Http4s: DigestAuth nonce map grows unbounded

The DigestAuth server middleware's stale-nonce cleanup uses an inverted comparison: it removes fresh nonces and stops at the first stale one. Because a new nonce is created for every unauthenticated challenge, an attacker can drive the nonce map to grow without bound until the JVM runs out of heap.

Impact

Unauthenticated remote denial of service (gradual heap exhaustion / OOM) against
any service using DigestAuth. The leak is persistent.

Preconditions

• Application uses DigestAuth on at least one route.

Workarounds

• Front the DigestAuth protected routes with a rate limiter to slow the leak.
• Restart periodically.

Fixes

• The eviction logic is corrected.
• A max cache size of 1000000 is imposed to protected against a burst between evictions. This limit is not yet configurable.