GHSA-9c4q-hq6p-c237: MinIO has an Unauthenticated Object Write via Missing Signature Verification in Unsigned-Trailer Uploads
ImpactTwo 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:Missing Signature Verification in PutObjectExtractHandler / Snowball (CWE-306)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: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 VersionsAll MinIO releases through the final release of the minio/minio open-source project.Both vulnerabilities were introduced in commit
[`76913a9fd`](https://github.com/minio/minio/commit/76913a9fd5c6e5c2dbd4e8c7faf56ed9e9e24091)
("Signed trailers for signature v4", [PR #16484](https://github.com/minio/minio/pull/16484)),
which added `authTypeStreamingUnsignedTrailer` support. The first affected release is
`RELEASE.2023-05-18T00-05-36Z`.Patches**Fixed in**: MinIO AIStor RELEASE.2026-04-11T03-20-12ZBinary Downloads| Platform | Architecture | Download                                                                    |
| -------- | ------------ | --------------------------------------------------------------------------- |
| Linux    | amd64        | [minio](https://dl.min.io/aistor/minio/release/linux-amd64/minio)           |
| Linux    | arm64        | [minio](https://dl.min.io/aistor/minio/release/linux-arm64/minio)           |
| macOS    | arm64        | [minio](https://dl.min.io/aistor/minio/release/darwin-arm64/minio)          |
| macOS    | amd64        | [minio](https://dl.min.io/aistor/minio/release/darwin-amd64/minio)          |
| Windows  | amd64        | [minio.exe](https://dl.min.io/aistor/minio/release/windows-amd64/minio.exe) |FIPS Binaries| Platform | Architecture | Download                                                                    |
| -------- | ------------ | --------------------------------------------------------------------------- |
| Linux    | amd64        | [minio.fips](https://dl.min.io/aistor/minio/release/linux-amd64/minio.fips) |
| Linux    | arm64        | [minio.fips](https://dl.min.io/aistor/minio/release/linux-arm64/minio.fips) |Package Downloads| Format | Architecture | Download                                                                                                                            |
| ------ | ------------ | ----------------------------------------------------------------------------------------------------------------------------------- |
| DEB    | amd64        | [minio_20260411032012.0.0_amd64.deb](https://dl.min.io/aistor/minio/release/linux-amd64/minio_20260411032012.0.0_amd64.deb)         |
| DEB    | arm64        | [minio_20260411032012.0.0_arm64.deb](https://dl.min.io/aistor/minio/release/linux-arm64/minio_20260411032012.0.0_arm64.deb)         |
| RPM    | amd64        | [minio-20260411032012.0.0-1.x86_64.rpm](https://dl.min.io/aistor/minio/release/linux-amd64/minio-20260411032012.0.0-1.x86_64.rpm)   |
| RPM    | arm64        | [minio-20260411032012.0.0-1.aarch64.rpm](https://dl.min.io/aistor/minio/release/linux-arm64/minio-20260411032012.0.0-1.aarch64.rpm) |Container ImagesHomebrew (macOS)Workarounds[Users of the open-source `minio/minio` project should upgrade to MinIO AIStor `RELEASE.2026-04-11T03-20-12Z` or later.](https://docs.min.io/enterprise/aistor-object-store/upgrade-aistor-server/community-edition/)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](https://github.com/ddd))ReferencesIntroducing commit: [`76913a9fd`](https://github.com/minio/minio/commit/76913a9fd5c6e5c2dbd4e8c7faf56ed9e9e24091) ([PR #16484](https://github.com/minio/minio/pull/16484))[MinIO AIStor](https://min.io/aistor)
CVSS v4.0: 8.8
CVSS v3.1: 8.2