CVE-2024-45337
CriticalCVSS 9.1Summary
A vulnerability in the golang.org/x/crypto/ssh library involves misuse of the ServerConfig.PublicKeyCallback function, potentially leading to an authorization bypass. Applications that store keys passed to this callback and make security decisions based on them may incorrectly assume that the client controls the key used for authentication. Version v0.31.0 introduces a partial mitigation by enforcing that the last key passed to the callback is the one used for authentication.
Risk Assessment
An attacker could exploit this vulnerability to bypass authorization mechanisms, impersonating another user or gaining unauthorized access to the system. The risk affects organizations using the golang.org/x/crypto/ssh library in SSH server applications that improperly use PublicKeyCallback for security decisions.
Recommendation
Update the golang.org/x/crypto/ssh library to version v0.31.0 or later. Review application code for usage of ServerConfig.PublicKeyCallback and replace storing keys in external state with using the Extensions field returned by authentication callbacks and the ServerConn.Permissions field.
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Original NVD description (English source)
Applications and libraries which misuse connection.serverAuthenticate (via callback field ServerConfig.PublicKeyCallback) 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/[email protected] 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 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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS

