http4k: `ServerFilters.DigestAuth` / `DigestAuthProvider` defaulted to an always-true nonce verifier, disabling replay protection in default deployments

Impact

ServerFilters.DigestAuth and the underlying DigestAuthProvider both defaulted their nonceVerifier parameter to { true } — i.e. every nonce was accepted regardless of value, age, or prior use. Any deployment using the default configuration had no replay protection on Digest authentication; a captured Authorization: Digest … response could be replayed indefinitely against the same protected resource.

The nonce-verification mechanism in Digest auth is the primary anti-replay control — without it, Digest reduces to a credential bound only to a stale nonce string.

Who is affected: any application using ServerFilters.DigestAuth or DigestAuthProvider with the default nonceVerifier. The broken default has been present since DigestAuthProvider was introduced (2021). Exploitation requires the attacker to first capture a valid Digest response (network observation, log access, etc.) — non-trivial in modern TLS deployments but not impossible. Anyone running Digest auth with default config should treat upgrade as urgent.

Patches

The fix ([Break]) removes the default value for nonceVerifier from both ServerFilters.DigestAuth and DigestAuthProvider. Callers must now supply a real verifier explicitly — the broken default cannot be silently inherited.

Workarounds

For deployments that cannot upgrade immediately: explicitly supply a nonceVerifier that tracks issued nonces, enforces a TTL, and rejects re-use. Do not rely on the default.