Http Request Smuggling in Buffalo

HTTP request smuggling is a high-impact vulnerability where an attacker sends an ambiguous HTTP request that is interpreted differently by a front-end proxy and the Buffalo backend. This discrepancy allows a second, malicious request to be smuggled past the proxy, leading to request hijacking, cache poisoning, or session takeover. The two primary variants are CL.TE (Content-Length / Transfer-Encoding) and TE.CL (Transfer-Encoding / Content-Length). The OWASP Web Security Testing Guide documents this attack pattern as a critical risk due to its potential for full system compromise.

In Buffalo applications, this vulnerability typically emerges from misconfigured reverse proxies (e.g., nginx, HAProxy) or custom Buffalo middleware that fails to normalize conflicting headers. Buffalo's underlying Go net/http server follows standard HTTP parsing, but when the proxy and backend disagree on header precedence, an attack surface appears.

Consider a Buffalo handler that proxies requests to an internal service without sanitizing headers:

func ProxyHandler(c buffalo.Context) error {
    req := c.Request()
    // Directly forward the request as-is
    proxy := httputil.NewSingleHostReverseProxy(targetURL)
    proxy.ServeHTTP(c.Response().Writer, req)
    return nil
}

If an attacker sends a request with both Content-Length: 4 and Transfer-Encoding: chunked, and the front-end uses Content-Length while Buffalo's Go server uses Transfer-Encoding, the proxy forwards the first 4 bytes as one request and the remainder as a smuggled second request. The smuggled request could access administrative endpoints or exfiltrate data.

middleBrick identifies HTTP request smuggling in Buffalo endpoints through active black-box scanning. The scanner sends crafted ambiguous requests to the target URL and analyzes responses for signs of request splitting. Since Buffalo APIs often follow RESTful patterns, middleBrick tests each endpoint with both CL.TE and TE.CL payloads.

The detection process involves:

• Crafting a request with conflicting Content-Length and Transfer-Encoding headers, where the body is designed to be interpreted as two separate requests by the backend.
• Sending the payload and then a follow-up probe to check if the smuggled request was processed (e.g., by including a unique token in the smuggled portion and verifying its presence in the response).
• If the smuggled request succeeds, middleBrick reports a high-severity finding with the exact payload, severity level, and remediation steps.

Scanning a Buffalo API is simple and requires no credentials:

middlebrick scan https://api.buffalo.example.com/v1/users

The scan completes in 5–15 seconds and leverages middleBrick's 12 parallel security checks, including Input Validation and others that probe for header inconsistencies. Results include a risk score (0–100, A–F) and a per-category breakdown, highlighting smuggling under Input Validation. Because middleBrick performs active probing, it accurately identifies environment-dependent vulnerabilities that static analysis might miss, such as those arising from proxy-Buffalo interactions.

Remediation focuses on normalizing conflicting headers before they reach Buffalo's HTTP server or within middleware. The goal is to ensure only one header (either Content-Length or Transfer-Encoding) controls the body length, eliminating ambiguity.

1. Add a Sanitization Middleware

Create a Buffalo middleware to strip conflicting headers. Register it early in the stack, before any body-reading handlers:

func SanitizeRequestHeaders(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        req := c.Request()
        // If Transfer-Encoding is present, remove Content-Length to avoid CL.TE
        if te := req.Header.Get("Transfer-Encoding"); te != "" {
            req.Header.Del("Content-Length")
        }
        // Alternatively, remove Transfer-Encoding if Content-Length exists
        return next(c)
    }
}

In app.go:

func init() {
    app := buffalo.New(&buffalo.Options{})
    app.Use(SanitizeRequestHeaders) // Register before routes
    // ... other middleware and routes
}

2. Configure the Reverse Proxy

If Buffalo runs behind nginx, normalize headers at the proxy layer:

location / {
    proxy_pass http://buffalo_backend;
    proxy_set_header Transfer-Encoding ""; # Remove Transfer-Encoding to prevent CL.TE
    # Or, force Content-Length removal if Transfer-Encoding is used
}

3. Customize ReverseProxy in Buffalo

For Buffalo apps that forward requests using httputil.ReverseProxy, adjust the Director:

proxy := httputil.NewSingleHostReverseProxy(targetURL)
proxy.Director = func(req *http.Request) {
    // Remove Transfer-Encoding if Content-Length exists
    if req.Header.Get("Content-Length") != "" {
        req.Header.Del("Transfer-Encoding")
    }
    // ... set URL, Host, etc.
}

4. Server-Level Limits

Configure the Buffalo server with timeouts and header limits as a secondary defense:

app := buffalo.New(&buffalo.Options{
    Server: &http.Server{
        ReadTimeout:   5 * time.Second,
        WriteTimeout:  10 * time.Second,
        MaxHeaderBytes: 1 << 20, // 1 MB
    },
})

These Buffalo-specific fixes eliminate header ambiguity. After remediation, rescanning with middleBrick should yield an improved Input Validation score, reflecting the mitigation.