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

### 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/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.2026-04-11T03-20-12Z #### Binary 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) |

CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L

Severity from

GitHub (reviewed advisory)

Weakness

CWE-287, CWE-306

Also known as

BIT-minio-2026-40344, CVE-2026-40344, GO-2026-5284

• github.com/minio/minio/security/advisories/GHSA-9c4q-hq6p-c237
• nvd.nist.gov/vuln/detail/CVE-2026-40344
• github.com/minio/minio/pull/16484
• github.com/minio/minio/commit/76913a9fd5c6e5c2dbd4e8c7faf56ed9e9e24091
• github.com/minio/minio