A flaw was found in Ceph RGW's SigV4 signature verification handler. When processing S3 requests, RGW verifies only the headers explicitly listed in the X-Amz-SignedHeaders field but does not reject requests that carry additional unsigned x-amz-* headers. This diverges from the AWS S3 specification, which requires all x-amz-* headers to be signed. As a result, anyone holding a presigned PUT URL can attach arbitrary unsigned x-amz-* headers that RGW will honor, effectively escalating their privileges beyond what the original URL signer authorized. This can lead to unauthorized access to and modification of S3 objects.