Executive brief
rclone's FTP server with auth-proxy support stores authenticated user credentials in a server-wide map keyed only by username, without binding them to individual FTP sessions. When two users with different credentials share the same username and have access to different backend storage locations, an authenticated attacker can gain unauthorized read, write, and delete access to another user's data by keeping their session open while a victim logs in, causing the attacker's session to be silently redirected to the victim's backend.
Technical details
The vulnerability is a session-isolation failure in rclone's FTP auth-proxy driver. When a user authenticates via FTP, the driver stores an obscured password in a driver-global map keyed only by username (cmd/serve/ftp/ftp.go:170-178), discarding the returned VFS and cache key from the authentication call. For each subsequent FTP operation, the driver retrieves whichever password was most recently stored for that username and calls the proxy again, rather than maintaining per-session authentication state. If two different credentials use the same username but resolve to different backend locations (a supported deployment model per cmd/serve/proxy:235-243), a later login overwrites the map entry. The first session's subsequent operations then execute against the second user's backend using the second user's credentials, without re-authentication. The attack requires: (1) an auth-proxy that accepts distinct credentials for the same username with different backend authority, (2) the attacker to maintain an active FTP session while a victim with a different credential logs in with the same username, and (3) the attacker to issue additional FTP commands after the victim's login. A mutex prevents data races but does not provide session isolation. Patch available in v1.75.1.
Affected products
- rclone rclone >=1.64.0, <=1.75.0
Timeline
- 2026-09-04: disclosed
- 2026-09-10: patched: patch released in v1.75.1
- 2026-09-10: advisory