CVE-2026-54291: PostgreSQL JDBC Driver: Silent channel-binding authentication downgrade via unsupported certificate algorithms

channelBinding=require connections can be silently downgraded from SCRAM-SHA-256-PLUS (with channel binding) to plain SCRAM-SHA-256 (without it), losing the man-in-the-middle protection the setting is meant to guarantee. An attacker who can intercept the TLS connection triggers the downgrade with a certificate whose signature algorithm has no tls-server-end-point channel-binding hash. Examples are Ed25519, Ed448, and post-quantum algorithms.

Two issues combine in releases 42.7.4 through 42.7.11:

1. The bundled com.ongres.scram:scram-client (3.1 or 3.2) returns an empty byte array instead of failing when it cannot derive the binding hash for such a certificate. This is the library issue tracked as GHSA-p9jg-fcr6-3mhf.
2. pgJDBC does not enforce channelBinding=require where it matters. ScramAuthenticator checks only that the server advertised a -PLUS mechanism; it neither rejects the empty binding nor checks that the negotiated mechanism uses channel binding. The connection therefore downgrades silently, and would do so even against a fixed scram-client, because the missing enforcement is in pgJDBC’s own code.

Only connections that set channelBinding=require are affected. Under the default prefer policy, and under allow or disable, falling back to plain SCRAM is the documented behaviour. Releases before 42.7.4 are unaffected, because they do not support channel binding.