Http Request Smuggling in Buffalo with Bearer Tokens

Http Request Smuggling is a protocol-level attack that exploits discrepancies in how request boundaries are parsed between frontend (e.g., a load balancer or reverse proxy) and backend HTTP servers. In the Buffalo web framework, which is a lightweight HTTP router and middleware stack for Go, this can occur when requests include Bearer Tokens in headers and the framing of those requests is ambiguous.

The vulnerability arises when Buffalo applications (or the infrastructure in front of them) handle requests with Transfer-Encoding and Content-Length headers inconsistently. For example, a request may include both Transfer-Encoding: chunked and a Content-Length header. If the frontend treats the request as chunked while the backend interprets it using Content-Length, the body boundary can be misaligned. This can cause one request’s body to be smuggled into the next request in the pipeline. Because Bearer Tokens are typically passed in HTTP headers (e.g., Authorization: Bearer <token>), a smuggled request may be able to inject an Authorization header into a downstream request, leading to authentication bypass or privilege escalation.

In Buffalo, this often manifests when applications use custom middleware that reads headers before routing, or when proxies normalize headers differently than the framework. An attacker can craft a request where the smuggled payload contains a valid Bearer Token intended for a privileged endpoint, effectively hijacking the token’s context without needing to know the token value in advance. The attack leverages the framework’s trust in the request’s header order and the proxy’s parsing rules. Because Buffalo does not inherently enforce strict header sanitization or body-length validation at the framework level, developers must ensure their proxy and application configurations agree on request parsing to mitigate this class of issue.

Remediation focuses on aligning how requests are parsed across the stack and ensuring Bearer Tokens are handled in a way that prevents header smuggling. Below are concrete steps and code examples for Buffalo applications.

1. Enforce consistent request body parsing: Disable ambiguous handling of both Transfer-Encoding and Content-Length. In your Buffalo application, ensure middleware does not process requests that contain both headers. You can reject such requests early.

package middleware

import (
    "net/http"
)

func RejectAmbiguousBody(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        transferEnc := r.Header.Get("Transfer-Encoding")
        contentLen := r.Header.Get("Content-Length")
        if transferEnc != "" && contentLen != "" {
            http.Error(w, "ambiguous body encoding", http.StatusBadRequest)
            return
        }
        next.ServeHTTP(w, r)
    })
}

2. Normalize and validate the Authorization header: Ensure that if a Bearer Token is present, it is validated and not duplicated or overridden by smuggled headers. Use strict header parsing and avoid merging headers from untrusted proxies.

package middleware

import (
    "net/http"
    "strings"
)

func ValidateBearerToken(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        auth := r.Header.Get("Authorization")
        if auth != "" {
            parts := strings.Split(auth, " ")
            if len(parts) != 2 || strings.ToLower(parts[0]) != "bearer" {
                http.Error(w, "invalid authorization header", http.StatusBadRequest)
                return
            }
            // Here you would validate the token (e.g., check format, verify with auth service)
            _ = parts[1] // token
        }
        next.ServeHTTP(w, r)
    })
}

3. Configure your proxy to strip or reject smuggling-indicative headers: Ensure your load balancer or reverse proxy does not allow requests where Transfer-Encoding and Content-Length coexist. If using a forward proxy, enforce header canonicalization before requests reach Buffalo.

4. Avoid merging headers from multiple sources: In Buffalo, do not append or merge headers that may have been injected by intermediate proxies. Treat incoming headers as authoritative only after validation.

By combining these practices, you reduce the risk that a smuggled request can manipulate the Authorization context of a downstream request. These controls are complementary to scanning with tools like middleBrick, which can detect misconfigurations in the unauthenticated attack surface and highlight inconsistencies between specification and runtime behavior, including how Bearer Tokens are exposed in headers.