Executive brief
Cloudreve, a self-hosted file management and sharing system, contains a security flaw in how it handles document viewing sessions. An attacker or a compromised third-party document viewer could bypass security checks to modify files even if they were only granted permission to view them. This could lead to unauthorized data modification or loss of integrity for files stored within the system.
Technical details
Cloudreve's WOPI implementation fails to properly validate access tokens and session permissions in the `ViewerSessionValidation` middleware. The system generates tokens in a `<session-id>.<random-secret>` format but only validates the session ID prefix, ignoring the cryptographic secret suffix. Furthermore, the system does not persist or enforce the requested viewer action (e.g., 'view' vs 'edit') within the `ViewerSessionCache`. Consequently, an attacker with a valid view-only session ID can forge the token suffix and invoke WOPI write routes (`PutFile`, `ModifyFile`) to modify underlying files. This issue is resolved in version 4.17.0 by implementing full token comparison and session-level permission enforcement.
Affected products
- cloudreve Cloudreve < 4.17.0
Timeline
- 2026-06-26: patched: Version 4.17.0 released
- 2026-07-23: advisory: GitHub Security Advisory published
- 2026-07-31: disclosed: NVD publication date