Description

rclone is a command-line program to sync files and directories to and from different cloud storage providers. Prior to 1.75.1, rclone serve s3 configured with --auth-proxy but without --auth-key allows authPairMiddleware to register any client-chosen accessKeyID with an empty ws.s3Secret. gofakes3 then verifies the request’s SigV4 signature against that same empty secret, while Server.auth passes the access key identifier as both the user and authentication value to the proxy without an independent per-identity secret. An unauthenticated network attacker can therefore choose an arbitrary access key, sign with an empty secret, and reach whatever backend the auth-proxy script resolves for that identity. This issue is fixed in version 1.75.1.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "9.8",
  "pubDate": "2026-09-10T16:18:08.913Z",
  "pubdate": "2026-09-10T16:18:08.913Z",
  "executiveSummary": "A critical authentication bypass vulnerability exists in rclone, a command-line program used for syncing files and directories to and from cloud storage providers. The flaw affects the 'rclone serve s3' component when configured with the '--auth-proxy' flag but lacking the '--auth-key' parameter, in versions prior to 1.75.1.\nThis misconfiguration allows unauthenticated remote attackers to bypass S3 signature verification and gain unauthorized access to backend storage. By exploiting this vulnerability, an attacker can specify an arbitrary access key identifier and sign the request using an empty secret key. The system validates this signature against the empty secret and subsequently forwards the request to the backend auth-proxy script under the assumed identity.\nConsequently, the risk implication is severe, potentially leading to unauthorized data access, modification, or deletion across any backend storage identity resolved by the proxy script. No special privileges or prior authentication are required to exploit this flaw, exposing affected installations to significant data compromise risk.",
  "technicalDetails": "The root cause of this vulnerability lies within the interaction between rclone's S3 serving middleware and the underlying gofakes3 library when '--auth-proxy' is enabled without '--auth-key'. Specifically, the 'authPairMiddleware' function is responsible for registering incoming client credentials.\nWhen '--auth-key' is omitted, the middleware registers any client-provided 'accessKeyID' with an empty string as the corresponding secret ('ws.s3Secret'). Subsequently, the request is processed by 'gofakes3', which performs AWS Signature Version 4 (SigV4) verification. Because the registered secret is empty, gofakes3 validates the signature of the incoming request against this empty secret.\nAn attacker can construct a valid SigV4 signature using an empty string as the signing key and any arbitrary 'accessKeyID'. Once the signature verification succeeds, 'Server.auth' processes the authenticated request. It passes the attacker-chosen access key identifier as both the user and the authentication value to the external 'auth-proxy' script. Since the 'auth-proxy' script does not perform an independent verification of a per-identity secret, it trusts the identifier provided by the rclone server and resolves the backend storage credentials for that identity.\nThe attack flow is executed in the following steps: First, the unauthenticated network attacker identifies an rclone instance running 'rclone serve s3' with '--auth-proxy' and no '--auth-key'. Second, the attacker generates an S3 request with an arbitrary target 'accessKeyID'. Third, the attacker signs the request using AWS SigV4, using an empty string ('') as the secret key. Fourth, rclone's 'authPairMiddleware' registers the key with the empty secret, and 'gofakes3' successfully verifies the signature. Fifth, 'Server.auth' forwards the arbitrary identity to the backend proxy script. Finally, the proxy script grants access to the backend storage mapped to that identity, allowing full read/write access.\nThis vulnerability affects all rclone versions prior to 1.75.1. It requires no privileges, can be executed completely unauthenticated over the network, and exposes the underlying storage systems configured within the proxy script to unauthorized control."
}