CVE-2026-40344: MinIO has an Unauthenticated Object Write via Missing Signature Verification in Unsigned-Trailer Uploads

Two authentication bypass vulnerabilities in MinIO’s STREAMING-UNSIGNED-PAYLOAD-TRAILER code path allow 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.

There are two vulnerabilities:

1. Missing Signature Verification in PutObjectExtractHandler / Snowball (CWE-306)
2. Signature Verification Bypass via Query-String Credentials (CWE-287)

Vulnerability 1 — Missing signature verification in PutObjectExtractHandler (Snowball)

When authTypeStreamingUnsignedTrailer support was added (commit 76913a9fd, PR #16484), the new auth type was handled in PutObjectHandler and PutObjectPartHandler but was never added to PutObjectExtractHandler. The snowball auto-extract handler’s switch rAuthType block has no case for authTypeStreamingUnsignedTrailer, so execution falls through with zero signature verification. The isPutActionAllowed call before the switch extracts the access key and checks IAM permissions, but does not verify the cryptographic signature.

An attacker sends a PUT request with X-Amz-Content-Sha256: STREAMING-UNSIGNED-PAYLOAD-TRAILER, X-Amz-Meta-Snowball-Auto-Extract: true, and an Authorization header containing a valid access key with a completely fabricated signature. The request is accepted and the tar payload is extracted into the bucket.

Affected component: cmd/object-handlers.go, function PutObjectExtractHandler.

Vulnerability 2 — Signature verification bypass via query-string credentials

PutObjectHandler and PutObjectPartHandler call newUnsignedV4ChunkedReader with a signature verification gate based solely on the presence of the Authorization header:

newUnsignedV4ChunkedReader(r, true, r.Header.Get(xhttp.Authorization) != "")

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.

Affected components: cmd/object-handlers.go (PutObjectHandler), cmd/object-multipart-handlers.go (PutObjectPartHandler).

CVSS v4.0 Score: 8.8 (High)

Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N

CWE: CWE-306 (Missing Authentication for Critical Function), CWE-287 (Improper Authentication)