GHSA-9c4q-hq6p-c237 MinIO has an Unauthenticated Object Write via Missing Signature Verification in Unsigned-Trailer Uploads

• Go github.com/minio/minio: >= 0.0.0-20230506025312-76913a9fd5c6, <= 0.0.0-20260212201848-7aac2a2c5b7c (no fix yet)

Impact 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: ```go 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) Affected Versions All MinIO releases through the final release of the minio/minio open-source project. Both vulnerabilities were introduced in commit `76913a9fd` ("Signed trailers for signature v4", PR #16484), which added `authTypeStreamingUnsignedTrailer` support. The first affected release is `RELEASE.2023-05-18T00-05-36Z`. Patches Fixed in: MinIO AIStor RELEASE.2026-04-11T03-20-12Z Binary Downloads | Platform | Architecture | Download | | -------- | ------------ | --------------------------------------------------------------------------- | | Linux | amd64 | minio | | Linux | arm64 | minio | | macOS | arm64 | minio | | macOS | amd64 | minio | | Windows | amd64 | minio.exe | FIPS Binaries | Platform | Architecture | Download | | -------- | ------------ | --------------------------------------------------------------------------- | | Linux | amd64 | minio.fips | | Linux | arm64 | minio.fips | Package Downloads | Format | Architecture | Download | | ------ | ------------ | ----------------------------------------------------------------------------------------------------------------------------------- | | DEB | amd64 | minio_20260411032012.0.0_amd64.deb | | DEB | arm64 | minio_20260411032012.0.0_arm64.deb | | RPM | amd64 | minio-20260411032012.0.0-1.x86_64.rpm | | RPM | arm64 | minio-20260411032012.0.0-1.aarch64.rpm | Container Images ```bash Standard docker pull quay.io/minio/aistor/minio:RELEASE.2026-04-11T03-20-12Z podman pull quay.io/minio/aistor/minio:RELEASE.2026-04-11T03-20-12Z FIPS docker pull quay.io/minio/aistor/minio:RELEASE.2026-04-11T03-20-12Z.fips podman pull quay.io/minio/aistor/minio:RELEASE.2026-04-11T03-20-12Z.fips ``` Homebrew (macOS) ```bash brew install minio/aistor/minio ``` Workarounds • 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. • 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. Credits • Finder: Arvin Shivram of Brutecat Security (@ddd) References • Introducing commit: `76913a9fd` (PR #16484) • MinIO AIStor