Gitea: OAuth2 access token scope enforcement bypass via HTTP Basic authentication

Summary

Gitea fails to enforce OAuth2 access token scopes when the token is submitted via HTTP Basic authentication instead of a Bearer token. An OAuth2 application granted only read:user can use the same token as Authorization: Basic base64(<token>:x-oauth-basic) and perform write actions, including modifying profiles, adding email addresses, creating repositories, and deleting repositories as the authorizing user.

Details

Root cause: services/auth/basic.go accepts OAuth2 access tokens through the Basic auth path but does not store the token scope in the request context:

// services/auth/basic.go
if uid != 0 {
    store.GetData()["LoginMethod"] = OAuth2TokenMethodName
    store.GetData()["IsApiToken"] = true   // scope is NOT set
    return u, nil
}

The scope enforcement middleware in routers/api/v1/api.go exits early when ApiTokenScope is absent:

// routers/api/v1/api.go — tokenRequiresScopes
scope, scopeExists := ctx.Data["ApiTokenScope"].(auth_model.AccessTokenScope)
if ctx.Data["IsApiToken"] != true || !scopeExists {
    return   //<- exits without checking scope, all actions permitted
}

When a token arrives via Bearer, ApiTokenScope is populated and scope checks apply normally. When the same token arrives via Basic auth, ApiTokenScope is never set, so tokenRequiresScopes returns immediately and no scope is enforced.

Suggested fix: When an OAuth2 access token is accepted in services/auth/basic.go, populate ApiTokenScope in the request context identically to the Bearer-token OAuth2 path.

PoC

1. Create an OAuth2 application in Gitea.
2. Authorize it as a normal user with scope read:user only.
3. Take the resulting access token and call a write endpoint both ways:

Bearer | correctly blocked:

Authorization: Bearer <token>
PATCH /api/v1/user/settings  ->  403 Forbidden

Basic | bypass:

Authorization: Basic base64(<token>:x-oauth-basic)
PATCH /api/v1/user/settings  ->  200 OK

All verified bypass endpoints using a read:user-only token:

Endpoint	Bearer	Basic
PATCH /api/v1/user/settings	403	200
POST /api/v1/user/emails	403	200
POST /api/v1/user/repos	403	200
PATCH /api/v1/repos/{owner}/{repo}	403	200
DELETE /api/v1/repos/{owner}/{repo}	403	200

The bypass respects the user's normal repository permissions, it does not grant access to repositories the user cannot otherwise reach, and does not escalate to admin.

Impact

Any OAuth2 application with any restricted scope can silently operate beyond its granted permissions by switching from Bearer to Basic auth. An attacker who obtains a token (e.g. via a malicious OAuth2 app a user authorized) can:

• Modify the victim's profile and settings
• Add attacker-controlled email addresses to the victim's account
• Create repositories as the victim
• Modify or delete the victim's private repositories

The entire OAuth2 scope system is effectively bypassed for any token submitted via Basic auth.