Email Injection in Aspnet with Hmac Signatures

Email Injection in ASP.NET occurs when user-controlled data is concatenated into email headers or message bodies without validation, enabling an attacker to inject additional headers or content. When HMAC signatures are used to protect integrity of a request or a token that includes email-related parameters, a mismatch between the expected and actual signature verification flow can expose logic that allows injection. For example, consider an endpoint that accepts email, action, and a signed payload:

POST /confirm?signature=HMAC(payload) HTTP/1.1
Content-Type: application/x-www-form-urlencoded

email=attacker%40evil.com&action=change_email&signature=....

If the server reuses the same HMAC key and verification logic across different contexts (e.g., trusting the email field after signature verification without canonicalization), an attacker may try to manipulate header interpretation downstream. In a web framework that builds email messages from user input (for example using SmtpClient or a third-party library), newlines in the email address or injected header fields can break the message format and enable header injection:

email: attacker%40evil.com%0D%0ACc:%20secret%40target.com
subject: Please confirm

Even with HMAC protecting the payload, if the server does not validate and sanitize the email string before using it in mail composition, the injected newline and subsequent lines can be interpreted as new SMTP headers. HMAC signatures themselves do not prevent this; they ensure the payload has not been altered, but they do not sanitize the content. The vulnerability arises when the application trusts the email value after signature verification and passes it directly to an email-sending routine that uses concatenation or unsafe formatting.

The LLM/AI Security checks in middleBrick specifically test for system prompt leakage and output handling that could expose sensitive instructions or data. Although LLM probes are not directly relevant to classic Email Injection, they highlight the importance of validating and encoding all outputs that leave the security boundary, including emails. middleBrick’s 12 security checks run in parallel and include Input Validation and Data Exposure, which would flag unsanitized email headers and risky data handling patterns that could lead to injection or unintended data disclosure.

To align with real-world findings, middleBrick’s scan would categorize issues like missing input validation and unsafe data handling under high severity, providing prioritized findings with remediation guidance. The scanner supports OpenAPI/Swagger spec analysis (2.0, 3.0, 3.1) with full $ref resolution, cross-referencing spec definitions with runtime behavior to detect mismatches between declared and actual input handling.

Remediation focuses on canonicalizing inputs before signing and strictly validating and encoding data before use in email composition. Do not rely on HMAC alone to sanitize content. Use a strict allowlist for email addresses and encode all user-supplied data before inserting it into headers or message bodies.

1. Canonicalize and validate before signing

Ensure the data used to compute the HMAC is canonical (e.g., sorted key-value pairs, trimmed strings) and validate the email format before processing. Example in C#:

using System;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Text.RegularExpressions;

public static class SecurityHelper
{
    private const string EmailPattern = @"^[^\s@]+@[^\s@]+\.[^\s@]+$";
    private static readonly Regex EmailRegex = new Regex(EmailPattern, RegexOptions.Compiled | RegexOptions.IgnoreCase);

    public static bool TryValidateEmail(string email, out string canonical)
    {
        canonical = null;
        if (string.IsNullOrWhiteSpace(email))
            return false;

        // Basic strict validation; consider using MailAddress for additional checks
        if (!EmailRegex.IsMatch(email))
            return false;

        canonical = email.Trim().ToLowerInvariant();
        return true;
    }

    public static string ComputeHmac(string data, string key)
    {
        using var hmac = new HMACSHA256(Encoding.UTF8.GetBytes(key));
        var hash = hmac.ComputeHash(Encoding.UTF8.GetBytes(data));
        return Convert.ToBase64String(hash);
    }

    public static string CanonicalizeAndSign(object payload, string key)
    {
        // Example: payload is a dictionary; sort keys for canonical form
        var sorted = payload.GetType().GetProperties()
            .Where(p => p.GetValue(payload) != null)
            .OrderBy(p => p.Name)
            .Select(p => $"{p.Name}={p.GetValue(payload)}");
        string canonical = string.Join("&", sorted);
        return ComputeHmac(canonical, key);
    }
}

2. Encode before email composition

Never directly concatenate user input into email headers. Use .NET’s built-in encoding for header fields and a robust email library. Example using MailMessage and AlternateView:

using System.Net;
using System.Net.Mail;

public void SendConfirmation(string rawEmail, string subject, string body)
{
    if (!SecurityHelper.TryValidateEmail(rawEmail, out string email))
        throw new ArgumentException("Invalid email address");

    var message = new MailMessage();
    message.From = new MailAddress("[email protected]");
    // Encode to prevent injection via headers
    message.To.Add(new MailAddress(email));
    message.Subject = subject; // subject should also be encoded if from user input
    message.Body = body;
    message.IsBodyHtml = false;

    using var client = new SmtpClient("smtp.example.com");
    // Configure credentials and port as needed
    client.Credentials = CredentialCache.DefaultNetworkCredentials;
    client.EnableSsl = true;
    client.Send(message);
}

// Example usage with HMAC verification
public bool HandleRequest(string email, string action, string signature, string secret)
{
    var payload = new { email = email, action = action };
    string expected = SecurityHelper.ComputeHmac(
        string.Join("&", payload.GetType().GetProperties()
            .OrderBy(p => p.Name)
            .Select(p => $"{p.Name}={p.GetValue(payload)}")),
        secret);

    if (!CryptographicOperations.FixedTimeEquals(
        Convert.FromBase64String(signature),
        Convert.FromBase64String(expected)))
    {
        throw new SecurityException("Invalid signature");
    }

    SendConfirmation(email, "Confirm", "Thank you for confirming.");
    return true;
}

3. Use middleware or filters for global validation

In ASP.NET Core, apply model validation and header encoding in pipeline filters to ensure consistent handling:

public class ValidateEmailAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext context)
    {
        if (context.ActionArguments.TryGetValue("email", out var value) && value is string email)
        {
            if (!SecurityHelper.TryValidateEmail(email, out _))
                context.ModelState.AddModelError("email", "Invalid email format");
        }
        base.OnActionExecuting(context);
    }
}

[ApiController]
[Route("api/[controller]")]
[ServiceFilter(typeof(ValidateEmailAttribute))]
public class AccountController : ControllerBase
{
    [HttpPost("confirm")]
    public IActionResult Confirm([FromQuery] string email, [FromQuery] string action, [FromQuery] string signature)
    {
        // Signature verification and safe email usage as shown above
        return Ok();
    }
}

By canonicalizing inputs before HMAC computation, validating and encoding email values, and using typed model binding with validation filters, you mitigate Email Injection risks while preserving the integrity guarantees provided by Hmac Signatures. middleBrick’s scans can verify that these controls are present and report findings with severity and remediation guidance mapped to standards such as OWASP API Top 10 and PCI-DSS.

The CLI tool (middlebrick scan <url>) and GitHub Action enable you to integrate checks into development workflows. The MCP Server allows scanning APIs directly from AI coding assistants, helping catch issues early while design decisions are made.