GitHub Security Advisory: GHSA-9c4q-hq6p-c237
Package: github.com/minio/minio (GO)
Severity: HIGH (CVSS 8.2)
CWE: CWE-287: Improper Authentication, CWE-306: Missing Authentication for Critical Function
Vulnerable versions: >= 0.0.0[PII_REDACTED]5312-76913a9fd5c6, <= 0.0.0[PII_REDACTED]1848-7aac2a2c5b7c
Patched in: unpatched

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

Details:
### 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`](https://github.com/minio