Cause: the router used gin.Default() without SetTrustedProxies, and gin
then trusts forwarding headers from every peer, so ClientIP() returned
whatever X-Forwarded-For a client sent: the IP whitelist and the IP bound
into JWTs could be spoofed.
Fix: new app.yaml option security.trusted_proxies (IP or CIDR list,
default empty = trust no proxy, validated at startup) passed to
SetTrustedProxies in Router(); an invalid list falls back to trusting
none. The nginx samples note that the proxy must be listed there.
Tests: TestRouter_IgnoresForwardedForByDefault,
TestRouter_HonoursForwardedForFromTrustedProxy, TestResolveTrustedProxies.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
