Email Injection in Aspnet with Bearer Tokens

Email Injection in ASP.NET occurs when user-controlled data is concatenated into email headers or message bodies without validation, allowing an attacker to inject additional headers or malformed content. When combined with Bearer Tokens used for API authentication, the risk pattern changes: the token itself is often handled in headers, and if the application reflects or logs token-related data into email workflows, it can expose token leakage or enable header smuggling.

Consider an ASP.NET Core web API that accepts a user email and an Authorization Bearer token, then sends a notification email including the token value for verification purposes. If the email construction does not sanitize the email address or the token, an attacker can provide a crafted email such as [email protected]\r\nCc: [email protected]. Depending on how the mail client or server parses the headers, this can lead to unintended recipients or header injection. In parallel, if the token is echoed into the email body or subject without encoding, a reflected token may be captured via phishing or log inspection.

In a black-box scan, middleBrick tests unauthenticated endpoints that accept user input used in email composition, checking whether newlines or special characters can break header boundaries. With Bearer Tokens, the scan also checks whether token values are improperly surfaced in email channels. For example, an endpoint accepting JSON like { "email": "[email protected]", "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9" } and then including the token in an email body could lead to sensitive data exposure if logs or mail servers are not properly secured.

Additionally, if the ASP.NET application uses the token to call downstream services and those responses are included in emails, an attacker may probe for Server Side Request Forgery (SSRF) via the email path, attempting to make the server fetch internal metadata or other services. The interplay between authentication via Bearer Tokens and email generation thus widens the attack surface: malformed email headers can bypass transport-level protections, and token inclusion can aid in tracking or session hijacking if logs are compromised.

middleBrick runs 12 security checks in parallel, including Input Validation, Data Exposure, and Unsafe Consumption, to detect whether email injection vectors exist when Bearer Tokens are involved. The scanner submits newline characters, encoding attempts, and header-like sequences into fields that flow into mail routines, while also checking whether tokens appear in outputs that reach email templates. Findings include severity ratings and remediation guidance mapped to frameworks such as OWASP API Top 10 and GDPR.

To remediate Email Injection when Bearer Tokens are part of the request, treat email headers and message bodies as untrusted input and apply strict validation and encoding. Do not concatenate user input directly into headers; use established mail libraries that enforce header safety. Ensure Bearer Tokens are never echoed into emails, logs, or client-side storage unless absolutely necessary and properly masked.

Example secure ASP.NET Core controller without injection risk:

using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;
using System.Net.Mail;

namespace SecureApi.Controllers
{
    [ApiController]
    [Route("api/[controller]")]
    public class NotifyController : ControllerBase
    {
        [HttpPost("send")]
        [Authorize(AuthenticationSchemes = "Bearer")]
        public IActionResult SendNotification([FromBody] NotifyRequest request)
        {
            // Validate email format strictly
            if (!System.Net.Mail.MailAddress.TryCreate(request.Email, out _))
            {
                return BadRequest("Invalid email address.");
            }

            // Build email using a safe library; do not inject headers from user input
            var mailMessage = new MailMessage("[email protected]", request.Email)
            {
                Subject = "Your notification",
                Body = $"Thank you. Your session remains secure.",
                IsBodyHtml = false
            };

            // Do NOT include the Bearer token in the email
            // Use a one-time lookup or a separate secure channel if verification is needed

            // Send via a safe SMTP client with enforced timeouts and TLS
            using var client = new SmtpClient("smtp.example.com")
            {
                Port = 587,
                Credentials = new System.Net.NetworkCredential("[email protected]", "AppSpecificPassword"),
                EnableSsl = true,
                DeliveryMethod = SmtpDeliveryMethod.Network,
                Timeout = 10000
            };
            client.Send(mailMessage);

            return Ok();
        }

        public class NotifyRequest
        {
            public string Email { get; set; } = string.Empty;
        }
    }
}

If you must reference a token in user-facing communication, use a short-lived, one-time token ID rather than the actual Bearer token, and transmit it via a separate secure channel (e.g., push notification). Store secrets securely using ASP.NET Core configuration and Azure Key Vault or similar, and never log raw Authorization headers.

In the CLI, you can verify your remediation by running:

middlebrick scan https://api.example.com/notify

In CI/CD, add the GitHub Action to fail builds if risk scores drop below your chosen threshold, and use the MCP Server to scan APIs directly from your IDE during development.