CVE-2026-88018 in rclone
Summary
by MITRE • 09/10/2026
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.
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Analysis
by VulDB Data Team • 09/10/2026
The vulnerability identified in rclone versions prior to 1.75.1 represents a critical authentication bypass within the S3 serving functionality when configured with an authentication proxy. Rclone, widely used for synchronizing data across various cloud storage providers, includes a feature that allows it to act as an Amazon S3-compatible server. When this service is deployed behind an external authentication proxy using the --auth-proxy flag, the system relies on the proxy to validate user credentials. However, a flaw in the middleware implementation allowed attackers to circumvent these controls entirely under specific configuration conditions where no secret key was provided via the --auth-key parameter. This scenario exposes a fundamental weakness in how identity and authentication secrets are managed during the request validation process.
The technical root cause lies in the interaction between rclone's authPairMiddleware, gofakes3 signature verification logic, and the external proxy server. When an S3 client initiates a connection, it typically provides an access key ID and signs requests using a corresponding secret access key according to AWS Signature Version 4 standards. In this vulnerable configuration, if no static --auth-key is set on the rclone instance, the authPairMiddleware permits any arbitrary string provided by the client in the AccessKeyId field to be registered as a valid identity without requiring proof of ownership via a pre-shared secret. Consequently, gofakes3 proceeds to verify the cryptographic signature of incoming requests against an empty or null secret value associated with that chosen key ID. Since HMAC-SHA256 signatures can be trivially computed when the signing key is known to be empty, any attacker on the network can generate valid-looking SigV4 signatures for their own arbitrary access keys.
Furthermore, the authentication proxy itself fails to enforce independent per-identity secrets. Instead of validating that a client possesses the correct secret corresponding to the claimed AccessKeyId, Server.auth simply passes the provided key identifier as both the username and password fields in its basic authentication handshake with the backend proxy. This design flaw means that if an attacker selects any access key ID and signs their request using an empty secret, rclone accepts this identity, forwards it to the auth-proxy, and the proxy authenticates based on the matching string rather than verifying actual credential possession. This effectively nullifies the intended security boundary provided by the authentication proxy, allowing unauthenticated users to impersonate arbitrary identities.
The operational impact of this vulnerability is severe, granting an unauthenticated network attacker full control over the S3 service backend associated with any resolved identity. An adversary can exploit this flaw to read sensitive data stored in buckets mapped to those identities or write malicious files, leading to potential data exfiltration, integrity compromise, and denial of service through resource exhaustion. Because the attack requires only network access to the rclone instance and knowledge of S3 signing protocols, it is easily executable by remote attackers without prior authentication. This aligns with CWE-287 Improper Authentication, specifically illustrating a failure in verifying credentials against known secrets, and maps to MITRE ATT&CK technique T1078 Valid Accounts, where an attacker uses legitimate-looking but forged or bypassed credentials to gain unauthorized access.
To mitigate this risk, organizations running rclone with S3 serving capabilities must upgrade immediately to version 1.75.1 or later, which corrects the middleware logic to enforce proper secret validation. In environments where upgrading is not immediately feasible, administrators should ensure that the --auth-key parameter is always configured with a strong, random secret string when using auth-proxy configurations. This forces gofakes3 and rclone to validate signatures against a known non-empty value, preventing attackers from exploiting empty-secret assumptions. Additionally, deploying network-level access controls such as firewalls or rate limiting can restrict exposure of the S3 endpoint to trusted IP ranges only, reducing the attack surface for remote exploitation attempts until the software patch is applied.