Header Injection in Buffalo with Hmac Signatures

Header Injection in the Buffalo framework when Hmac Signatures are used for request authentication can occur when untrusted input is reflected into HTTP headers without proper sanitization or canonicalization. This typically happens when developers build authorization or custom headers using string concatenation with user-controlled data, such as query parameters, form values, or headers from upstream proxies. An attacker can inject additional header lines (e.g., via CRLF characters) to smuggle headers like X-Original-URL or Set-Cookie, which may bypass intended authorization checks that rely on Hmac Signatures if the canonical request fed into the signature does not tightly constrain which headers are included.

In Buffalo, Hmac Signatures are commonly implemented by computing a signature over a canonical string that includes selected headers (e.g., X-Request-ID, X-Timestamp, and the request method/path). If the application includes attacker-influenced header values in the signature base string without strict validation, an attacker can inject crafted header lines that alter the signed canonical request. This can lead to signature mismatches being mishandled or enable logic where injected headers change routing or authentication behavior. For example, an attacker-supplied X-Forwarded-Host could be reflected into the canonical string and then used to manipulate the effective endpoint while the Hmac Signature appears valid if the server is not strict about which headers are considered authoritative.

The risk is compounded when the server relies on header values for routing or content negotiation without re-validating them against the Hmac Signature context. A malicious request with sequences like \r\nSet-Cookie: session=attacker or \r\nX-Backend: malicious could be processed if the framework does not reject or drop such injected headers before building the canonical string for signing. Because Buffalo applications often use middleware to parse and forward headers, insecure handling of untrusted inputs in these stages can expose the Hmac-based scheme to tampering or bypass. This does not break the cryptographic integrity of Hmac itself, but it subverts the intended security boundary by allowing unauthorized instructions to be introduced via injected headers.

Remediation focuses on strict header validation, canonicalization discipline, and avoiding the inclusion of untrusted header values in the Hmac computation unless explicitly required. Always treat incoming headers as untrusted and filter them before they influence the canonical request used for signature verification.

Example of a safe Hmac signature verification flow in a Buffalo action:

// Verify Hmac signature safely in a Buffalo action
import (
  "crypto/hmac"
  "crypto/sha256"
  "encoding/hex"
  "net/http"
  "strings"
)

func VerifyHmac(next http.Handler) http.Handler {
  return http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
    // Only allow specific trusted headers in canonical string
    const timestampHeader = "X-Request-Timestamp"
    const nonceHeader = "X-Request-Nonce"
    const signatureHeader = "X-Request-Signature"

    timestamp := req.Header.Get(timestampHeader)
    nonce := req.Header.Get(nonceHeader)
    receivedSig := req.Header.Get(signatureHeader)

    // Reject if any critical header is missing
    if timestamp == "" || nonce == "" || receivedSig == "" {
      http.Error(rw, "missing required headers", http.StatusBadRequest)
      return
    }

    // Build canonical string explicitly, do not include untrusted headers
    canonical := strings.Join([]string{req.Method, req.URL.RequestURI(), timestamp, nonce}, "|")

    secret := []byte("your-secure-server-secret") // use env/config
    mac := hmac.New(sha256.New, secret)
    mac.Write([]byte(canonical))
    expected := hex.EncodeToString(mac.Sum(nil))

    // Use constant-time comparison
    if !hmac.Equal([]byte(expected), []byte(receivedSig)) {
      http.Error(rw, "invalid signature", http.StatusUnauthorized)
      return
    }

    // At this point, the request is authenticated; proceed safely
    next.ServeHTTP(rw, req)
  })
}

Key practices:

• Do not include request headers that are set by clients (except those explicitly whitelisted) in the canonical input for Hmac computation.
• Normalize and validate header values (e.g., reject CR/LF, enforce length limits) before using them in signature logic.
• Use a strict allowlist of headers that influence the canonical string; drop or ignore any unexpected headers before routing.
• Perform constant-time signature comparisons to avoid timing-based side-channels.
• Consider adding replay protection (e.g., short timestamp windows and nonce tracking) to mitigate injected or replayed requests.