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

What is CVE-2026-40344?

CVE-2026-40344 is a high-severity authentication bypass vulnerability in minio, published on 22 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

MinIO is a high-performance object storage system. Starting in RELEASE.2023-05-18T00-05-36Z and prior to RELEASE.2026-04-11T03-20-12Z, an authentication bypass vulnerability in MinIO's Snowball auto-extract handler (`PutObjectExtractHandler`) allows 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. When `authTypeStreamingUnsignedTrailer` support was added, 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. 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. Alternatively, 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.

Weakness: CWE-306, CWE-287

Which versions are affected by CVE-2026-40344?

3 open source packages ship an affected version, including github.com/minio/minio. It affects 1 product.

Products include Minio Minio.

How do I fix CVE-2026-40344?

Upgrade to a fixed version.

Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.

Is CVE-2026-40344 being exploited?

There is no known exploitation of CVE-2026-40344 in the wild and no public exploit code.

Other vulnerabilities in minio

• CVE-2023-28434 MinIO Security Feature Bypass Exploited High 8.8
• CVE-2023-28432 MinIO Information Disclosure Exploited High 7.5
• CVE-2020-11012 Authentication bypass MinIO Admin API Critical 9.3
• CVE-2026-33322 MinIO: JWT Algorithm Confusion in OIDC Authentication Critical 9.2
• CVE-2026-33419 MinIO: LDAP login brute-force via user enumeration and missing rate limit Critical 9.1

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-9c4q-hq6p-c237, GO-2026-5284, ROOT-APP-GOBINARY-CVE-2026-40344.