CVE-2026-77637: Cloudreve: Privilege Scope Bypass: State-Mutating Admin Operations Accessible via Read-Only OAuth Scope

Summary There is a privilege scope bypass in Cloudreve's admin API where two endpoints that mutate server state are missing the write-scope enforcement that their neighboring endpoints correctly apply. Specifically, the WOPI configuration fetch endpoint and the SMTP test/mail endpoint can both be triggered by an OAuth token that only has Admin.Read authorization — no Admin.Write needed. This breaks the intended OAuth scope boundary in a way that's subtle enough to have slipped through but meaningful enough to matter in a real deployment.

Details The root cause is an inconsistency in how the admin tool routes are wired up in routers/router.go. The admin route group sets a baseline of ScopeAdminRead for everything underneath it (around line 868), and most write-capable endpoints inside the tool sub-group correctly layer on an additional RequiredScopes(types.ScopeAdminWrite) check on top of that. For example, the thumbnail executable setter (line 929) and the entity URL cache deletion (line 937) both do this properly.

The two endpoints that don't follow this pattern are tool.GET('wopi') (line 925) and tool.POST('mail') (line 933). Despite POST /mail clearly triggering an outbound email and GET /wopi fetching or probing WOPI service connectivity, neither has the ScopeAdminWrite guard. What that means in practice is that any OAuth application granted only Admin.Read — a scope that should be limited to inspecting configuration, not changing anything — can silently invoke both of these operations. The developer looking at the route definitions would reasonably assume all the write-adjacent tool endpoints were protected, because the ones right above and below them are. It's the kind of gap that's easy to miss in a code review.

PoC bash OAuth app with ONLY Admin.Read scope can send emails: curl -s -X POST -H 'Content-Type: application/json' \ -H 'Authorization: Bearer <ADMINREADONLYTOKEN>' \ 'https://cloudreve.example.com/api/v4/admin/tool/mail' \ -d '{"settings":{"smtpHost":"smtp.gmail.com","smtpPort":"587",...},"to":"victim@example.com"}' Expected: 403 Forbidden (if Admin.Write was required) Actual: 200 OK, email is sent

Impact An OAuth application or API key scoped to Admin.Read can send arbitrary emails through the server's configured SMTP account and probe internal WOPI service endpoints — both actions that should require elevated write authorization. In multi-party deployments where Admin.Read tokens are issued more liberally (e.g., to monitoring integrations or third-party plugins), a compromised or malicious token holder gains capabilities well beyond what the scope contract implies. Scope separation in OAuth 2.0 is a security boundary, not just a convention, and its violation here could factor into broader attack chains.

Fix Add middleware.RequiredScopes(types.ScopeAdminWrite) as the first handler argument to both the tool.GET('wopi') and tool.POST('mail') route definitions in routers/router.go. This brings them in line with tool.POST('thumbExecutable') at line 929 and tool.DELETE('entityUrlCache') at line 937, which already follow the correct pattern. No logic changes are needed — it's purely an additive middleware insertion.

If possible, please apply for a CVE number when publishing. I would greatly appreciate it.