Rate Limit Bypass in Buffalo

Rate limiting is a critical defense against brute-force attacks, credential stuffing, and denial-of-service. In Buffalo applications, misconfigurations often lead to bypasses. This section details common patterns.

1. Inconsistent middleware
Buffalo's routing allows global or group-specific middleware. If rate limiting is applied only to the main app or only to certain groups, endpoints like /api/v1/login may be left unprotected. Ensure the rate limiter runs for all sensitive routes.

2. Proxy IP handling
c.Request().RemoteAddr returns the immediate peer IP. Behind a reverse proxy, this is the proxy's IP, not the client's. Using it directly defeats per-IP limits. Trust X-Forwarded-For only from configured trusted proxies and extract the original client IP correctly.

3. Mutable keys
Basing limits on User-Agent, Referer, or other mutable fields lets attackers rotate values. Use immutable identifiers like API keys, user IDs, or properly derived client IPs.

4. Distributed deployments
In-memory stores (e.g., default in many Go limiters) are per-instance. Scale horizontally and the global limit is split. Use a shared store like Redis.

5. Method/path gaps
Applying limits only to GET but not POST, or not including resource-specific parameters (e.g., user ID) allows bypass. Ensure all methods and relevant parameters are covered.

These issues map to OWASP API Top 10's Unrestricted Resource Consumption (API4:2023). Understanding Buffalo's middleware chain and request context is key to fixing them.

// Vulnerable: uses RemoteAddr without proxy consideration
func RateLimit(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        ip := c.Request().RemoteAddr
        if !allowRequest(ip) {
            c.Response().WriteHeader(429)
            return c.Render(429, r.JSON(map[string]string{
                "error": "rate limit exceeded",
            }))
        }
        return next(c)
    }
}

middleBrick automatically detects rate limit bypass by testing each endpoint. It first looks for standard rate limiting headers (X-RateLimit-Limit, X-RateLimit-Remaining, Retry-After). Then it performs a burst test: sending many requests (e.g., 50 in 5 seconds) and checking for HTTP 429 responses. Critical endpoints like login, password reset, and token refresh are explicitly verified for protection. The scanner also tests multiple HTTP methods on the same endpoint to uncover partial coverage. While it cannot directly see server-side IP logic, it can infer proxy-related bypasses by observing if rate limits reset when the X-Forwarded-For header changes. All findings contribute to a risk score (0–100) with a letter grade (A–F) and include prioritized remediation guidance. Use the web dashboard, CLI (middlebrick scan <url>), or GitHub Action to run scans.

To fix rate limit bypass in Buffalo, implement a robust middleware using a distributed store and proper client IP handling.

Step 1: Use a shared store
In-memory stores fail across instances. Use Redis with ulule/limiter:

import (
    "net"
    "strings"
    "time"
    "github.com/ulule/limiter/v3"
    "github.com/ulule/limiter/v3/drivers/store/redis"
    "github.com/ulule/limiter/v3/ratelimit"
)

store, _ := redis.NewStore(&redis.Options{
    Network:  "tcp",
    Address:  "localhost:6379",
    Password: "",
    DB:       0,
})
rateLimiter := limiter.New(ratelimit.NewPerIPRateLimiter(store, 100, time.Minute))

Step 2: Build middleware that respects trusted proxies
Extract the real client IP only from trusted proxies:

func RateLimitMiddleware(rl *limiter.Limiter) buffalo.MiddlewareFunc {
    return func(next buffalo.Handler) buffalo.Handler {
        return func(c buffalo.Context) error {
            ip := getClientIP(c)
            if _, err := rl.Get(ip); err != nil {
                c.Response().WriteHeader(429)
                return c.Render(429, r.JSON(map[string]string{
                    "error": "rate limit exceeded",
                }))
            }
            return next(c)
        }
    }
}

func getClientIP(c buffalo.Context) string {
    remote := c.Request().RemoteAddr
    host, _, _ := net.SplitHostPort(remote)
    // List trusted proxy IPs (exact match for simplicity)
    trusted := []string{"10.0.0.1", "192.168.1.1"}
    for _, p := range trusted {
        if host == p {
            if xff := c.Request().Header.Get("X-Forwarded-For"); xff != "" {
                return strings.Split(xff, ",")[0]
            }
        }
    }
    return host
}

Step 3: Apply globally
In actions/app.go:

func (as *App) Middleware() []middleware.Middleware {
    return []middleware.Middleware{
        middleware.RequestID(),
        middleware.CORS(),
        RateLimitMiddleware(rateLimiter),
    }
}

Step 4: Optional headers
Add X-RateLimit-* headers using a separate middleware if desired. For authenticated APIs, key limits by user ID or API key for finer control.

After changes, re-scan with middleBrick to confirm the issue is resolved.