Fix: Prefer X-Real-IP over XFF[0] in get_client_ip to close spoof bypass

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>

api/app/auth/rate_limiter.py

@@ -14,20 +14,30 @@ def get_client_ip(
     request: Request,
     trusted_networks: list[IPv4Network | IPv6Network],
 ) -> str:
-    """Return the resolved client IP, honouring X-Forwarded-For when the
-    TCP peer is a trusted upstream proxy. Falls back to the TCP peer address
-    when no trusted networks are configured or the peer is not in the list."""
+    """Return the resolved client IP.
+
+    Prefers X-Real-IP over X-Forwarded-For when the TCP peer is a trusted
+    proxy. ingress-nginx sets X-Real-IP via its realip module using an
+    authoritative CIDR allowlist; it overwrites any client-supplied value, so
+    it cannot be spoofed via XFF injection. XFF[0] is the fallback for paths
+    that lack nginx (none currently exist, but kept for defence in depth).
+    """
     peer = request.client.host if request.client else "unknown"
     if trusted_networks and peer != "unknown":
         try:
             peer_addr = ipaddress.ip_address(peer)
             if any(peer_addr in net for net in trusted_networks):
-                xff = request.headers.get("X-Forwarded-For", "").split(",")[0].strip()
-                if xff:
-                    return xff
                 real_ip = request.headers.get("X-Real-IP", "").strip()
                 if real_ip:
                     return real_ip
+                # XFF[0] fallback — warn because this path should not be
+                # reached in production (nginx always sets X-Real-IP).
+                xff = request.headers.get("X-Forwarded-For", "").split(",")[0].strip()
+                if xff:
+                    logger.warning(
+                        "X-Real-IP absent from trusted peer %s; falling back to XFF[0]", peer
+                    )
+                    return xff
         except ValueError:
             pass
     return peer

api/tests/unit/test_rate_limiter.py

@@ -80,10 +80,17 @@ def test_get_client_ip_no_trusted_networks_returns_peer():
     assert get_client_ip(req, []) == "203.0.113.1"
 
 
-def test_get_client_ip_trusted_peer_uses_xff():
-    req = make_request("10.0.0.1", {"X-Forwarded-For": "203.0.113.5"})
+def test_get_client_ip_trusted_peer_uses_real_ip():
+    req = make_request("10.0.0.1", {"X-Real-IP": "203.0.113.9"})
     nets = [ipaddress.ip_network("10.0.0.0/8")]
-    assert get_client_ip(req, nets) == "203.0.113.5"
+    assert get_client_ip(req, nets) == "203.0.113.9"
+
+
+def test_get_client_ip_real_ip_wins_over_xff():
+    # Regression: spoofed XFF must not override nginx-set X-Real-IP.
+    req = make_request("10.0.0.1", {"X-Real-IP": "203.0.113.9", "X-Forwarded-For": "1.2.3.4"})
+    nets = [ipaddress.ip_network("10.0.0.0/8")]
+    assert get_client_ip(req, nets) == "203.0.113.9"
 
 
 def test_get_client_ip_untrusted_peer_ignores_xff():
@@ -92,7 +99,7 @@ def test_get_client_ip_untrusted_peer_ignores_xff():
     assert get_client_ip(req, nets) == "8.8.8.8"
 
 
-def test_get_client_ip_trusted_peer_falls_back_to_real_ip():
-    req = make_request("10.0.0.1", {"X-Real-IP": "203.0.113.9"})
+def test_get_client_ip_trusted_peer_falls_back_to_xff_when_no_real_ip():
+    req = make_request("10.0.0.1", {"X-Forwarded-For": "203.0.113.5"})
     nets = [ipaddress.ip_network("10.0.0.0/8")]
-    assert get_client_ip(req, nets) == "203.0.113.9"
+    assert get_client_ip(req, nets) == "203.0.113.5"