Crlf Injection in Buffalo with Hmac Signatures

Crlf Injection occurs when an attacker can inject CRLF sequences (%0D%0A or literal \r\n) into HTTP headers or header-like values. In the Buffalo web framework for Go, this commonly arises when dynamic values are used to construct headers, Set-Cookie attributes, or redirect locations without proper sanitization. When Hmac Signatures are used—typically to sign query parameters, form values, or API tokens—the signature is often computed over the raw, user-influenced input. If the application includes user-controlled data directly in a header or redirect target and then signs that data, an attacker can inject newline characters to create a second header (e.g., Set-Cookie) or split responses, while the Hmac Signature may still validate because the server recomputes the signature over the tainted input.

In Buffalo, Hmac Signatures are often implemented using packages like github.com/gobuffalo/buffalo alongside a signing library (e.g., HMAC-SHA256) to ensure integrity of tamper-sensitive values such as password reset tokens or webhook payloads. If the application places a user-supplied string—such as a callback URL or a locale parameter—into a header and signs the original or modified value, the CRLF injection can alter the semantic structure of the message. For example, an attacker might provide redirect_url=https://example.com%0D%0ASet-Cookie:%20session=attacker; the server includes this in a Location header, and the injected CRLF causes the Set-Cookie to be interpreted as an additional header. Because the Hmac Signature was computed over the attacker-controlled payload, the server may still consider the signature valid, leading to response splitting, session fixation, or cache poisoning.

The risk is compounded when the signature verification does not canonicalize or reject newline characters in the signed data. Buffalo applications that use Hmac Signatures for webhook validation or signed cookies must ensure that the signed payload does not include carriage returns or line feeds. Security checks such as input validation and Data Exposure scans in middleBrick can surface these weaknesses by identifying unsanitized use of user input in headers and missing rejection of control characters before signature computation.

Remediation focuses on preventing CRLF injection by sanitizing inputs before they are used in headers, redirects, or signed payloads, and by ensuring Hmac Signatures are computed over safe, canonical data.

• Reject or sanitize newline characters in user input used in headers and redirects: before placing a value into a Set-Cookie header or a redirect location, remove or encode \r and \n.
• Canonicalize before signing: normalize the input (trim, remove control characters) before computing the Hmac Signature so that the signature does not cover injected delimiters.
• Validate redirect and cookie targets against an allowlist or strict pattern (e.g., same-origin or a set of trusted domains) and avoid directly concatenating user input into header strings.

Example: secure handling of a redirect URL with Hmac Signature in Buffalo:

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

    "github.com/gobuffalo/buffalo"
)

// sanitizeInput removes CRLF characters to prevent header injection.
func sanitizeInput(s string) string {
    s = strings.ReplaceAll(s, "\r", "")
    s = strings.ReplaceAll(s, "\n", "")
    return s
}

// computeHMAC returns a hex-encoded HMAC-SHA256 of the message using the secret key.
func computeHMAC(message string, secret []byte) string {
    mac := hmac.New(sha256.New, secret)
    mac.Write([]byte(message))
    return hex.EncodeToString(mac.Sum(nil))
}

// Example handler for a redirect with a signed token.
func redirectHandler(c buffalo.Context) error {
    rawURL := c.Param("url")          // user-supplied, e.g., from query
    rawURL = sanitizeInput(rawURL)     // critical: strip \r\n before use
    secret := []byte("your-secure-secret-key")
    token := computeHMAC(rawURL, secret)

    // Build a safe redirect location; ensure no newline in final URL.
    location := rawURL + "?token=" + token
    // Validate location to ensure it is same-origin or matches allowed hosts.
    http.Redirect(c.Response(), c.Request(), location, http.StatusFound)
    return nil
}

// Example: validating a signed cookie value with Hmac Signature.
func validateSignedCookie(c buffalo.Context) error {
    cookie, err := c.Request().Cookie("session_token")
    if err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": "missing cookie"}))
    }
    // The cookie value should be: baseValue|hmac
    parts := strings.Split(cookie.Value, "|")
    if len(parts) != 2 {
        return c.Render(400, r.JSON(map[string]string{"error": "invalid cookie format"}))
    }
    baseValue := sanitizeInput(parts[0])
    receivedMAC := parts[1]
    expectedMAC := computeHMAC(baseValue, []byte("your-secure-secret-key"))
    if !hmac.Equal([]byte(expectedMAC), []byte(receivedMAC)) {
        return c.Render(403, r.JSON(map[string]string{"error": "invalid signature"}))
    }
    // Use baseValue safely; it has been sanitized.
    _ = baseValue
    return c.Render(200, r.JSON(map[string]string{"status": "ok"}))
}

These examples demonstrate how to integrate Hmac Signatures safely in Buffalo by removing CRLF characters before signing and before header usage, thereby mitigating injection risks while preserving integrity verification.