S3API middleware in OpenStack Swift versions through 2.38.0 fails to enforce that semantic x-amz-* headers are covered by the SigV4 signature on presigned URL requests. An attacker who obtains a presigned PUT URL can inject an unsigned X-Amz-Copy-Source header. The middleware then performs a server‑side copy from an arbitrary source object using the signer's authorization context, allowing the attacker to read any object the signer has access to. The vulnerability therefore compromises confidentiality by enabling read access to objects whose project, container, and object names are known.

The flaw affects deployments of OpenStack Swift, specifically the Swift S3API middleware in versions up to 2.38.0. All installations using the default configuration where s3_acl is set to false are vulnerable.

The CVSS score of 6.0 indicates a moderate severity. EPSS data is not available, and the vulnerability is not listed in CISA KEV. The likely attack vector is the use of presigned PUT URLs; an attacker needs to receive a valid presigned URL and then inject an unsigned header in a separate request. Because the vulnerability relies on URL‑based authorization and a missing header check, it is exploitable from the network, and the impact is confined to the signer's authorized bucket and objects whose identifiers are known.