Crlf Injection in Gin with Hmac Signatures

Crlf Injection occurs when an attacker can inject CRLF sequences (\r\n) into HTTP headers or the response body, allowing header injection or response splitting. In the Gin web framework, this risk arises when user-controlled input is directly reflected in headers such as Location, Content-Type, or custom headers without validation. When Hmac Signatures are used—commonly to sign requests or validate webhook payloads—the interaction can amplify issues if the signature is computed over a header value that includes injected CRLF characters. For example, an attacker may supply a Location header value like https://example.com\r\nSet-Cookie: malicious=1. If Gin passes this value into a redirect response and the Hmac signature is derived from the raw header string, the signature may still verify (assuming the secret is known or the check is weak), allowing the injected header to be processed. In server-side request forgery or log-poisoning scenarios, CRLF Injection can manipulate how responses are parsed, especially when logs or downstream systems split entries by \r\n. The problem is not with Hmac itself, but with how input is handled before signing and how signatures are verified across multiple header fields. Because Hmac Signatures bind integrity to specific header values, failing to sanitize or canonicalize those values before hashing can lead to bypasses where an attacker injects additional headers while maintaining a valid signature. This is particularly dangerous in APIs that both consume and produce signed messages, such as webhook endpoints in microservices using Gin, where an attacker might forge a webhook with injected headers that appear authentic due to a valid Hmac. The OWASP API Security Top 10 category '2023-A1: Broken Object Level Authorization' and related injection risks can intersect with these header manipulation techniques. Tools like middleBrick, which scans for BOLA/IDOR, Input Validation, and Data Exposure, can detect unsafe header usage and signature misuse. Its LLM/AI Security checks also look for patterns where unchecked input reaches headers or logging. Note that middleBrick detects and reports these issues but does not fix them; it provides remediation guidance to guide developers toward safer implementations.

To remediate Crlf Injection risks when using Hmac Signatures in Gin, you must validate and sanitize all user-controlled data before it is included in signed payloads or headers. Canonicalize inputs by removing or encoding CRLF and other control characters, and enforce a strict allowlist for header values where possible. When constructing the data to sign, use a deterministic, normalized representation to avoid discrepancies between signing and verification. Below are concrete code examples demonstrating secure handling in Gin.

First, define a helper to strip dangerous characters from header values:

// sanitize.go
package main

import (
    "net/http"
    "strings"
)

// sanitizeHeaderValue removes CR and LF to prevent header injection.
func sanitizeHeaderValue(value string) string {
    // Remove carriage return and line feed characters.
    value = strings.ReplaceAll(value, "\r", "")
    value = strings.ReplaceAll(value, "\n", "")
    return value
}

Second, apply sanitization before using the value in both the Hmac computation and the HTTP response:

// handlers.go
package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "fmt"
    "net/http"
    "strings"
)

// computeHmac returns a hex-encoded HMAC-SHA256 of the message using the secret.
func computeHmac(message string, secret string) string {
    key := []byte(secret)
    h := hmac.New(sha256.New, key)
    h.Write([]byte(message))
    return fmt.Sprintf("%x", h.Sum(nil))
}

// webhookHandler demonstrates a safe Gin handler using Hmac signatures.
func webhookHandler(secret string) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // Assume a header "X-Webhook-Data" is provided by the sender.
        rawData := r.Header.Get("X-Webhook-Data")
        // Canonicalize: trim spaces and remove CRLF from the data used in signing.
        canonicalData := strings.TrimSpace(sanitizeHeaderValue(rawData))

        // The sender must also send X-Webhook-Signature with the Hmac of the canonical data.
        receivedSig := r.Header.Get("X-Webhook-Signature")
        expectedSig := computeHmac(canonicalData, secret)

        if !hmac.Equal([]byte(expectedSig), []byte(receivedSig)) {
            http.Error(w, "invalid signature", http.StatusBadRequest)
            return
        }

        // Use sanitized data for further processing.
        _ = canonicalData // proceed with business logic

        // If setting a Location header in a redirect, sanitize user input.
        userLocation := r.Header.Get("X-Requested-Location")
        safeLocation := sanitizeHeaderValue(userLocation)
        // Ensure safeLocation is a valid URL in practice; omitted for brevity.
        http.Redirect(w, r, safeLocation, http.StatusFound)
    }
}

For outgoing requests where you sign headers, sanitize before signing and set headers after:

// client.go
package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "fmt"
    "net/http"
    "strings"
)

// sendSignedRequest builds a request with Hmac-signed headers safely.
func sendSignedRequest(url string, secret string, userHeader string) (*http.Response, error) {
    // Sanitize user input before it touches the signature.
    safeHeader := sanitizeHeaderValue(userHeader)

    // Build the message to sign using the sanitized value.
    message := safeHeader
    key := []byte(secret)
    h := hmac.New(sha256.New, key)
    h.Write([]byte(message))
    signature := fmt.Sprintf("%x", h.Sum(nil))

    req, err := http.NewRequest("POST", url, nil)
    if err != nil {
        return nil, err
    }
    // Set sanitized header and its signature.
    req.Header.Set("X-Secure-Data", safeHeader)
    req.Header.Set("X-Signature", signature)

    client := &http.Client{}
    return client.Do(req)
}

These practices ensure that CRLF characters cannot split headers or alter the message format. By sanitizing inputs, canonicalizing before signing, and validating on receipt, you maintain the integrity guarantees of Hmac Signatures while preventing injection attacks. middleBrick can support this workflow by scanning for input handling issues and header-related vulnerabilities, offering findings mapped to standards such as OWASP API Top 10. Its CLI and GitHub Action integrations allow you to automate checks in development and CI/CD, while the Dashboard helps track improvements over time.