Comment # 1 on bug 1234562 from Andrea Mattiazzo

Applications and libraries which 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/cry...@v0.31.0 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 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.

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2024-45337
https://www.cve.org/CVERecord?id=CVE-2024-45337
https://github.com/golang/crypto/commit/b4f1988a35dee11ec3e05d6bf3e90b695fbd8909
https://go.dev/cl/635315
https://go.dev/issue/70779
https://groups.google.com/g/golang-announce/c/-nPEi39gI4Q/m/cGVPJCqdAQAJ
https://pkg.go.dev/vuln/GO-2024-3321
http://www.openwall.com/lists/oss-security/2024/12/11/2
https://bugzilla.redhat.com/show_bug.cgi?id=2331720
https://github.com/CVEProject/cvelistV5/blob/main//cves/2024/45xxx/CVE-2024-45337.json