，仅供本地测试
+    # 请在实际应用中替换为真实的处理逻辑
+    return input('从回调 URL 中提取出 code，粘贴到此处并按回车：').strip()
+
+# 第三步：使用 code 参数获取访问令牌
+def get_access_token(code):
+    try:
+        data = {
+            'client_id': CLIENT_ID,
+            'client_secret': CLIENT_SECRET,
+            'code': code,
+            'redirect_uri': REDIRECT_URI,
+            'grant_type': 'authorization_code'
+        }
+        # 提醒：需正确配置请求头，否则无法正常获取访问令牌
+        headers = {
+            'Content-Type': 'application/x-www-form-urlencoded',
+            'Accept': 'application/json'
+        }
+        response = requests.post(TOKEN_URL, data=data, headers=headers)
+        response.raise_for_status()
+        return response.json()
+    except requests.exceptions.RequestException as e:
+        print(f"获取访问令牌失败：{e}")
+        return None
+
+# 第四步：使用访问令牌获取用户信息
+def get_user_info(access_token):
+    try:
+        headers = {
+            'Authorization': f'Bearer {access_token}'
+        }
+        response = requests.get(USER_INFO_URL, headers=headers)
+        response.raise_for_status()
+        return response.json()
+    except requests.exceptions.RequestException as e:
+        print(f"获取用户信息失败：{e}")
+        return None
+
+# 主流程
+if __name__ == '__main__':
+    # 1. 生成授权 URL，前端引导用户访问授权页
+    auth_url = get_auth_url()
+    print(f'请访问此 URL 授权：{auth_url}
+')
+
+    # 2. 用户授权后，从回调 URL 获取 code 参数
+    code = get_code()
+
+    # 3. 使用 code 参数获取访问令牌
+    token_data = get_access_token(code)
+    if token_data:
+        access_token = token_data.get('access_token')
+
+        # 4. 使用访问令牌获取用户信息
+        if access_token:
+            user_info = get_user_info(access_token)
+            if user_info:
+                print(f"
+获取用户信息成功：{json.dumps(user_info, indent=2)}")
+            else:
+                print("
+获取用户信息失败")
+        else:
+            print(f"
+获取访问令牌失败：{json.dumps(token_data, indent=2)}")
+    else:
+        print("
+获取访问令牌失败")
+```
+PHP
+```php
+// 通过 OAuth2 获取 Linux Do 用户信息的参考流程
+
+// 配置信息
+$CLIENT_ID = '你的 Client ID';
+$CLIENT_SECRET = '你的 Client Secret';
+$REDIRECT_URI = '你的回调地址';
+$AUTH_URL = 'https://connect.linux.do/oauth2/authorize';
+$TOKEN_URL = 'https://connect.linux.do/oauth2/token';
+$USER_INFO_URL = 'https://connect.linux.do/api/user';
+
+// 生成授权 URL
+function getAuthUrl($clientId, $redirectUri) {
+    global $AUTH_URL;
+    return $AUTH_URL . '?' . http_build_query([
+        'client_id' => $clientId,
+        'redirect_uri' => $redirectUri,
+        'response_type' => 'code',
+        'scope' => 'user'
+    ]);
+}
+
+// 使用 code 参数获取用户信息（合并获取令牌和获取用户信息的步骤）
+function getUserInfoWithCode($code, $clientId, $clientSecret, $redirectUri) {
+    global $TOKEN_URL, $USER_INFO_URL;
+
+    // 1. 获取访问令牌
+    $ch = curl_init($TOKEN_URL);
+    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+    curl_setopt($ch, CURLOPT_POST, true);
+    curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query([
+        'client_id' => $clientId,
+        'client_secret' => $clientSecret,
+        'code' => $code,
+        'redirect_uri' => $redirectUri,
+        'grant_type' => 'authorization_code'
+    ]));
+    curl_setopt($ch, CURLOPT_HTTPHEADER, [
+        'Content-Type: application/x-www-form-urlencoded',
+        'Accept: application/json'
+    ]);
+
+    $tokenResponse = curl_exec($ch);
+    curl_close($ch);
+
+    $tokenData = json_decode($tokenResponse, true);
+    if (!isset($tokenData['access_token'])) {
+        return ['error' => '获取访问令牌失败', 'details' => $tokenData];
+    }
+
+    // 2. 获取用户信息
+    $ch = curl_init($USER_INFO_URL);
+    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+    curl_setopt($ch, CURLOPT_HTTPHEADER, [
+        'Authorization: Bearer ' . $tokenData['access_token']
+    ]);
+
+    $userResponse = curl_exec($ch);
+    curl_close($ch);
+
+    return json_decode($userResponse, true);
+}
+
+// 主流程
+// 1. 生成授权 URL
+$authUrl = getAuthUrl($CLIENT_ID, $REDIRECT_URI);
+echo "<a href='$authUrl'>使用 Linux Do 登录</a>";
+
+// 2. 处理回调并获取用户信息
+if (isset($_GET['code'])) {
+    $userInfo = getUserInfoWithCode(
+        $_GET['code'],
+        $CLIENT_ID,
+        $CLIENT_SECRET,
+        $REDIRECT_URI
+    );
+
+    if (isset($userInfo['error'])) {
+        echo '错误: ' . $userInfo['error'];
+    } else {
+        echo '欢迎, ' . $userInfo['name'] . '!';
+        // 处理用户登录逻辑...
+    }
+}
+```
+
+## 使用说明
+
+### 授权流程
+
+1. 用户点击应用中的’使用 Linux Do 登录’按钮
+2. 系统将用户重定向至 Linux Do 的授权页面
+3. 用户完成授权后，系统自动重定向回应用并携带授权码
+4. 应用使用授权码获取访问令牌
+5. 使用访问令牌获取用户信息
+
+### 安全建议
+
+- 切勿在前端代码中暴露 Client Secret
+- 对所有用户输入数据进行严格验证
+- 确保使用 HTTPS 协议传输数据
+- 定期更新并妥善保管 Client Secret
\ No newline at end of file

backend/cmd/server/VERSION

-0.1.1
+0.1.46

backend/internal/config/config.go

@@ -6,6 +6,7 @@ import (
 	"encoding/hex"
 	"fmt"
 	"log"
+	"net/url"
 	"os"
 	"strings"
 	"time"
@@ -35,24 +36,25 @@ const (
 )
 
 type Config struct {
-	Server       ServerConfig       `mapstructure:"server"`
-	CORS         CORSConfig         `mapstructure:"cors"`
-	Security     SecurityConfig     `mapstructure:"security"`
-	Billing      BillingConfig      `mapstructure:"billing"`
-	Turnstile    TurnstileConfig    `mapstructure:"turnstile"`
-	Database     DatabaseConfig     `mapstructure:"database"`
-	Redis        RedisConfig        `mapstructure:"redis"`
-	JWT          JWTConfig          `mapstructure:"jwt"`
-	Default      DefaultConfig      `mapstructure:"default"`
-	RateLimit    RateLimitConfig    `mapstructure:"rate_limit"`
-	Pricing      PricingConfig      `mapstructure:"pricing"`
-	Gateway      GatewayConfig      `mapstructure:"gateway"`
-	Concurrency  ConcurrencyConfig  `mapstructure:"concurrency"`
-	TokenRefresh TokenRefreshConfig `mapstructure:"token_refresh"`
-	RunMode      string             `mapstructure:"run_mode" yaml:"run_mode"`
-	Timezone     string             `mapstructure:"timezone"` // e.g. "Asia/Shanghai", "UTC"
-	Gemini       GeminiConfig       `mapstructure:"gemini"`
-	Update       UpdateConfig       `mapstructure:"update"`
+	Server       ServerConfig         `mapstructure:"server"`
+	CORS         CORSConfig           `mapstructure:"cors"`
+	Security     SecurityConfig       `mapstructure:"security"`
+	Billing      BillingConfig        `mapstructure:"billing"`
+	Turnstile    TurnstileConfig      `mapstructure:"turnstile"`
+	Database     DatabaseConfig       `mapstructure:"database"`
+	Redis        RedisConfig          `mapstructure:"redis"`
+	JWT          JWTConfig            `mapstructure:"jwt"`
+	LinuxDo      LinuxDoConnectConfig `mapstructure:"linuxdo_connect"`
+	Default      DefaultConfig        `mapstructure:"default"`
+	RateLimit    RateLimitConfig      `mapstructure:"rate_limit"`
+	Pricing      PricingConfig        `mapstructure:"pricing"`
+	Gateway      GatewayConfig        `mapstructure:"gateway"`
+	Concurrency  ConcurrencyConfig    `mapstructure:"concurrency"`
+	TokenRefresh TokenRefreshConfig   `mapstructure:"token_refresh"`
+	RunMode      string               `mapstructure:"run_mode" yaml:"run_mode"`
+	Timezone     string               `mapstructure:"timezone"` // e.g. "Asia/Shanghai", "UTC"
+	Gemini       GeminiConfig         `mapstructure:"gemini"`
+	Update       UpdateConfig         `mapstructure:"update"`
 }
 
 // UpdateConfig 在线更新相关配置
@@ -322,6 +324,30 @@ type TurnstileConfig struct {
 	Required bool `mapstructure:"required"`
 }
 
+// LinuxDoConnectConfig controls LinuxDo Connect OAuth login (end-user SSO).
+//
+// Note: This is NOT the same as upstream account OAuth (e.g. OpenAI/Gemini).
+// It is used for logging in to Sub2API itself.
+type LinuxDoConnectConfig struct {
+	Enabled             bool   `mapstructure:"enabled"`
+	ClientID            string `mapstructure:"client_id"`
+	ClientSecret        string `mapstructure:"client_secret"`
+	AuthorizeURL        string `mapstructure:"authorize_url"`
+	TokenURL            string `mapstructure:"token_url"`
+	UserInfoURL         string `mapstructure:"userinfo_url"`
+	Scopes              string `mapstructure:"scopes"`
+	RedirectURL         string `mapstructure:"redirect_url"`          // backend callback URL registered at the provider
+	FrontendRedirectURL string `mapstructure:"frontend_redirect_url"` // frontend route to receive token (default: /auth/linuxdo/callback)
+	TokenAuthMethod     string `mapstructure:"token_auth_method"`     // client_secret_post / client_secret_basic / none
+	UsePKCE             bool   `mapstructure:"use_pkce"`
+
+	// Optional: gjson paths to extract fields from userinfo JSON.
+	// When empty, the server tries a set of common keys.
+	UserInfoEmailPath    string `mapstructure:"userinfo_email_path"`
+	UserInfoIDPath       string `mapstructure:"userinfo_id_path"`
+	UserInfoUsernamePath string `mapstructure:"userinfo_username_path"`
+}
+
 type DefaultConfig struct {
 	AdminEmail      string  `mapstructure:"admin_email"`
 	AdminPassword   string  `mapstructure:"admin_password"`
@@ -388,6 +414,18 @@ func Load() (*Config, error) {
 		cfg.Server.Mode = "debug"
 	}
 	cfg.JWT.Secret = strings.TrimSpace(cfg.JWT.Secret)
+	cfg.LinuxDo.ClientID = strings.TrimSpace(cfg.LinuxDo.ClientID)
+	cfg.LinuxDo.ClientSecret = strings.TrimSpace(cfg.LinuxDo.ClientSecret)
+	cfg.LinuxDo.AuthorizeURL = strings.TrimSpace(cfg.LinuxDo.AuthorizeURL)
+	cfg.LinuxDo.TokenURL = strings.TrimSpace(cfg.LinuxDo.TokenURL)
+	cfg.LinuxDo.UserInfoURL = strings.TrimSpace(cfg.LinuxDo.UserInfoURL)
+	cfg.LinuxDo.Scopes = strings.TrimSpace(cfg.LinuxDo.Scopes)
+	cfg.LinuxDo.RedirectURL = strings.TrimSpace(cfg.LinuxDo.RedirectURL)
+	cfg.LinuxDo.FrontendRedirectURL = strings.TrimSpace(cfg.LinuxDo.FrontendRedirectURL)
+	cfg.LinuxDo.TokenAuthMethod = strings.ToLower(strings.TrimSpace(cfg.LinuxDo.TokenAuthMethod))
+	cfg.LinuxDo.UserInfoEmailPath = strings.TrimSpace(cfg.LinuxDo.UserInfoEmailPath)
+	cfg.LinuxDo.UserInfoIDPath = strings.TrimSpace(cfg.LinuxDo.UserInfoIDPath)
+	cfg.LinuxDo.UserInfoUsernamePath = strings.TrimSpace(cfg.LinuxDo.UserInfoUsernamePath)
 	cfg.CORS.AllowedOrigins = normalizeStringSlice(cfg.CORS.AllowedOrigins)
 	cfg.Security.ResponseHeaders.AdditionalAllowed = normalizeStringSlice(cfg.Security.ResponseHeaders.AdditionalAllowed)
 	cfg.Security.ResponseHeaders.ForceRemove = normalizeStringSlice(cfg.Security.ResponseHeaders.ForceRemove)
@@ -426,6 +464,77 @@ func Load() (*Config, error) {
 	return &cfg, nil
 }
 
+func validateAbsoluteHTTPURL(raw string) error {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return fmt.Errorf("empty url")
+	}
+	u, err := url.Parse(raw)
+	if err != nil {
+		return err
+	}
+	if !u.IsAbs() {
+		return fmt.Errorf("must be absolute")
+	}
+	if !isHTTPScheme(u.Scheme) {
+		return fmt.Errorf("unsupported scheme: %s", u.Scheme)
+	}
+	if strings.TrimSpace(u.Host) == "" {
+		return fmt.Errorf("missing host")
+	}
+	if u.Fragment != "" {
+		return fmt.Errorf("must not include fragment")
+	}
+	return nil
+}
+
+func validateFrontendRedirectURL(raw string) error {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return fmt.Errorf("empty url")
+	}
+	if strings.ContainsAny(raw, "\r\n") {
+		return fmt.Errorf("contains invalid characters")
+	}
+	if strings.HasPrefix(raw, "/") {
+		if strings.HasPrefix(raw, "//") {
+			return fmt.Errorf("must not start with //")
+		}
+		return nil
+	}
+	u, err := url.Parse(raw)
+	if err != nil {
+		return err
+	}
+	if !u.IsAbs() {
+		return fmt.Errorf("must be absolute http(s) url or relative path")
+	}
+	if !isHTTPScheme(u.Scheme) {
+		return fmt.Errorf("unsupported scheme: %s", u.Scheme)
+	}
+	if strings.TrimSpace(u.Host) == "" {
+		return fmt.Errorf("missing host")
+	}
+	if u.Fragment != "" {
+		return fmt.Errorf("must not include fragment")
+	}
+	return nil
+}
+
+func isHTTPScheme(scheme string) bool {
+	return strings.EqualFold(scheme, "http") || strings.EqualFold(scheme, "https")
+}
+
+func warnIfInsecureURL(field, raw string) {
+	u, err := url.Parse(strings.TrimSpace(raw))
+	if err != nil {
+		return
+	}
+	if strings.EqualFold(u.Scheme, "http") {
+		log.Printf("Warning: %s uses http scheme; use https in production to avoid token leakage.", field)
+	}
+}
+
 func setDefaults() {
 	viper.SetDefault("run_mode", RunModeStandard)
 
@@ -475,6 +584,22 @@ func setDefaults() {
 	// Turnstile
 	viper.SetDefault("turnstile.required", false)
 
+	// LinuxDo Connect OAuth login (end-user SSO)
+	viper.SetDefault("linuxdo_connect.enabled", false)
+	viper.SetDefault("linuxdo_connect.client_id", "")
+	viper.SetDefault("linuxdo_connect.client_secret", "")
+	viper.SetDefault("linuxdo_connect.authorize_url", "https://connect.linux.do/oauth2/authorize")
+	viper.SetDefault("linuxdo_connect.token_url", "https://connect.linux.do/oauth2/token")
+	viper.SetDefault("linuxdo_connect.userinfo_url", "https://connect.linux.do/api/user")
+	viper.SetDefault("linuxdo_connect.scopes", "user")
+	viper.SetDefault("linuxdo_connect.redirect_url", "")
+	viper.SetDefault("linuxdo_connect.frontend_redirect_url", "/auth/linuxdo/callback")
+	viper.SetDefault("linuxdo_connect.token_auth_method", "client_secret_post")
+	viper.SetDefault("linuxdo_connect.use_pkce", false)
+	viper.SetDefault("linuxdo_connect.userinfo_email_path", "")
+	viper.SetDefault("linuxdo_connect.userinfo_id_path", "")
+	viper.SetDefault("linuxdo_connect.userinfo_username_path", "")
+
 	// Database
 	viper.SetDefault("database.host", "localhost")
 	viper.SetDefault("database.port", 5432)
@@ -586,6 +711,60 @@ func (c *Config) Validate() error {
 	if c.Security.CSP.Enabled && strings.TrimSpace(c.Security.CSP.Policy) == "" {
 		return fmt.Errorf("security.csp.policy is required when CSP is enabled")
 	}
+	if c.LinuxDo.Enabled {
+		if strings.TrimSpace(c.LinuxDo.ClientID) == "" {
+			return fmt.Errorf("linuxdo_connect.client_id is required when linuxdo_connect.enabled=true")
+		}
+		if strings.TrimSpace(c.LinuxDo.AuthorizeURL) == "" {
+			return fmt.Errorf("linuxdo_connect.authorize_url is required when linuxdo_connect.enabled=true")
+		}
+		if strings.TrimSpace(c.LinuxDo.TokenURL) == "" {
+			return fmt.Errorf("linuxdo_connect.token_url is required when linuxdo_connect.enabled=true")
+		}
+		if strings.TrimSpace(c.LinuxDo.UserInfoURL) == "" {
+			return fmt.Errorf("linuxdo_connect.userinfo_url is required when linuxdo_connect.enabled=true")
+		}
+		if strings.TrimSpace(c.LinuxDo.RedirectURL) == "" {
+			return fmt.Errorf("linuxdo_connect.redirect_url is required when linuxdo_connect.enabled=true")
+		}
+		method := strings.ToLower(strings.TrimSpace(c.LinuxDo.TokenAuthMethod))
+		switch method {
+		case "", "client_secret_post", "client_secret_basic", "none":
+		default:
+			return fmt.Errorf("linuxdo_connect.token_auth_method must be one of: client_secret_post/client_secret_basic/none")
+		}
+		if method == "none" && !c.LinuxDo.UsePKCE {
+			return fmt.Errorf("linuxdo_connect.use_pkce must be true when linuxdo_connect.token_auth_method=none")
+		}
+		if (method == "" || method == "client_secret_post" || method == "client_secret_basic") && strings.TrimSpace(c.LinuxDo.ClientSecret) == "" {
+			return fmt.Errorf("linuxdo_connect.client_secret is required when linuxdo_connect.enabled=true and token_auth_method is client_secret_post/client_secret_basic")
+		}
+		if strings.TrimSpace(c.LinuxDo.FrontendRedirectURL) == "" {
+			return fmt.Errorf("linuxdo_connect.frontend_redirect_url is required when linuxdo_connect.enabled=true")
+		}
+
+		if err := validateAbsoluteHTTPURL(c.LinuxDo.AuthorizeURL); err != nil {
+			return fmt.Errorf("linuxdo_connect.authorize_url invalid: %w", err)
+		}
+		if err := validateAbsoluteHTTPURL(c.LinuxDo.TokenURL); err != nil {
+			return fmt.Errorf("linuxdo_connect.token_url invalid: %w", err)
+		}
+		if err := validateAbsoluteHTTPURL(c.LinuxDo.UserInfoURL); err != nil {
+			return fmt.Errorf("linuxdo_connect.userinfo_url invalid: %w", err)
+		}
+		if err := validateAbsoluteHTTPURL(c.LinuxDo.RedirectURL); err != nil {
+			return fmt.Errorf("linuxdo_connect.redirect_url invalid: %w", err)
+		}
+		if err := validateFrontendRedirectURL(c.LinuxDo.FrontendRedirectURL); err != nil {
+			return fmt.Errorf("linuxdo_connect.frontend_redirect_url invalid: %w", err)
+		}
+
+		warnIfInsecureURL("linuxdo_connect.authorize_url", c.LinuxDo.AuthorizeURL)
+		warnIfInsecureURL("linuxdo_connect.token_url", c.LinuxDo.TokenURL)
+		warnIfInsecureURL("linuxdo_connect.userinfo_url", c.LinuxDo.UserInfoURL)
+		warnIfInsecureURL("linuxdo_connect.redirect_url", c.LinuxDo.RedirectURL)
+		warnIfInsecureURL("linuxdo_connect.frontend_redirect_url", c.LinuxDo.FrontendRedirectURL)
+	}
 	if c.Billing.CircuitBreaker.Enabled {
 		if c.Billing.CircuitBreaker.FailureThreshold <= 0 {
 			return fmt.Errorf("billing.circuit_breaker.failure_threshold must be positive")

backend/internal/config/config_test.go

 package config
 
 import (
+	"strings"
 	"testing"
 	"time"
 
@@ -90,3 +91,53 @@ func TestLoadDefaultSecurityToggles(t *testing.T) {
 		t.Fatalf("ResponseHeaders.Enabled = true, want false")
 	}
 }
+
+func TestValidateLinuxDoFrontendRedirectURL(t *testing.T) {
+	viper.Reset()
+
+	cfg, err := Load()
+	if err != nil {
+		t.Fatalf("Load() error: %v", err)
+	}
+
+	cfg.LinuxDo.Enabled = true
+	cfg.LinuxDo.ClientID = "test-client"
+	cfg.LinuxDo.ClientSecret = "test-secret"
+	cfg.LinuxDo.RedirectURL = "https://example.com/api/v1/auth/oauth/linuxdo/callback"
+	cfg.LinuxDo.TokenAuthMethod = "client_secret_post"
+	cfg.LinuxDo.UsePKCE = false
+
+	cfg.LinuxDo.FrontendRedirectURL = "javascript:alert(1)"
+	err = cfg.Validate()
+	if err == nil {
+		t.Fatalf("Validate() expected error for javascript scheme, got nil")
+	}
+	if !strings.Contains(err.Error(), "linuxdo_connect.frontend_redirect_url") {
+		t.Fatalf("Validate() expected frontend_redirect_url error, got: %v", err)
+	}
+}
+
+func TestValidateLinuxDoPKCERequiredForPublicClient(t *testing.T) {
+	viper.Reset()
+
+	cfg, err := Load()
+	if err != nil {
+		t.Fatalf("Load() error: %v", err)
+	}
+
+	cfg.LinuxDo.Enabled = true
+	cfg.LinuxDo.ClientID = "test-client"
+	cfg.LinuxDo.ClientSecret = ""
+	cfg.LinuxDo.RedirectURL = "https://example.com/api/v1/auth/oauth/linuxdo/callback"
+	cfg.LinuxDo.FrontendRedirectURL = "/auth/linuxdo/callback"
+	cfg.LinuxDo.TokenAuthMethod = "none"
+	cfg.LinuxDo.UsePKCE = false
+
+	err = cfg.Validate()
+	if err == nil {
+		t.Fatalf("Validate() expected error when token_auth_method=none and use_pkce=false, got nil")
+	}
+	if !strings.Contains(err.Error(), "linuxdo_connect.use_pkce") {
+		t.Fatalf("Validate() expected use_pkce error, got: %v", err)
+	}
+}

backend/internal/handler/auth_linuxdo_oauth.go

+package handler
+
+import (
+	"context"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"log"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+	"unicode/utf8"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/oauth"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
+
+	"github.com/gin-gonic/gin"
+	"github.com/imroc/req/v3"
+	"github.com/tidwall/gjson"
+)
+
+const (
+	linuxDoOAuthCookiePath        = "/api/v1/auth/oauth/linuxdo"
+	linuxDoOAuthStateCookieName   = "linuxdo_oauth_state"
+	linuxDoOAuthVerifierCookie    = "linuxdo_oauth_verifier"
+	linuxDoOAuthRedirectCookie    = "linuxdo_oauth_redirect"
+	linuxDoOAuthCookieMaxAgeSec   = 10 * 60 // 10 minutes
+	linuxDoOAuthDefaultRedirectTo = "/dashboard"
+	linuxDoOAuthDefaultFrontendCB = "/auth/linuxdo/callback"
+
+	linuxDoOAuthMaxRedirectLen      = 2048
+	linuxDoOAuthMaxFragmentValueLen = 512
+	linuxDoOAuthMaxSubjectLen       = 64 - len("linuxdo-")
+)
+
+type linuxDoTokenResponse struct {
+	AccessToken  string `json:"access_token"`
+	TokenType    string `json:"token_type"`
+	ExpiresIn    int64  `json:"expires_in"`
+	RefreshToken string `json:"refresh_token,omitempty"`
+	Scope        string `json:"scope,omitempty"`
+}
+
+// LinuxDoOAuthStart starts the LinuxDo Connect OAuth login flow.
+// GET /api/v1/auth/oauth/linuxdo/start?redirect=/dashboard
+func (h *AuthHandler) LinuxDoOAuthStart(c *gin.Context) {
+	cfg, err := linuxDoOAuthConfig(h.cfg)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	state, err := oauth.GenerateState()
+	if err != nil {
+		response.ErrorFrom(c, infraerrors.InternalServer("OAUTH_STATE_GEN_FAILED", "failed to generate oauth state").WithCause(err))
+		return
+	}
+
+	redirectTo := sanitizeFrontendRedirectPath(c.Query("redirect"))
+	if redirectTo == "" {
+		redirectTo = linuxDoOAuthDefaultRedirectTo
+	}
+
+	secureCookie := isRequestHTTPS(c)
+	setCookie(c, linuxDoOAuthStateCookieName, encodeCookieValue(state), linuxDoOAuthCookieMaxAgeSec, secureCookie)
+	setCookie(c, linuxDoOAuthRedirectCookie, encodeCookieValue(redirectTo), linuxDoOAuthCookieMaxAgeSec, secureCookie)
+
+	codeChallenge := ""
+	if cfg.UsePKCE {
+		verifier, err := oauth.GenerateCodeVerifier()
+		if err != nil {
+			response.ErrorFrom(c, infraerrors.InternalServer("OAUTH_PKCE_GEN_FAILED", "failed to generate pkce verifier").WithCause(err))
+			return
+		}
+		codeChallenge = oauth.GenerateCodeChallenge(verifier)
+		setCookie(c, linuxDoOAuthVerifierCookie, encodeCookieValue(verifier), linuxDoOAuthCookieMaxAgeSec, secureCookie)
+	}
+
+	redirectURI := strings.TrimSpace(cfg.RedirectURL)
+	if redirectURI == "" {
+		response.ErrorFrom(c, infraerrors.InternalServer("OAUTH_CONFIG_INVALID", "oauth redirect url not configured"))
+		return
+	}
+
+	authURL, err := buildLinuxDoAuthorizeURL(cfg, state, codeChallenge, redirectURI)
+	if err != nil {
+		response.ErrorFrom(c, infraerrors.InternalServer("OAUTH_BUILD_URL_FAILED", "failed to build oauth authorization url").WithCause(err))
+		return
+	}
+
+	c.Redirect(http.StatusFound, authURL)
+}
+
+// LinuxDoOAuthCallback handles the OAuth callback, creates/logins the user, then redirects to frontend.
+// GET /api/v1/auth/oauth/linuxdo/callback?code=...&state=...
+func (h *AuthHandler) LinuxDoOAuthCallback(c *gin.Context) {
+	cfg, cfgErr := linuxDoOAuthConfig(h.cfg)
+	if cfgErr != nil {
+		response.ErrorFrom(c, cfgErr)
+		return
+	}
+
+	frontendCallback := strings.TrimSpace(cfg.FrontendRedirectURL)
+	if frontendCallback == "" {
+		frontendCallback = linuxDoOAuthDefaultFrontendCB
+	}
+
+	if providerErr := strings.TrimSpace(c.Query("error")); providerErr != "" {
+		redirectOAuthError(c, frontendCallback, "provider_error", providerErr, c.Query("error_description"))
+		return
+	}
+
+	code := strings.TrimSpace(c.Query("code"))
+	state := strings.TrimSpace(c.Query("state"))
+	if code == "" || state == "" {
+		redirectOAuthError(c, frontendCallback, "missing_params", "missing code/state", "")
+		return
+	}
+
+	secureCookie := isRequestHTTPS(c)
+	defer func() {
+		clearCookie(c, linuxDoOAuthStateCookieName, secureCookie)
+		clearCookie(c, linuxDoOAuthVerifierCookie, secureCookie)
+		clearCookie(c, linuxDoOAuthRedirectCookie, secureCookie)
+	}()
+
+	expectedState, err := readCookieDecoded(c, linuxDoOAuthStateCookieName)
+	if err != nil || expectedState == "" || state != expectedState {
+		redirectOAuthError(c, frontendCallback, "invalid_state", "invalid oauth state", "")
+		return
+	}
+
+	redirectTo, _ := readCookieDecoded(c, linuxDoOAuthRedirectCookie)
+	redirectTo = sanitizeFrontendRedirectPath(redirectTo)
+	if redirectTo == "" {
+		redirectTo = linuxDoOAuthDefaultRedirectTo
+	}
+
+	codeVerifier := ""
+	if cfg.UsePKCE {
+		codeVerifier, _ = readCookieDecoded(c, linuxDoOAuthVerifierCookie)
+		if codeVerifier == "" {
+			redirectOAuthError(c, frontendCallback, "missing_verifier", "missing pkce verifier", "")
+			return
+		}
+	}
+
+	redirectURI := strings.TrimSpace(cfg.RedirectURL)
+	if redirectURI == "" {
+		redirectOAuthError(c, frontendCallback, "config_error", "oauth redirect url not configured", "")
+		return
+	}
+
+	tokenResp, err := linuxDoExchangeCode(c.Request.Context(), cfg, code, redirectURI, codeVerifier)
+	if err != nil {
+		log.Printf("[LinuxDo OAuth] token exchange failed: %v", err)
+		redirectOAuthError(c, frontendCallback, "token_exchange_failed", "failed to exchange oauth code", "")
+		return
+	}
+
+	email, username, _, err := linuxDoFetchUserInfo(c.Request.Context(), cfg, tokenResp)
+	if err != nil {
+		log.Printf("[LinuxDo OAuth] userinfo fetch failed: %v", err)
+		redirectOAuthError(c, frontendCallback, "userinfo_failed", "failed to fetch user info", "")
+		return
+	}
+
+	jwtToken, _, err := h.authService.LoginOrRegisterOAuth(c.Request.Context(), email, username)
+	if err != nil {
+		// Avoid leaking internal details to the client; keep structured reason for frontend.
+		redirectOAuthError(c, frontendCallback, "login_failed", infraerrors.Reason(err), infraerrors.Message(err))
+		return
+	}
+
+	fragment := url.Values{}
+	fragment.Set("access_token", jwtToken)
+	fragment.Set("token_type", "Bearer")
+	fragment.Set("redirect", redirectTo)
+	redirectWithFragment(c, frontendCallback, fragment)
+}
+
+func linuxDoOAuthConfig(cfg *config.Config) (config.LinuxDoConnectConfig, error) {
+	if cfg == nil {
+		return config.LinuxDoConnectConfig{}, infraerrors.ServiceUnavailable("CONFIG_NOT_READY", "config not loaded")
+	}
+	if !cfg.LinuxDo.Enabled {
+		return config.LinuxDoConnectConfig{}, infraerrors.NotFound("OAUTH_DISABLED", "oauth login is disabled")
+	}
+	return cfg.LinuxDo, nil
+}
+
+func linuxDoExchangeCode(
+	ctx context.Context,
+	cfg config.LinuxDoConnectConfig,
+	code string,
+	redirectURI string,
+	codeVerifier string,
+) (*linuxDoTokenResponse, error) {
+	client := req.C().SetTimeout(30 * time.Second)
+
+	form := url.Values{}
+	form.Set("grant_type", "authorization_code")
+	form.Set("client_id", cfg.ClientID)
+	form.Set("code", code)
+	form.Set("redirect_uri", redirectURI)
+	if cfg.UsePKCE {
+		form.Set("code_verifier", codeVerifier)
+	}
+
+	r := client.R().
+		SetContext(ctx).
+		SetHeader("Accept", "application/json")
+
+	switch strings.ToLower(strings.TrimSpace(cfg.TokenAuthMethod)) {
+	case "", "client_secret_post":
+		form.Set("client_secret", cfg.ClientSecret)
+	case "client_secret_basic":
+		r.SetBasicAuth(cfg.ClientID, cfg.ClientSecret)
+	case "none":
+	default:
+		return nil, fmt.Errorf("unsupported token_auth_method: %s", cfg.TokenAuthMethod)
+	}
+
+	var tokenResp linuxDoTokenResponse
+	resp, err := r.SetFormDataFromValues(form).SetSuccessResult(&tokenResp).Post(cfg.TokenURL)
+	if err != nil {
+		return nil, fmt.Errorf("request token: %w", err)
+	}
+	if !resp.IsSuccessState() {
+		return nil, fmt.Errorf("token exchange status=%d", resp.StatusCode)
+	}
+	if strings.TrimSpace(tokenResp.AccessToken) == "" {
+		return nil, errors.New("token response missing access_token")
+	}
+	if strings.TrimSpace(tokenResp.TokenType) == "" {
+		tokenResp.TokenType = "Bearer"
+	}
+	return &tokenResp, nil
+}
+
+func linuxDoFetchUserInfo(
+	ctx context.Context,
+	cfg config.LinuxDoConnectConfig,
+	token *linuxDoTokenResponse,
+) (email string, username string, subject string, err error) {
+	client := req.C().SetTimeout(30 * time.Second)
+	authorization, err := buildBearerAuthorization(token.TokenType, token.AccessToken)
+	if err != nil {
+		return "", "", "", fmt.Errorf("invalid token for userinfo request: %w", err)
+	}
+
+	resp, err := client.R().
+		SetContext(ctx).
+		SetHeader("Accept", "application/json").
+		SetHeader("Authorization", authorization).
+		Get(cfg.UserInfoURL)
+	if err != nil {
+		return "", "", "", fmt.Errorf("request userinfo: %w", err)
+	}
+	if !resp.IsSuccessState() {
+		return "", "", "", fmt.Errorf("userinfo status=%d", resp.StatusCode)
+	}
+
+	return linuxDoParseUserInfo(resp.String(), cfg)
+}
+
+func linuxDoParseUserInfo(body string, cfg config.LinuxDoConnectConfig) (email string, username string, subject string, err error) {
+	email = firstNonEmpty(
+		getGJSON(body, cfg.UserInfoEmailPath),
+		getGJSON(body, "email"),
+		getGJSON(body, "user.email"),
+		getGJSON(body, "data.email"),
+		getGJSON(body, "attributes.email"),
+	)
+	username = firstNonEmpty(
+		getGJSON(body, cfg.UserInfoUsernamePath),
+		getGJSON(body, "username"),
+		getGJSON(body, "preferred_username"),
+		getGJSON(body, "name"),
+		getGJSON(body, "user.username"),
+		getGJSON(body, "user.name"),
+	)
+	subject = firstNonEmpty(
+		getGJSON(body, cfg.UserInfoIDPath),
+		getGJSON(body, "sub"),
+		getGJSON(body, "id"),
+		getGJSON(body, "user_id"),
+		getGJSON(body, "uid"),
+		getGJSON(body, "user.id"),
+	)
+
+	subject = strings.TrimSpace(subject)
+	if subject == "" {
+		return "", "", "", errors.New("userinfo missing id field")
+	}
+	if !isSafeLinuxDoSubject(subject) {
+		return "", "", "", errors.New("userinfo returned invalid id field")
+	}
+
+	email = strings.TrimSpace(email)
+	if email == "" {
+		// LinuxDo Connect userinfo does not necessarily provide email. To keep compatibility with the
+		// existing user schema (email is required/unique), use a stable synthetic email.
+		email = fmt.Sprintf("linuxdo-%s@linuxdo-connect.invalid", subject)
+	}
+
+	username = strings.TrimSpace(username)
+	if username == "" {
+		username = "linuxdo_" + subject
+	}
+
+	return email, username, subject, nil
+}
+
+func buildLinuxDoAuthorizeURL(cfg config.LinuxDoConnectConfig, state string, codeChallenge string, redirectURI string) (string, error) {
+	u, err := url.Parse(cfg.AuthorizeURL)
+	if err != nil {
+		return "", fmt.Errorf("parse authorize_url: %w", err)
+	}
+
+	q := u.Query()
+	q.Set("response_type", "code")
+	q.Set("client_id", cfg.ClientID)
+	q.Set("redirect_uri", redirectURI)
+	if strings.TrimSpace(cfg.Scopes) != "" {
+		q.Set("scope", cfg.Scopes)
+	}
+	q.Set("state", state)
+	if cfg.UsePKCE {
+		q.Set("code_challenge", codeChallenge)
+		q.Set("code_challenge_method", "S256")
+	}
+
+	u.RawQuery = q.Encode()
+	return u.String(), nil
+}
+
+func redirectOAuthError(c *gin.Context, frontendCallback string, code string, message string, description string) {
+	fragment := url.Values{}
+	fragment.Set("error", truncateFragmentValue(code))
+	if strings.TrimSpace(message) != "" {
+		fragment.Set("error_message", truncateFragmentValue(message))
+	}
+	if strings.TrimSpace(description) != "" {
+		fragment.Set("error_description", truncateFragmentValue(description))
+	}
+	redirectWithFragment(c, frontendCallback, fragment)
+}
+
+func redirectWithFragment(c *gin.Context, frontendCallback string, fragment url.Values) {
+	u, err := url.Parse(frontendCallback)
+	if err != nil {
+		// Fallback: best-effort redirect.
+		c.Redirect(http.StatusFound, linuxDoOAuthDefaultRedirectTo)
+		return
+	}
+	if u.Scheme != "" && !strings.EqualFold(u.Scheme, "http") && !strings.EqualFold(u.Scheme, "https") {
+		c.Redirect(http.StatusFound, linuxDoOAuthDefaultRedirectTo)
+		return
+	}
+	u.Fragment = fragment.Encode()
+	c.Header("Cache-Control", "no-store")
+	c.Header("Pragma", "no-cache")
+	c.Redirect(http.StatusFound, u.String())
+}
+
+func firstNonEmpty(values ...string) string {
+	for _, v := range values {
+		v = strings.TrimSpace(v)
+		if v != "" {
+			return v
+		}
+	}
+	return ""
+}
+
+func getGJSON(body string, path string) string {
+	path = strings.TrimSpace(path)
+	if path == "" {
+		return ""
+	}
+	res := gjson.Get(body, path)
+	if !res.Exists() {
+		return ""
+	}
+	return res.String()
+}
+
+func sanitizeFrontendRedirectPath(path string) string {
+	path = strings.TrimSpace(path)
+	if path == "" {
+		return ""
+	}
+	if len(path) > linuxDoOAuthMaxRedirectLen {
+		return ""
+	}
+	// Only allow same-origin relative paths (avoid open redirect).
+	if !strings.HasPrefix(path, "/") {
+		return ""
+	}
+	if strings.HasPrefix(path, "//") {
+		return ""
+	}
+	if strings.Contains(path, "://") {
+		return ""
+	}
+	if strings.ContainsAny(path, "\r\n") {
+		return ""
+	}
+	return path
+}
+
+func isRequestHTTPS(c *gin.Context) bool {
+	if c.Request.TLS != nil {
+		return true
+	}
+	proto := strings.ToLower(strings.TrimSpace(c.GetHeader("X-Forwarded-Proto")))
+	return proto == "https"
+}
+
+func encodeCookieValue(value string) string {
+	return base64.RawURLEncoding.EncodeToString([]byte(value))
+}
+
+func decodeCookieValue(value string) (string, error) {
+	raw, err := base64.RawURLEncoding.DecodeString(value)
+	if err != nil {
+		return "", err
+	}
+	return string(raw), nil
+}
+
+func readCookieDecoded(c *gin.Context, name string) (string, error) {
+	ck, err := c.Request.Cookie(name)
+	if err != nil {
+		return "", err
+	}
+	return decodeCookieValue(ck.Value)
+}
+
+func setCookie(c *gin.Context, name string, value string, maxAgeSec int, secure bool) {
+	http.SetCookie(c.Writer, &http.Cookie{
+		Name:     name,
+		Value:    value,
+		Path:     linuxDoOAuthCookiePath,
+		MaxAge:   maxAgeSec,
+		HttpOnly: true,
+		Secure:   secure,
+		SameSite: http.SameSiteLaxMode,
+	})
+}
+
+func clearCookie(c *gin.Context, name string, secure bool) {
+	http.SetCookie(c.Writer, &http.Cookie{
+		Name:     name,
+		Value:    "",
+		Path:     linuxDoOAuthCookiePath,
+		MaxAge:   -1,
+		HttpOnly: true,
+		Secure:   secure,
+		SameSite: http.SameSiteLaxMode,
+	})
+}
+
+func truncateFragmentValue(value string) string {
+	value = strings.TrimSpace(value)
+	if value == "" {
+		return ""
+	}
+	if len(value) > linuxDoOAuthMaxFragmentValueLen {
+		value = value[:linuxDoOAuthMaxFragmentValueLen]
+		for !utf8.ValidString(value) {
+			value = value[:len(value)-1]
+		}
+	}
+	return value
+}
+
+func buildBearerAuthorization(tokenType, accessToken string) (string, error) {
+	tokenType = strings.TrimSpace(tokenType)
+	if tokenType == "" {
+		tokenType = "Bearer"
+	}
+	if !strings.EqualFold(tokenType, "Bearer") {
+		return "", fmt.Errorf("unsupported token_type: %s", tokenType)
+	}
+
+	accessToken = strings.TrimSpace(accessToken)
+	if accessToken == "" {
+		return "", errors.New("missing access_token")
+	}
+	if strings.ContainsAny(accessToken, " \t\r\n") {
+		return "", errors.New("access_token contains whitespace")
+	}
+	return "Bearer " + accessToken, nil
+}
+
+func isSafeLinuxDoSubject(subject string) bool {
+	subject = strings.TrimSpace(subject)
+	if subject == "" || len(subject) > linuxDoOAuthMaxSubjectLen {
+		return false
+	}
+	for _, r := range subject {
+		switch {
+		case r >= '0' && r <= '9':
+		case r >= 'a' && r <= 'z':
+		case r >= 'A' && r <= 'Z':
+		case r == '_' || r == '-':
+		default:
+			return false
+		}
+	}
+	return true
+}

backend/internal/handler/auth_linuxdo_oauth_test.go

+package handler
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSanitizeFrontendRedirectPath(t *testing.T) {
+	require.Equal(t, "/dashboard", sanitizeFrontendRedirectPath("/dashboard"))
+	require.Equal(t, "/dashboard", sanitizeFrontendRedirectPath(" /dashboard "))
+	require.Equal(t, "", sanitizeFrontendRedirectPath("dashboard"))
+	require.Equal(t, "", sanitizeFrontendRedirectPath("//evil.com"))
+	require.Equal(t, "", sanitizeFrontendRedirectPath("https://evil.com"))
+	require.Equal(t, "", sanitizeFrontendRedirectPath("/\nfoo"))
+
+	long := "/" + strings.Repeat("a", linuxDoOAuthMaxRedirectLen)
+	require.Equal(t, "", sanitizeFrontendRedirectPath(long))
+}
+
+func TestBuildBearerAuthorization(t *testing.T) {
+	auth, err := buildBearerAuthorization("", "token123")
+	require.NoError(t, err)
+	require.Equal(t, "Bearer token123", auth)
+
+	auth, err = buildBearerAuthorization("bearer", "token123")
+	require.NoError(t, err)
+	require.Equal(t, "Bearer token123", auth)
+
+	_, err = buildBearerAuthorization("MAC", "token123")
+	require.Error(t, err)
+
+	_, err = buildBearerAuthorization("Bearer", "token 123")
+	require.Error(t, err)
+}
+
+func TestLinuxDoParseUserInfoParsesIDAndUsername(t *testing.T) {
+	cfg := config.LinuxDoConnectConfig{
+		UserInfoURL: "https://connect.linux.do/api/user",
+	}
+
+	email, username, subject, err := linuxDoParseUserInfo(`{"id":123,"username":"alice"}`, cfg)
+	require.NoError(t, err)
+	require.Equal(t, "123", subject)
+	require.Equal(t, "alice", username)
+	require.Equal(t, "linuxdo-123@linuxdo-connect.invalid", email)
+}
+
+func TestLinuxDoParseUserInfoDefaultsUsername(t *testing.T) {
+	cfg := config.LinuxDoConnectConfig{
+		UserInfoURL: "https://connect.linux.do/api/user",
+	}
+
+	email, username, subject, err := linuxDoParseUserInfo(`{"id":"123"}`, cfg)
+	require.NoError(t, err)
+	require.Equal(t, "123", subject)
+	require.Equal(t, "linuxdo_123", username)
+	require.Equal(t, "linuxdo-123@linuxdo-connect.invalid", email)
+}
+
+func TestLinuxDoParseUserInfoRejectsUnsafeSubject(t *testing.T) {
+	cfg := config.LinuxDoConnectConfig{
+		UserInfoURL: "https://connect.linux.do/api/user",
+	}
+
+	_, _, _, err := linuxDoParseUserInfo(`{"id":"123@456"}`, cfg)
+	require.Error(t, err)
+
+	tooLong := strings.Repeat("a", linuxDoOAuthMaxSubjectLen+1)
+	_, _, _, err = linuxDoParseUserInfo(`{"id":"`+tooLong+`"}`, cfg)
+	require.Error(t, err)
+}

backend/internal/handler/dto/settings.go

@@ -50,5 +50,6 @@ type PublicSettings struct {
 	APIBaseURL          string `json:"api_base_url"`
 	ContactInfo         string `json:"contact_info"`
 	DocURL              string `json:"doc_url"`
+	LinuxDoOAuthEnabled bool   `json:"linuxdo_oauth_enabled"`
 	Version             string `json:"version"`
 }

backend/internal/handler/setting_handler.go

@@ -42,6 +42,7 @@ func (h *SettingHandler) GetPublicSettings(c *gin.Context) {
 		APIBaseURL:          settings.APIBaseURL,
 		ContactInfo:         settings.ContactInfo,
 		DocURL:              settings.DocURL,
+		LinuxDoOAuthEnabled: settings.LinuxDoOAuthEnabled,
 		Version:             h.version,
 	})
 }

backend/internal/server/routes/auth.go

@@ -19,6 +19,8 @@ func RegisterAuthRoutes(
 		auth.POST("/register", h.Auth.Register)
 		auth.POST("/login", h.Auth.Login)
 		auth.POST("/send-verify-code", h.Auth.SendVerifyCode)
+		auth.GET("/oauth/linuxdo/start", h.Auth.LinuxDoOAuthStart)
+		auth.GET("/oauth/linuxdo/callback", h.Auth.LinuxDoOAuthCallback)
 	}
 
 	// 公开设置（无需认证）

backend/internal/service/auth_service.go

@@ -2,9 +2,13 @@ package service
 
 import (
 	"context"
+	"crypto/rand"
+	"encoding/hex"
 	"errors"
 	"fmt"
 	"log"
+	"net/mail"
+	"strings"
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
@@ -18,6 +22,7 @@ var (
 	ErrInvalidCredentials  = infraerrors.Unauthorized("INVALID_CREDENTIALS", "invalid email or password")
 	ErrUserNotActive       = infraerrors.Forbidden("USER_NOT_ACTIVE", "user is not active")
 	ErrEmailExists         = infraerrors.Conflict("EMAIL_EXISTS", "email already exists")
+	ErrEmailReserved       = infraerrors.BadRequest("EMAIL_RESERVED", "email is reserved")
 	ErrInvalidToken        = infraerrors.Unauthorized("INVALID_TOKEN", "invalid token")
 	ErrTokenExpired        = infraerrors.Unauthorized("TOKEN_EXPIRED", "token has expired")
 	ErrTokenTooLarge       = infraerrors.BadRequest("TOKEN_TOO_LARGE", "token too large")
@@ -27,6 +32,8 @@ var (
 	ErrServiceUnavailable  = infraerrors.ServiceUnavailable("SERVICE_UNAVAILABLE", "service temporarily unavailable")
 )
 
+const linuxDoSyntheticEmailDomain = "@linuxdo-connect.invalid"
+
 // maxTokenLength 限制 token 大小，避免超长 header 触发解析时的异常内存分配。
 const maxTokenLength = 8192
 
@@ -80,6 +87,11 @@ func (s *AuthService) RegisterWithVerification(ctx context.Context, email, passw
 		return "", nil, ErrRegDisabled
 	}
 
+	// Prevent users from registering emails reserved for synthetic OAuth accounts.
+	if isReservedEmail(email) {
+		return "", nil, ErrEmailReserved
+	}
+
 	// 检查是否需要邮件验证
 	if s.settingService != nil && s.settingService.IsEmailVerifyEnabled(ctx) {
 		// 如果邮件验证已开启但邮件服务未配置，拒绝注册
@@ -161,6 +173,10 @@ func (s *AuthService) SendVerifyCode(ctx context.Context, email string) error {
 		return ErrRegDisabled
 	}
 
+	if isReservedEmail(email) {
+		return ErrEmailReserved
+	}
+
 	// 检查邮箱是否已存在
 	existsEmail, err := s.userRepo.ExistsByEmail(ctx, email)
 	if err != nil {
@@ -195,6 +211,10 @@ func (s *AuthService) SendVerifyCodeAsync(ctx context.Context, email string) (*S
 		return nil, ErrRegDisabled
 	}
 
+	if isReservedEmail(email) {
+		return nil, ErrEmailReserved
+	}
+
 	// 检查邮箱是否已存在
 	existsEmail, err := s.userRepo.ExistsByEmail(ctx, email)
 	if err != nil {
@@ -319,6 +339,101 @@ func (s *AuthService) Login(ctx context.Context, email, password string) (string
 	return token, user, nil
 }
 
+// LoginOrRegisterOAuth logs a user in by email (trusted from an OAuth provider) or creates a new user.
+//
+// This is used by end-user OAuth/SSO login flows (e.g. LinuxDo Connect), and intentionally does
+// NOT require the local password. A random password hash is generated for new users to satisfy
+// the existing database constraint.
+func (s *AuthService) LoginOrRegisterOAuth(ctx context.Context, email, username string) (string, *User, error) {
+	email = strings.TrimSpace(email)
+	if email == "" || len(email) > 255 {
+		return "", nil, infraerrors.BadRequest("INVALID_EMAIL", "invalid email")
+	}
+	if _, err := mail.ParseAddress(email); err != nil {
+		return "", nil, infraerrors.BadRequest("INVALID_EMAIL", "invalid email")
+	}
+
+	username = strings.TrimSpace(username)
+	if len([]rune(username)) > 100 {
+		username = string([]rune(username)[:100])
+	}
+
+	user, err := s.userRepo.GetByEmail(ctx, email)
+	if err != nil {
+		if errors.Is(err, ErrUserNotFound) {
+			// Treat OAuth-first login as registration.
+			if s.settingService != nil && !s.settingService.IsRegistrationEnabled(ctx) {
+				return "", nil, ErrRegDisabled
+			}
+
+			randomPassword, err := randomHexString(32)
+			if err != nil {
+				log.Printf("[Auth] Failed to generate random password for oauth signup: %v", err)
+				return "", nil, ErrServiceUnavailable
+			}
+			hashedPassword, err := s.HashPassword(randomPassword)
+			if err != nil {
+				return "", nil, fmt.Errorf("hash password: %w", err)
+			}
+
+			// Defaults for new users.
+			defaultBalance := s.cfg.Default.UserBalance
+			defaultConcurrency := s.cfg.Default.UserConcurrency
+			if s.settingService != nil {
+				defaultBalance = s.settingService.GetDefaultBalance(ctx)
+				defaultConcurrency = s.settingService.GetDefaultConcurrency(ctx)
+			}
+
+			newUser := &User{
+				Email:        email,
+				Username:     username,
+				PasswordHash: hashedPassword,
+				Role:         RoleUser,
+				Balance:      defaultBalance,
+				Concurrency:  defaultConcurrency,
+				Status:       StatusActive,
+			}
+
+			if err := s.userRepo.Create(ctx, newUser); err != nil {
+				if errors.Is(err, ErrEmailExists) {
+					// Race: user created between GetByEmail and Create.
+					user, err = s.userRepo.GetByEmail(ctx, email)
+					if err != nil {
+						log.Printf("[Auth] Database error getting user after conflict: %v", err)
+						return "", nil, ErrServiceUnavailable
+					}
+				} else {
+					log.Printf("[Auth] Database error creating oauth user: %v", err)
+					return "", nil, ErrServiceUnavailable
+				}
+			} else {
+				user = newUser
+			}
+		} else {
+			log.Printf("[Auth] Database error during oauth login: %v", err)
+			return "", nil, ErrServiceUnavailable
+		}
+	}
+
+	if !user.IsActive() {
+		return "", nil, ErrUserNotActive
+	}
+
+	// Best-effort: fill username when empty.
+	if user.Username == "" && username != "" {
+		user.Username = username
+		if err := s.userRepo.Update(ctx, user); err != nil {
+			log.Printf("[Auth] Failed to update username after oauth login: %v", err)
+		}
+	}
+
+	token, err := s.GenerateToken(user)
+	if err != nil {
+		return "", nil, fmt.Errorf("generate token: %w", err)
+	}
+	return token, user, nil
+}
+
 // ValidateToken 验证JWT token并返回用户声明
 func (s *AuthService) ValidateToken(tokenString string) (*JWTClaims, error) {
 	// 先做长度校验，尽早拒绝异常超长 token，降低 DoS 风险。
@@ -361,6 +476,22 @@ func (s *AuthService) ValidateToken(tokenString string) (*JWTClaims, error) {
 	return nil, ErrInvalidToken
 }
 
+func randomHexString(byteLength int) (string, error) {
+	if byteLength <= 0 {
+		byteLength = 16
+	}
+	buf := make([]byte, byteLength)
+	if _, err := rand.Read(buf); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(buf), nil
+}
+
+func isReservedEmail(email string) bool {
+	normalized := strings.ToLower(strings.TrimSpace(email))
+	return strings.HasSuffix(normalized, linuxDoSyntheticEmailDomain)
+}
+
 // GenerateToken 生成JWT token
 func (s *AuthService) GenerateToken(user *User) (string, error) {
 	now := time.Now()

backend/internal/service/auth_service_register_test.go

@@ -182,6 +182,14 @@ func TestAuthService_Register_CheckEmailError(t *testing.T) {
 	require.ErrorIs(t, err, ErrServiceUnavailable)
 }
 
+func TestAuthService_Register_ReservedEmail(t *testing.T) {
+	repo := &userRepoStub{}
+	service := newAuthService(repo, nil, nil)
+
+	_, _, err := service.Register(context.Background(), "linuxdo-123@linuxdo-connect.invalid", "password")
+	require.ErrorIs(t, err, ErrEmailReserved)
+}
+
 func TestAuthService_Register_CreateError(t *testing.T) {
 	repo := &userRepoStub{createErr: errors.New("create failed")}
 	service := newAuthService(repo, map[string]string{

backend/internal/service/setting_service.go

@@ -82,6 +82,7 @@ func (s *SettingService) GetPublicSettings(ctx context.Context) (*PublicSettings
 		APIBaseURL:          settings[SettingKeyAPIBaseURL],
 		ContactInfo:         settings[SettingKeyContactInfo],
 		DocURL:              settings[SettingKeyDocURL],
+		LinuxDoOAuthEnabled: s.cfg != nil && s.cfg.LinuxDo.Enabled,
 	}, nil
 }
 

backend/internal/service/settings_view.go

@@ -51,5 +51,6 @@ type PublicSettings struct {
 	APIBaseURL          string
 	ContactInfo         string
 	DocURL              string
+	LinuxDoOAuthEnabled bool
 	Version             string
 }

deploy/config.example.yaml

@@ -234,6 +234,31 @@ jwt:
   # 令牌过期时间（小时，最大 24）
   expire_hour: 24
 
+# =============================================================================
+# LinuxDo Connect OAuth Login (SSO)
+# LinuxDo Connect OAuth 登录（用于 Sub2API 用户登录）
+# =============================================================================
+linuxdo_connect:
+  enabled: false
+  client_id: ""
+  client_secret: ""
+  authorize_url: "https://connect.linux.do/oauth2/authorize"
+  token_url: "https://connect.linux.do/oauth2/token"
+  userinfo_url: "https://connect.linux.do/api/user"
+  scopes: "user"
+  # 示例: "https://your-domain.com/api/v1/auth/oauth/linuxdo/callback"
+  redirect_url: ""
+  # 安全提示：
+  # - 建议使用同源相对路径（以 / 开头），避免把 token 重定向到意外的第三方域名
+  # - 该地址不应包含 #fragment（本实现使用 URL fragment 传递 access_token）
+  frontend_redirect_url: "/auth/linuxdo/callback"
+  token_auth_method: "client_secret_post" # client_secret_post | client_secret_basic | none
+  # 注意：当 token_auth_method=none（public client）时，必须启用 PKCE
+  use_pkce: false
+  userinfo_email_path: ""
+  userinfo_id_path: ""
+  userinfo_username_path: ""
+
 # =============================================================================
 # Default Settings
 # 默认设置

frontend/src/i18n/locales/en.ts

@@ -229,6 +229,15 @@ export default {
     sendingCode: 'Sending...',
     clickToResend: 'Click to resend code',
     resendCode: 'Resend verification code',
+    linuxdo: {
+      signIn: 'Continue with Linux.do',
+      orContinue: 'or continue with email',
+      callbackTitle: 'Signing you in',
+      callbackProcessing: 'Completing login, please wait...',
+      callbackHint: 'If you are not redirected automatically, go back to the login page and try again.',
+      callbackMissingToken: 'Missing login token, please try again.',
+      backToLogin: 'Back to Login'
+    },
     oauth: {
       code: 'Code',
       state: 'State',

frontend/src/i18n/locales/zh.ts

@@ -227,6 +227,15 @@ export default {
     sendingCode: '发送中...',
     clickToResend: '点击重新发送验证码',
     resendCode: '重新发送验证码',
+    linuxdo: {
+      signIn: '使用 Linux.do 登录',
+      orContinue: '或使用邮箱密码继续',
+      callbackTitle: '正在完成登录',
+      callbackProcessing: '正在验证登录信息，请稍候...',
+      callbackHint: '如果页面未自动跳转，请返回登录页重试。',
+      callbackMissingToken: '登录信息缺失，请返回重试。',
+      backToLogin: '返回登录'
+    },
     oauth: {
       code: '授权码',
       state: '状态',

frontend/src/router/index.ts

@@ -67,6 +67,15 @@ const routes: RouteRecordRaw[] = [
       title: 'OAuth Callback'
     }
   },
+  {
+    path: '/auth/linuxdo/callback',
+    name: 'LinuxDoOAuthCallback',
+    component: () => import('@/views/auth/LinuxDoCallbackView.vue'),
+    meta: {
+      requiresAuth: false,
+      title: 'LinuxDo OAuth Callback'
+    }
+  },
 
   // ==================== User Routes ====================
   {

frontend/src/stores/app.ts

@@ -282,23 +282,24 @@ export const useAppStore = defineStore('app', () => {
    * Fetch public settings (uses cache unless force=true)
    * @param force - Force refresh from API
    */
-  async function fetchPublicSettings(force = false): Promise<PublicSettings | null> {
-    // Return cached data if available and not forcing refresh
-    if (publicSettingsLoaded.value && !force) {
-      return {
-        registration_enabled: false,
-        email_verify_enabled: false,
-        turnstile_enabled: false,
-        turnstile_site_key: '',
-        site_name: siteName.value,
-        site_logo: siteLogo.value,
-        site_subtitle: '',
-        api_base_url: apiBaseUrl.value,
-        contact_info: contactInfo.value,
-        doc_url: docUrl.value,
-        version: siteVersion.value
-      }
-    }
+	  async function fetchPublicSettings(force = false): Promise<PublicSettings | null> {
+	    // Return cached data if available and not forcing refresh
+	    if (publicSettingsLoaded.value && !force) {
+	      return {
+	        registration_enabled: false,
+	        email_verify_enabled: false,
+	        turnstile_enabled: false,
+	        turnstile_site_key: '',
+	        site_name: siteName.value,
+	        site_logo: siteLogo.value,
+	        site_subtitle: '',
+	        api_base_url: apiBaseUrl.value,
+	        contact_info: contactInfo.value,
+	        doc_url: docUrl.value,
+	        linuxdo_oauth_enabled: false,
+	        version: siteVersion.value
+	      }
+	    }
 
     // Prevent duplicate requests
     if (publicSettingsLoading.value) {

frontend/src/stores/auth.ts

@@ -159,6 +159,27 @@ export const useAuthStore = defineStore('auth', () => {
     }
   }
 
+  /**
+   * Set token directly (OAuth/SSO callback) and load current user profile.
+   * @param newToken - JWT access token issued by backend
+   */
+  async function setToken(newToken: string): Promise<User> {
+    // Clear any previous state first (avoid mixing sessions)
+    clearAuth()
+
+    token.value = newToken
+    localStorage.setItem(AUTH_TOKEN_KEY, newToken)
+
+    try {
+      const userData = await refreshUser()
+      startAutoRefresh()
+      return userData
+    } catch (error) {
+      clearAuth()
+      throw error
+    }
+  }
+
   /**
    * User logout
    * Clears all authentication state and persisted data
@@ -233,6 +254,7 @@ export const useAuthStore = defineStore('auth', () => {
     // Actions
     login,
     register,
+    setToken,
     logout,
     checkAuth,
     refreshUser

frontend/src/types/index.ts

@@ -73,6 +73,7 @@ export interface PublicSettings {
   api_base_url: string
   contact_info: string
   doc_url: string
+  linuxdo_oauth_enabled: boolean
   version: string
 }