CVE-2026-55874: SeaweedFS: Path traversal in the S3 gateway X-Amz-Copy-Source header allows cross-bucket object read
Summary The SeaweedFS S3 API gateway did not reject .. path segments in the X-Amz-Copy-Source header used by CopyObject and UploadPartCopy. The request URL path was hardened against traversal in 4.30 (CVE-2026-54917), but the copy-source header was only checked for emptiness, so a .. segment in the copy source survived into the server-side filer path and resolved into a different bucket.
Impact A confused-deputy authorization bypass that breaks bucket isolation. IAM evaluates the caller's policy against the bucket named in the request URL (the destination the caller owns), while the copy reads its source from the traversed target bucket. An identity scoped to a single bucket (Read + Write on one bucket it controls) can therefore read any object in any bucket on the instance and land the result in its own bucket.
For example, a caller authorized only for bucket-a issues a CopyObject into bucket-a with copy source bucket-a/../<victim-bucket>/<key>; the gateway reads <victim-bucket>/<key> and writes it to the attacker-controlled destination, from which the caller reads it normally. UploadPartCopy (CopyObjectPartHandler) is affected by the same vector.
Affected versions All releases prior to 4.34. The 4.30 fix for CVE-2026-54917 hardened the request URL path but not the X-Amz-Copy-Source header.
Patched version 4.34 and later.
Remediation Upgrade to 4.34 or later. The fix validates the copy-source bucket and object key with the same IsValidBucketName / IsValidObjectKey guards already applied to the request URL, rejecting traversal segments before the handler runs, and applies the same check to UploadPartCopy.
Workaround No configuration workaround. For deployments that cannot upgrade immediately, front the gateway with a reverse proxy that rejects requests whose X-Amz-Copy-Source header contains .., %2e%2e, or backslash sequences.
References - Fix: https://github.com/seaweedfs/seaweedfs/pull/9929 (commit b44cf51fe931bd75aa4d37ae766bea90d7f85ccd)
Credit Reported responsibly by @47Cid.
Other sources
SeaweedFS is a distributed storage system. Prior to 4.34, the S3 API gateway does not reject dot-dot path segments in the X-Amz-Copy-Source header used by CopyObject and UploadPartCopy, allowing an authenticated identity scoped to one bucket to read objects from other buckets through server-side copy. This issue is fixed in version 4.34.
— NVD
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
go/github.com/seaweedfs/seaweedfsto a version that resolves this vulnerability.Fixed in 0.0.0-20260612000715-b44cf51fe931 - Upgrade
Upgrade
seaweedfs/seaweedfsto a version that resolves this vulnerability.Fixed in 4.34 - Compensating control
For deployments that cannot upgrade immediately, front the SeaweedFS S3 gateway with a reverse proxy that rejects CopyObject/UploadPartCopy requests whose `X-Amz-Copy-Source` header contains `..`, `%2e%2e`, or backslash sequences.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-55874?
The severity of CVE-2026-55874 is rated as high with a score of 7.7.
How do I fix CVE-2026-55874?
To fix CVE-2026-55874, upgrade to SeaweedFS version 4.34 or later, where the vulnerability has been addressed.
What does CVE-2026-55874 allow attackers to do?
CVE-2026-55874 allows attackers to perform path traversal via the X-Amz-Copy-Source header, enabling them to read objects from unauthorized buckets.
Which software is affected by CVE-2026-55874?
CVE-2026-55874 affects SeaweedFS versions prior to 4.34.
Is authentication required to exploit CVE-2026-55874?
Yes, CVE-2026-55874 requires that the attacker has an authenticated identity scoped to one bucket to exploit the vulnerability.