Email Injection in Gin with Hmac Signatures

Email Injection occurs when user-controlled input is placed into email headers without proper validation, enabling attackers to inject additional headers or malformed content. In the Gin framework (a minimal and performant HTTP web framework for Go), this typically arises when building email-related endpoints that accept user-supplied fields such as To, From, Subject, or Reply-To. If these values are concatenated into raw email text or headers without sanitization, an attacker can inject newline characters (\r\n) to add arbitrary headers like Cc: or Bcc:, or manipulate the message body.

Using Hmac Signatures in Gin is a common pattern to ensure the integrity and authenticity of incoming requests, for example when a webhook or callback endpoint accepts email actions from a third party. A server computes an HMAC over the request payload (or selected headers) using a shared secret and expects the client to provide a matching signature (e.g., in a header like X-Signature). If the implementation only validates the HMAC but does not validate or sanitize the email content, an attacker can still exploit Email Injection by controlling the payload used to generate the signature. The signature validates the payload’s integrity, but the server processes the payload unsafely, allowing newline injection in headers or body. This combination creates a scenario where authentication and integrity checks pass, but the application remains vulnerable to header manipulation and potential email redirection or spam.

For instance, consider a Gin endpoint that accepts JSON containing email and signature, verifies the HMAC, and then directly uses the email value in an SMTP transaction or a mail library call. If the email field contains sequences like \r\nCc: attacker@example.com, the underlying mail library may interpret it as an additional recipient header. Because Hmac Signature verification focuses on tampering, not on safe handling of structured data, the developer must separately enforce strict validation on email fields. OWASP API Top 10’s Input Validation and Data Exposure checks highlight this class of risk when structured data is consumed without canonicalization and constraint checks.

Real-world attack patterns include injecting a Bcc: header to silently copy emails, or adding Content-Type overrides to facilitate phishing payloads. In an API that returns templated emails or processes bulk sends, an injected newline can break the header/body delimiter (\r\n\r\n), causing undefined behavior or information leakage. Even when Hmac Signatures prevent unauthorized callers from crafting requests, they do not mitigate logic flaws where valid requests carry malicious data. Therefore, defense requires canonicalizing and validating email inputs—rejecting or encoding newlines and controlling characters—regardless of signature status.

Remediation centers on strict input validation and safe handling of email data, independent of Hmac verification. In Gin, you should validate email fields before using them in any mail-related operations, and treat the Hmac as a separate integrity gate rather than a substitute for content safety. Below are concrete Go code examples using the standard library and a common Hmac approach.

First, validate email addresses using strict rules (allowlist alphanumerics and a limited set of symbols, and reject control characters including carriage return and line feed). Then compute and verify Hmac on the raw request body to ensure integrity, and only proceed if both checks pass.

// validateEmail ensures the email contains no dangerous newline or control characters.
func validateEmail(email string) bool {
    // Reject if contains CR or LF which can be used for header injection.
    if strings.ContainsAny(email, "\r\n") {
        return false
    }
    // Basic format check: local@domain with simple rules.
    parts := strings.Split(email, "@")
    if len(parts) != 2 {
        return false
    }
    local, domain := parts[0], parts[1]
    if local == "" || domain == "" {
        return false
    }
    // Domain must contain a dot and no spaces/newlines.
    if !strings.Contains(domain, ".") || strings.ContainsAny(domain, " \r\n") {
        return false
    }
    return true
}

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

// verifyHmac compares provided signature with computed HMAC in constant time.
func verifyHmac(body []byte, provided, secret string) bool {
    expected := computeHmac(body, secret)
    return hmac.Equal([]byte(expected), []byte(provided))
}

// Gin handler example combining validation and Hmac verification.
func emailHandler(c *gin.Context) {
    // Read raw body for Hmac verification.
    rawBody, err := c.GetRawData()
    if err != nil {
        c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "unable to read body"})
        return
    }

    // Parse JSON payload.
    var payload struct {
        Email    string `json:"email"`
        Signature string `json:"signature"`
    }
    if err := json.Unmarshal(rawBody, &payload); err != nil {
        c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "invalid JSON"})
        return
    }

    // Verify Hmac before processing.
    secret := os.Getenv("HMAC_SECRET")
    if secret == "" {
        c.AbortWithStatusJSON(http.StatusInternalServerError, gin.H{"error": "server misconfiguration"})
        return
    }
    if !verifyHmac(rawBody, payload.Signature, secret) {
        c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "invalid signature"})
        return
    }

    // Strict email validation to prevent injection.
    if !validateEmail(payload.Email) {
        c.AbortWithStatusJSON(http.StatusBadRequest, gin.H{"error": "invalid email"})
        return
    }

    // Safe to use payload.Email in mail logic (e.g., SMTP or mail library).
    // Ensure mail library uses parameterized headers to avoid any injection.
    _ = payload.Email // placeholder for actual email sending logic
    c.JSON(http.StatusOK, gin.H{"status": "accepted"})
}

Key points in this remediation:

• Validate email with an allowlist approach that explicitly rejects CR/LF and other control characters before any header construction.
• Perform Hmac verification on the raw request body to ensure integrity, but treat it as a separate security layer, not a content filter.
• Use constant-time comparison for Hmac verification to avoid timing attacks.
• When sending email, prefer mail libraries that accept structured recipients (e.g., separate To, Cc slices) rather than raw header strings; never directly concatenate user input into header lines.

By combining precise input validation with integrity checks, you mitigate Email Injection while preserving the authenticity guarantees provided by Hmac Signatures. This approach aligns with best practices for secure API consumption and helps satisfy checks under OWASP API Top 10 and related compliance mappings.