A flaw was found in rgw. This flaw allows an unprivileged user to write to any bucket(s) accessible by a given key if a POST's form-data contains a key called 'bucket' with a value matching the bucket's name used to sign the request. This issue results in a user being able to upload to any bucket accessible by the specified access key as long as the bucket in the POST policy matches the bucket in the said POST form part.