SigV4 Authentication Bypass in rclone serve s3

The rclone serve s3 command contains a critical vulnerability (CVE-2026-88018) in its authentication middleware chain. When a user configures rclone serve s3 with the --auth-proxy flag but fails to provide an --auth-key, the application incorrectly handles the credential registration process. Specifically, the authPairMiddleware parses the AccessKeyID directly from the client-controlled Authorization header and registers it into the internal credential store with an empty string as the secret key.

Because an empty string is a valid HMAC key for SigV4, any network-reachable attacker can compute a valid signature for an arbitrary AccessKeyID using an empty secret. The application subsequently accepts these forged signatures as authenticated requests. This effectively bypasses all authentication for the S3 interface, granting attackers the ability to interact with the backend as any identity accepted by the configured auth-proxy script. This vulnerability affects all rclone versions prior to 1.75.1.

Attack Chain

1. Attacker identifies a network-accessible rclone instance running serve s3 with --auth-proxy enabled.
2. Attacker verifies the configuration lacks an --auth-key (often inferred by testing unauthorized access).
3. Attacker selects an arbitrary AccessKeyID to impersonate a target user or administrative account.
4. Attacker constructs an S3 API request (e.g., ListAllMyBuckets) using the chosen AccessKeyID.
5. Attacker computes a valid SigV4 signature for the request payload using an empty string as the secret key.
6. Attacker sends the forged request to the rclone instance.
7. The authPairMiddleware registers the attacker-supplied key with an empty secret in the internal store.
8. The gofakes3 handler validates the forged signature against the registered empty secret, granting full access to the requested S3 backend.

Impact

Successful exploitation allows unauthenticated remote attackers to gain full access to the S3 interface of an rclone instance. This can lead to unauthorized data exfiltration, deletion, or modification of stored objects, depending on the permissions of the identities handled by the auth-proxy script. No prior credentials or user interaction are required for exploitation.

Recommendation

• Upgrade rclone to version 1.75.1 or later immediately to apply the fix that prevents the server from starting with an insecure configuration.
• Audit existing rclone deployments for the usage of --auth-proxy in serve s3 commands.
• Ensure that all serve s3 instances utilizing --auth-proxy are configured with a robust --auth-key if immediate upgrading is not feasible, although upgrading is the only supported mitigation.
• Review logs for S3 requests that appear to be authenticated using anomalous or unknown AccessKeyID values.