CVE-2026-88044

rclone versions 1.70.0 up to (but not including) 1.75.1 contain an incorrect-authorization flaw (CWE-863) in the FTP and S3 server constructors used when servers are launched through the Remote Control (RC) 'serve/start' interface: the code reads the process-global proxy.Opt.AuthProxy setting instead of the per-server proxyOpt.AuthProxy value supplied with the request. When the global option is empty and a request-local authentication proxy is configured, the local proxy is silently ignored, so an rclone FTP server falls back to serving a fixed filesystem and accepts the username 'anonymous' with any password, and an rclone S3 server with AuthKey serves the fixed RC filesystem rather than the backend selected by the proxy. An attacker with network access to such a server gains unauthorized access to the files on the served filesystem (high confidentiality and integrity impact per the 9.1 CVSS score). Only deployments that start FTP or S3 servers via the RC serve/start interface with a per-server auth proxy while the global auth proxy is unset are affected; dedicated command-line servers that configure the global option are not affected. There is no known public proof-of-concept, the issue is not in CISA KEV, and no exploitation has been reported; the fix is in rclone 1.75.1.

What to do: Upgrade to rclone 1.75.1 or later. As interim mitigations, set the global proxy.Opt.AuthProxy option (command-line servers configured this way are not affected), avoid per-server AuthProxy settings on RC-started FTP/S3 servers, or restrict network access to those servers. Check whether any exposed rclone FTP server accepts the username 'anonymous' with an arbitrary password, which indicates the vulnerable configuration.