In OpenStack Swift before 2.38.2, the tempurl middleware does not reject the X-Copy-From header on PUT requests. A TempURL signature only covers the method, expiry, and path, and thus the list of disallowed headers is the only defense against a signed PUT request changing what the request does. An attacker holding a PUT TempURL for a single object can add an X-Copy-From header naming any object in the same account; the copy middleware copies that object to the destination, and the attacker then reads the victim's data back with a GET TempURL for the destination object. Copies across account boundaries are rejected. Only deployments using the shipped default proxy pipeline (tempurl and copy middleware) with account-level TempURL keys are affected.