XFF[0] is attacker-controllable; a crafted X-Forwarded-For header could
attribute login failures to a victim IP, triggering their lockout while
the attacker accumulates none. ingress-nginx sets X-Real-IP via its
realip module using an authoritative CIDR allowlist and overwrites any
client-supplied value, making it spoof-resistant. Fallback to XFF[0]
is retained for defence in depth but now emits a warning if reached.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>