Email Injection in Aspnet

Email injection attacks in Aspnet applications typically exploit how the framework handles SMTP and email composition. Attackers craft malicious input that manipulates email headers or message bodies to send unauthorized emails, spam, or phishing messages through your application's mail server.

The most common Aspnet-specific manifestation occurs when user input flows directly into email headers without proper validation. Consider this vulnerable Aspnet Core pattern:

var email = new MimeMessage();
email.From.Add(new MailboxAddress(fromAddress));
email.To.Add(new MailboxAddress(toAddress));
email.Subject = subject;
email.Body = new TextPart("plain") { Text = body };

using var client = new SmtpClient();
client.Connect("smtp.example.com", 587);
client.Authenticate("username", "password");
client.Send(email);
client.Disconnect(true);

An attacker could supply a subject like Subject: Important Bcc: victim@example.com Actual subject here to silently BCC a third party. The newline characters (\n) create new header fields that the SMTP client processes.

Another Aspnet-specific vector involves model binding in MVC applications. When using [BindProperty] or [FromBody] with email-related models:

public class ContactFormModel
{
    public string Name { get; set; }
    public string Email { get; set; }
    public string Subject { get; set; }
    public string Message { get; set; }
}

If the Subject property contains newline characters, model binding doesn't sanitize them, and they flow directly to the email library. This is particularly dangerous in Aspnet Core's default model binding behavior, which preserves special characters unless explicitly configured otherwise.

QueryString-based email injection also affects Aspnet applications. When parameters are read from Request.Query without validation:

var subject = Request.Query["subject"];
var body = Request.Query["body"];
// No validation before using in email

Attackers can URL-encode newline sequences (%0A%0D) to bypass basic input filtering. Aspnet's query string parsing preserves these encoded characters unless explicitly decoded and sanitized.

Form collection attacks exploit Aspnet's Request.Form handling. When processing multipart form data for file uploads with email functionality:

var form = await Request.ReadFormAsync();
var subject = form["subject"];
// Subject passed directly to email without sanitization

The multipart parser in Aspnet doesn't inherently validate header content, making it vulnerable to crafted submissions that inject additional email headers.

Detecting email injection in Aspnet applications requires examining both code patterns and runtime behavior. Start with static analysis of your Aspnet MVC controllers and API endpoints:

var emailInjectionPatterns = new[]
{
    @"\r\n", // CRLF
    @"\n",   // LF
    @"%0A",  // URL-encoded newline
    @"%0D"   // URL-encoded carriage return
};

Search for these patterns in any user-controlled input that flows to email headers or bodies. In Aspnet Core, use middleware to inspect requests:

public class EmailInjectionDetectionMiddleware
{
    private readonly RequestDelegate _next;
    
    public EmailInjectionDetectionMiddleware(RequestDelegate next)
    {
        _next = next;
    }
    
    public async Task InvokeAsync(HttpContext context)
    {
        var suspicious = CheckForInjectionPatterns(context.Request);
        if (suspicious)
        {
            // Log and potentially block
            context.Response.StatusCode = 400;
            await context.Response.WriteAsync("Invalid input detected");
            return;
        }
        
        await _next(context);
    }
    
    private bool CheckForInjectionPatterns(HttpRequest request)
    {
        var injectionPatterns = new[] { "\r\n", "\n", "%0A", "%0D" };
        
        foreach (var pattern in injectionPatterns)
        {
            if (request.QueryString.HasValue && 
                request.QueryString.Value.Contains(pattern))
                return true;
                
            if (request.HasFormContentType)
            {
                var form = request.Form;
                foreach (var val in form.Values)
                {
                    if (val.Contains(pattern)) return true;
                }
            }
        }
        
        return false;
    }
}

Register this middleware in Configure before MVC routing:

app.UseMiddleware();
app.UseRouting();
app.UseEndpoints(endpoints =>
{
    endpoints.MapControllers();
});

For runtime detection, middleBrick's black-box scanning specifically tests for email injection vulnerabilities by sending payloads containing newline sequences to your Aspnet API endpoints. The scanner checks if these inputs create additional email headers or modify email behavior.

middleBrick tests 12 security categories including authentication bypass and input validation. For email injection specifically, it sends crafted payloads to your endpoints and analyzes SMTP server responses to detect if unauthorized emails were sent or if header injection occurred.

The scanner's LLM security checks also detect if your Aspnet application has AI endpoints that might be vulnerable to prompt injection attacks that could manipulate email generation in AI-powered email features.

Remediating email injection in Aspnet requires a defense-in-depth approach. Start with input validation at the model level using data annotations:

public class SecureContactFormModel
{
    [StringLength(100)]
    public string Name { get; set; }
    
    [EmailAddress]
    [StringLength(254)]
    public string Email { get; set; }
    
    [RegularExpression(@"^[^
]*$", ErrorMessage = "Subject cannot contain newlines")]
    [StringLength(200)]
    public string Subject { get; set; }
    
    [StringLength(5000)]
    public string Message { get; set; }
}

The RegularExpression attribute blocks newline characters in the subject field. For more comprehensive validation, create a custom validation attribute:

public class NoNewLineAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        if (value == null) return true;
        
        var str = value.ToString();
        return !str.Contains("\r") && !str.Contains("\n");
    }
}

Apply this to all email-related properties:

public class SecureEmailModel
{
    [NoNewLine]
    public string From { get; set; }
    
    [NoNewLine]
    public string To { get; set; }
    
    [NoNewLine]
    public string Subject { get; set; }
    
    public string Body { get; set; }
}

For Aspnet Core's built-in email validation, use the EmailAddressAttribute and custom sanitization:

public class EmailService
{
    public void SendSecureEmail(SecureEmailModel model)
    {
        // Sanitize inputs
        var sanitizedSubject = SanitizeInput(model.Subject);
        var sanitizedBody = SanitizeInput(model.Body);
        
        var email = new MimeMessage();
        email.From.Add(new MailboxAddress(model.From));
        email.To.Add(new MailboxAddress(model.To));
        email.Subject = sanitizedSubject;
        email.Body = new TextPart("plain") { Text = sanitizedBody };
        
        using var client = new SmtpClient();
        client.Connect("smtp.example.com", 587);
        client.Authenticate("username", "password");
        client.Send(email);
        client.Disconnect(true);
    }
    
    private string SanitizeInput(string input)
    {
        if (string.IsNullOrEmpty(input)) return input;
        
        // Remove all newline characters
        return input.Replace("\r", string.Empty)
                    .Replace("\n", string.Empty)
                    .Replace("%0A", string.Empty)
                    .Replace("%0D", string.Empty);
    }
}

For Aspnet Core MVC, use action filters to centralize email injection prevention:

public class EmailInjectionFilter : IActionFilter
{
    public void OnActionExecuting(ActionExecutingContext context)
    {
        var injectionPatterns = new[] { "\r\n", "\n", "%0A", "%0D" };
        
        foreach (var kvp in context.ActionArguments)
        {
            var value = kvp.Value?.ToString();
            if (value != null && injectionPatterns.Any(p => value.Contains(p)))
            {
                context.Result = new BadRequestObjectResult(
                    $"Invalid characters in {kvp.Key}");
                return;
            }
        }
    }
    
    public void OnActionExecuted(ActionExecutedContext context) { }
}

Apply this filter globally or to specific controllers:

[ServiceFilter(typeof(EmailInjectionFilter))]
public class ContactController : Controller
{
    // Controller actions
}

For production deployments, integrate middleBrick's continuous monitoring to automatically scan your Aspnet APIs for email injection vulnerabilities. The Pro plan includes scheduled scans that test your endpoints with injection payloads and alert you if vulnerabilities are detected.

middleBrick's GitHub Action can be configured to scan your Aspnet APIs during CI/CD pipelines, failing builds if email injection vulnerabilities are found before deployment to production.