MinIO is a high-performance object storage system. Starting in RELEASE.2023-05-18T00-05-36Z and prior to RELEASE.2026-04-11T03-20-12Z, an authentication bypass vulnerability in MinIO's STREAMING-UNSIGNED-PAYLOAD-TRAILER code path allows any user who knows a valid access key to write arbitrary objects to any bucket without knowing the secret key or providing a valid cryptographic signature. Any MinIO deployment is impacted.

The attack requires only a valid access key (the well-known default minioadmin, or any key with WRITE permission on a bucket) and a target bucket name. PutObjectHandler and PutObjectPartHandler call newUnsignedV4ChunkedReader with a signature verification gate based solely on the presence of the Authorization header. Meanwhile, isPutActionAllowed extracts credentials from either the Authorization header or the X-Amz-Credential query parameter, and trusts whichever it finds. An attacker omits the Authorization header and supplies credentials exclusively via the query string.

The signature gate evaluates to false, doesSignatureMatch is never called, and the request proceeds with the permissions of the impersonated access key. This affects PutObjectHandler (standard and tables/warehouse bucket paths) and PutObjectPartHandler (multipart uploads). Users of the open-source minio/minio project should upgrade to MinIO AIStor RELEASE.2026-04-11T03-20-12Z or later.

If upgrading is not immediately possible, block unsigned-trailer requests at the load balancer. Reject any request containing X-Amz-Content-Sha256: STREAMING-UNSIGNED-PAYLOAD-TRAILER at the reverse proxy or WAF layer. Clients can use STREAMING-AWS4-HMAC-SHA256-PAYLOAD-TRAILER (the signed variant) instead.

Alternatively, restrict WRITE permissions. Limit s3:PutObject grants to trusted principals. While this reduces the attack surface, it does not eliminate the vulnerability since any user with WRITE permission can exploit it with only their access key.