{"id":"CVE-2026-40344","aliases":["BIT-minio-2026-40344","GHSA-9c4q-hq6p-c237","GO-2026-5284"],"url":"https://o3.security/vulnerability/CVE-2026-40344","summary":"MinIO has an Unauthenticated Object Write via Missing Signature Verification in Unsigned-Trailer Uploads","details":"### Impact\n\nTwo authentication bypass vulnerabilities in MinIO's `STREAMING-UNSIGNED-PAYLOAD-TRAILER` code path\nallow any user who knows a valid access key to write arbitrary objects to any bucket without knowing\nthe secret key or providing a valid cryptographic signature.\n\nAny MinIO deployment is impacted. The attack requires only a valid access key (the well-known default\n`minioadmin`, or any key with WRITE permission on a bucket) and a target bucket name.\n\nThere are two vulnerabilities:\n\n1. Missing Signature Verification in PutObjectExtractHandler / Snowball (CWE-306)\n2. Signature Verification Bypass via Query-String Credentials (CWE-287)\n\n**Vulnerability 1 — Missing signature verification in PutObjectExtractHandler (Snowball)**\n\nWhen `authTypeStreamingUnsignedTrailer` support was added (commit 76913a9fd, PR #16484), the new auth\ntype was handled in `PutObjectHandler` and `PutObjectPartHandler` but was never added to\n`PutObjectExtractHandler`. The snowball auto-extract handler's `switch rAuthType` block has no case for\n`authTypeStreamingUnsignedTrailer`, so execution falls through with zero signature verification. The\n`isPutActionAllowed` call before the switch extracts the access key and checks IAM permissions, but\ndoes not verify the cryptographic signature.\n\nAn attacker sends a PUT request with `X-Amz-Content-Sha256: STREAMING-UNSIGNED-PAYLOAD-TRAILER`,\n`X-Amz-Meta-Snowball-Auto-Extract: true`, and an `Authorization` header containing a valid access key\nwith a completely fabricated signature. The request is accepted and the tar payload is extracted into\nthe bucket.\n\n**Affected component:** `cmd/object-handlers.go`, function `PutObjectExtractHandler`.\n\n**Vulnerability 2 — Signature verification bypass via query-string credentials**\n\n`PutObjectHandler` and `PutObjectPartHandler` call `newUnsignedV4ChunkedReader` with a signature\nverification gate based solely on the presence of the `Authorization` header:\n\n```go\nnewUnsignedV4ChunkedReader(r, true, r.Header.Get(xhttp.Authorization) != \"\")\n```\n\nMeanwhile, `isPutActionAllowed` extracts credentials from either the `Authorization` header or the\n`X-Amz-Credential` query parameter, and trusts whichever it finds. An attacker omits the\n`Authorization` header and supplies credentials exclusively via the query string. The signature gate\nevaluates to `false`, `doesSignatureMatch` is never called, and the request proceeds with the\npermissions of the impersonated access key.\n\n**Affected components:** `cmd/object-handlers.go` (`PutObjectHandler`),\n`cmd/object-multipart-handlers.go` (`PutObjectPartHandler`).\n\n**CVSS v4.0 Score:** 8.8 (High)\n\n**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`\n\n**CWE:** CWE-306 (Missing Authentication for Critical Function), CWE-287 (Improper Authentication)\n\n### Affected Versions\n\nAll MinIO releases through the final release of the minio/minio open-source project.\n\nBoth vulnerabilities were introduced in commit\n[`76913a9fd`](https://github.com/minio/minio/commit/76913a9fd5c6e5c2dbd4e8c7faf56ed9e9e24091)\n(\"Signed trailers for signature v4\", [PR #16484](https://github.com/minio/minio/pull/16484)),\nwhich added `authTypeStreamingUnsignedTrailer` support. The first affected release is\n`RELEASE.2023-05-18T00-05-36Z`.\n\n### Patches\n\n**Fixed in**: MinIO AIStor RELEASE.2026-04-11T03-20-12Z\n\n#### Binary Downloads\n\n| Platform | Architecture | Download                                                                    |\n| -------- | ------------ | --------------------------------------------------------------------------- |\n| Linux    | amd64        | [minio](https://dl.min.io/aistor/minio/release/linux-amd64/minio)           |\n| Linux    | arm64        | [minio](https://dl.min.io/aistor/minio/release/linux-arm64/minio)           |\n| macOS    | arm64        | [minio](https://dl.min.io/aistor/minio/release/darwin-arm64/minio)          |\n| macOS    | amd64        | [minio](https://dl.min.io/aistor/minio/release/darwin-amd64/minio)          |\n| Windows  | amd64        | [minio.exe](https://dl.min.io/aistor/minio/release/windows-amd64/minio.exe) |\n\n#### FIPS Binaries\n\n| Platform | Architecture | Download                                                                    |\n| -------- | ------------ | --------------------------------------------------------------------------- |\n| Linux    | amd64        | [minio.fips](https://dl.min.io/aistor/minio/release/linux-amd64/minio.fips) |\n| Linux    | arm64        | [minio.fips](https://dl.min.io/aistor/minio/release/linux-arm64/minio.fips) |\n\n#### Package Downloads\n\n| Format | Architecture | Download                                                                                                                            |\n| ------ | ------------ | ----------------------------------------------------------------------------------------------------------------------------------- |\n| DEB    | amd64        | [minio_20260411032012.0.0_amd64.deb](https://dl.min.io/aistor/minio/release/linux-amd64/minio_20260411032012.0.0_amd64.deb)         |\n| DEB    | arm64        | [minio_20260411032012.0.0_arm64.deb](https://dl.min.io/aistor/minio/release/linux-arm64/minio_20260411032012.0.0_arm64.deb)         |\n| 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)   |\n| 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) |\n\n#### Container Images\n\n```bash\n# Standard\ndocker pull quay.io/minio/aistor/minio:RELEASE.2026-04-11T03-20-12Z\npodman pull quay.io/minio/aistor/minio:RELEASE.2026-04-11T03-20-12Z\n\n# FIPS\ndocker pull quay.io/minio/aistor/minio:RELEASE.2026-04-11T03-20-12Z.fips\npodman pull quay.io/minio/aistor/minio:RELEASE.2026-04-11T03-20-12Z.fips\n```\n\n#### Homebrew (macOS)\n\n```bash\nbrew install minio/aistor/minio\n```\n\n### Workarounds\n\n- [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/)\n\nIf upgrading is not immediately possible:\n\n- **Block unsigned-trailer requests at the load balancer.** Reject any request containing\n  `X-Amz-Content-Sha256: STREAMING-UNSIGNED-PAYLOAD-TRAILER` at the reverse proxy or WAF layer.\n  Clients can use `STREAMING-AWS4-HMAC-SHA256-PAYLOAD-TRAILER` (the signed variant) instead.\n\n- **Restrict WRITE permissions.** Limit `s3:PutObject` grants to trusted principals. While this\n  reduces the attack surface, it does not eliminate the vulnerability since any user with WRITE\n  permission can exploit it with only their access key.\n\n### Credits\n\n- **Finder:** Arvin Shivram of Brutecat Security ([@ddd](https://github.com/ddd))\n\n### References\n\n- Introducing commit: [`76913a9fd`](https://github.com/minio/minio/commit/76913a9fd5c6e5c2dbd4e8c7faf56ed9e9e24091) ([PR #16484](https://github.com/minio/minio/pull/16484))\n- [MinIO AIStor](https://min.io/aistor)","published":"2026-04-22T00:49:30.137Z","modified":"2026-08-12T03:51:17.851923380Z","cvss":null,"epss":{"score":0.00418,"percentile":0.34594,"asOf":"2026-08-24"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/minio/minio","fixedVersion":null}],"fix":{"url":"https://github.com/minio/minio/commit/76913a9fd5c6e5c2dbd4e8c7faf56ed9e9e24091","label":"minio/minio@76913a9"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/40xxx/CVE-2026-40344.json"},{"type":"ADVISORY","url":"https://github.com/minio/minio/security/advisories/GHSA-9c4q-hq6p-c237"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-40344"},{"type":"FIX","url":"https://github.com/minio/minio/commit/76913a9fd5c6e5c2dbd4e8c7faf56ed9e9e24091"},{"type":"FIX","url":"https://github.com/minio/minio/pull/16484"},{"type":"PACKAGE","url":"https://github.com/minio/minio"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:17.851923380Z"}}