Incorrect Authorization in github.com/golang/crypto

Summary

Applications and libraries that misuse the 'ServerConfig.PublicKeyCallback' callback may be susceptible to an authorization bypass. The documentation for 'ServerConfig.PublicKeyCallback' says that a call to this function does not guarantee that the key offered is in fact used to authenticate. Specifically, the SSH protocol allows clients to inquire about whether a public key is acceptable before proving control of the corresponding private key. 'PublicKeyCallback' may be called with multiple keys, and the order in which the keys were provided cannot be used to infer which key the client successfully authenticated with, if any. Some applications, which store the key(s) passed to 'PublicKeyCallback' (or derived information) and make security-relevant determinations based on it once the connection is established, may make incorrect assumptions. For example, an attacker may send public keys A and B and then authenticate with A. 'PublicKeyCallback' would be called only twice, first with A and then with B. A vulnerable application may then make authorization decisions based on key B for which the attacker does not actually control the private key. Since this API is widely misused, as a partial mitigation, golang.org/x/crypto prior to 0.31.0 enforces the property that, when successfully authenticating via public key, the last key passed to 'ServerConfig.PublicKeyCallback' will be the key used to authenticate the connection. 'PublicKeyCallback' will now be called multiple times with the same key, if necessary. Note that the client may still not control the last key passed to 'PublicKeyCallback' if the connection is then authenticated with a different method, such as 'PasswordCallback', 'KeyboardInteractiveCallback', or 'NoClientAuth'. Users should be using the 'Extensions' field of the 'Permissions' return value from the various authentication callbacks to record data associated with the authentication attempt instead of referencing the external state. Once the connection is established, the state corresponding to the successful authentication attempt can be retrieved via the 'ServerConn.Permissions' field. Note that some third-party libraries misuse the Permissions type by sharing it across authentication attempts; users of third-party libraries should refer to the relevant projects for guidance.