TWO critical vulnerabilities have been disclosed in Rclone that together enable a complete auth proxy bypass, potentially letting unauthenticated users read, modify, or delete data across connected cloud backends. Public PoC details accompany two separate flaws: a SigV4 signature bypass tied to configuring the –auth-proxy flag without an –auth-key, and a per-server option parsing mismatch that ignores the intended per-server configuration when a process-global proxy setting is used.
The security advisory notes that an empty string remains a valid HMAC key, meaning an attacker can forge a valid SigV4 signature for any access key ID without a real secret, allowing fully authenticated bucket listings. The second issue arises from the server startup logic: when the process-global option is empty, the supplied authentication proxy is silently ignored, causing FTP servers to permit anonymous read/write access and S3 servers to expose a fixed filesystem rather than the backend. The combined effect is that remote attackers can bypass authentication entirely on vulnerable deployments.
Affected versions span multiple Rclone releases. The SigV4 bypass affects all versions up to and including 1.68.0, while the per-server auth proxy bypass covers 1.70.0 through 1.75.0 and the development commit 5629f2668c69149bf3d9d8e2a25bb32a2648606e. An official fix has been released: upgrade to Rclone 1.75.1 at once. If upgrading isn’t possible immediately, ensure that –auth-key is always explicitly defined, and avoid exposing remotely controlled FTP or S3 servers to untrusted networks.
Administrators should audit access logs for suspicious activity such as unexpected bucket listings. The article also references GHSA advisories GHSA-xwwr-4h3p-r22c and GHSA-p569-5gjg-9cmj.