Request Smuggling in Buffalo with Hmac Signatures

Request smuggling occurs when an intermediary (such as a load balancer or reverse proxy) processes HTTP requests differently than the origin server, allowing attackers to smuggle a request across security boundaries. In Buffalo, when Hmac Signatures are used for request authentication but the framework or server does not enforce strict message integrity checks before routing or buffering, smuggling can be introduced.

Buffalo applications often place a reverse proxy or load balancer in front that terminates TLS and forwards requests. If the proxy normalizes or parses headers differently—such as handling duplicate Content-Length or Transfer-Encoding headers—and then forwards the altered request to Buffalo, the application may interpret the request body differently than intended. Hmac Signatures are typically computed over a canonical set of headers and the body. When the proxy modifies the body or header ordering without updating the signature, Buffalo may still accept the request because it validates the signature against the altered but still signed request, or it may process two separate requests from a single connection.

This creates a vulnerability where an attacker can smuggle a request containing an authenticated action (e.g., a payment or admin operation) so that it is interpreted by a different route or consumer. For example, an attacker might send a request with two Content-Length headers: one indicating a small body with a valid Hmac Signature, and another indicating a larger body that includes a hidden malicious request. If Buffalo trusts the first length and validates the signature against the smaller body, the proxy may forward the remaining bytes as a new request, which Buffalo processes without signature validation.

The interaction between Buffalo’s routing and Hmac Signature validation is particularly risky when the application uses middleware that reads the request body before passing it to the signature verification logic. If the body is read partially or buffered inconsistently, the canonical string used to compute or verify the Hmac may not match what the origin server expects. This mismatch can allow a smuggled request to bypass intended access controls or execute unintended operations, even though Hmac Signatures are in place.

Real-world attack patterns mirror known CVEs in other frameworks that involve header parsing ambiguities and body handling. Although the scanner does not test for request smuggling directly, the findings related to Input Validation and BOLA/IDOR can indicate inconsistent request handling that may be exploited when Hmac Signatures are used in a multi-hop setup.

To mitigate request smuggling when using Hmac Signatures in Buffalo, ensure the signature is computed over a canonical representation of the entire request before any routing or body buffering occurs, and enforce strict header handling on both the proxy and the application.

First, configure your proxy to not modify headers that are part of the Hmac computation, and avoid sending multiple Content-Length or Transfer-Encoding headers. On the Buffalo side, compute the Hmac over the raw request body and a normalized set of headers, and verify the signature before any middleware consumes the body.

Example Hmac signature generation and verification in Buffalo (using Go):

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
)

func computeSignature(secret, body []byte, headers http.Header) string {
    mac := hmac.New(sha256.New, secret)
    // Canonical header list: include only headers relevant to authentication
    mac.Write([]byte(headers.Get("X-Request-ID")))
    mac.Write([]byte(headers.Get("Content-Type")))
    mac.Write(body)
    return hex.EncodeToString(mac.Sum(nil))
}

func verifySignature(secret, body []byte, headers http.Header, provided string) bool {
    expected := computeSignature(secret, body, headers)
    return hmac.Equal([]byte(expected), []byte(provided))
}

func MyHandler(secret []byte) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        body := r.Body // read once and cache if needed
        // Ensure no duplicate headers interfere with canonical form
        r.Header.Del("X-Forwarded-For") // remove potentially injected headers
        if !verifySignature(secret, body, r.Header, r.Header.Get("X-Signature")) {
            http.Error(w, "invalid signature", http.StatusUnauthorized)
            return
        }
        // proceed with routing and business logic
    }
}

Ensure your proxy does not alter the request body or the headers included in the Hmac computation. If you must normalize headers, update the signature accordingly on the proxy side or reject requests where header modifications could lead to smuggling. Use a single Content-Length and avoid chunked encoding when Hmac Signatures protect the request.

For continuous protection, add the GitHub Action to your CI/CD pipeline to fail builds if security checks related to Input Validation or BOLA/IDOR raise concerns about request handling. The MCP Server in your IDE can also highlight suspicious header usage during development.