Server Side Template Injection in Aspnet with Basic Auth

Server Side Template Injection (SSTI) in ASP.NET becomes particularly concerning when combined with Basic Authentication. In this scenario, an attacker provides malformed credentials containing template syntax which are then reflected into server-side rendering logic. Because Basic Authentication credentials are often decoded and used directly in string concatenation or passed into view components, unsanitized input can reach Razor or other templating engines.

Consider an endpoint that greets a user by decoding the Authorization header without validation:

// Example controller action using Basic Auth credentials
[ApiController]
[Route("api/[controller]")]
public class GreetController : ControllerBase
{
    [HttpGet]
    public IActionResult Greet()
    {
        var authHeader = Request.Headers["Authorization"].ToString();
        if (authHeader.StartsWith("Basic ", StringComparison.OrdinalIgnoreCase))
        {
            var token = authHeader.Substring("Basic ".Length).Trim();
            var credentialString = Encoding.UTF8.GetString(Convert.FromBase64String(token));
            var parts = credentialString.Split(':', 2);
            var username = parts[0];
            // Vulnerable: username used directly in a view component or rendered string
            return Content($"Hello {username}, welcome back.");
        }
        return Unauthorized();
    }
}

If the username contains Razor syntax such as {{7*7}} or {% import namespace %} depending on the rendering pipeline, the server may evaluate it. In ASP.NET MVC with Razor, an attacker could attempt payloads like {% system("id") %} when the template engine is misconfigured to parse user input. The risk is amplified when developers use string interpolation to build dynamic content that is later processed by a template engine, effectively turning the Basic Auth credential into a vector for remote code execution or information disclosure.

middleBrick detects this class of issue under BOLA/IDOR and Unsafe Consumption checks, noting when authentication data flows into rendering contexts. The scanner evaluates unauthenticated endpoints and maps findings to frameworks such as OWASP API Top 10 and CWE-90, emphasizing input validation failures.

Remediation focuses on strict validation, avoiding direct reflection of credentials, and using structured authentication mechanisms. Never concatenate decoded Basic Auth credentials directly into output or templates. Instead, treat credentials as opaque tokens for authentication only.

Use ASP.NET Core Identity or built-in authentication handlers rather than manual parsing. Below is a secure pattern that validates and uses credentials without exposing them in rendering:

// Secure approach: authenticate without reflecting credentials in output
[ApiController]
[Route("api/[controller]")]
public class SecureGreetController : ControllerBase
{
    [HttpGet]
    public IActionResult Greet()
    {
        var authHeader = Request.Headers["Authorization"].ToString();
        if (authHeader.StartsWith("Basic ", StringComparison.OrdinalIgnoreCase))
        {
            var token = authHeader.Substring("Basic ".Length).Trim();
            var credentialString = Encoding.UTF8.GetString(Convert.FromBase64String(token));
            var parts = credentialString.Split(':', 2);
            var username = parts[0];
            var password = parts[1];

            // Validate credentials against a secure store (example placeholder)
            if (IsValidUser(username, password))
            {

            // Use claims or identity rather than raw credential strings
            var claims = new[]
            {
                new Claim(ClaimTypes.Name, username)
            };
            var identity = new ClaimsIdentity(claims, "Basic");
            var principal = new ClaimsPrincipal(identity);
            HttpContext.User = principal;

            return Ok(new { Message = $"Welcome {username}" });
        }
        return Unauthorized();
    }

    private bool IsValidUser(string user, string pass)
    {
        // Implement proper password hashing and user lookup
        return user == "alice" && pass == "correcthorsebatterystaple";
    }
}

Additional measures include enforcing HTTPS to prevent credential interception, setting strict Content-Security-Policy headers to limit template execution contexts, and applying input validation libraries such as FluentValidation. middleBrick’s Pro plan supports continuous monitoring for such misconfigurations, integrating with CI/CD pipelines via the GitHub Action to fail builds if risky patterns are detected in staging environments.

For development teams, the CLI tool (middlebrick scan <url>) can be incorporated into local workflows to catch unsafe credential handling before deployment. The MCP Server allows AI coding assistants to trigger scans directly, promoting secure coding practices during implementation.