Heap Overflow in Aspnet with Basic Auth

A heap-based buffer overflow in an ASP.NET application that uses HTTP Basic Authentication can occur when untrusted input is copied into fixed-size buffers on the managed heap without proper length validation. Although .NET runtime features such as the garbage collector and bounds-checked arrays reduce the risk of classic stack-based overflows, attackers can still trigger out-of-memory conditions or process instability by sending extremely large or malformed credentials.

With Basic Authentication, credentials are transmitted in the Authorization header as base64-encoded username:password. If the server decodes this header into a fixed-size buffer (for example using a byte[] of predetermined length) or performs string manipulation without validating the decoded length, an oversized payload can lead to excessive memory allocations, fragmentation, or exceptions that may be observable as denial-of-service or as a precursor to more severe memory corruption in unmanaged interop code. In a black-box scan, this pattern may be detected as an input validation failure when the API responds inconsistently to large or unusual Basic Auth credentials.

ASP.NET endpoints that accept custom headers or route data alongside Basic Auth are especially at risk if they pass these values to unsafe native components via P/Invoke or COM interop. For instance, if the application forwards the decoded credentials to native image processing or compression libraries, a heap overflow in the unmanaged layer can be triggered. Even when no direct interop exists, an attacker sending very long base64 strings can probe rate limiting and logging behavior, potentially amplifying the impact through resource exhaustion. Security checks such as Input Validation and Unsafe Consumption in middleBrick scans for these patterns by correlating runtime behavior with OpenAPI specifications, highlighting endpoints where oversized Basic Auth data reaches unsafe code paths.

Because Basic Authentication sends credentials with every request, an exploit can be repeatedly triggered without needing prior authentication. This makes the attack surface persistent across sessions. Findings from middleBrick’s 12 parallel checks will surface input validation anomalies and point to the related authentication context, enabling teams to prioritize remediation based on observable risk rather than assumed obscurity.

Remediation focuses on validating the length and format of Basic Auth credentials before decoding and avoiding fixed-size buffers. Always prefer framework-managed types such as string and ReadOnlyMemory<char> and reject credentials that exceed a reasonable threshold.

Example 1: Safe Basic Auth parsing with length validation

using Microsoft.AspNetCore.Http;
using System;
using System.Text;
using System.Threading.Tasks;

public class BasicAuthMiddleware
{
    private const int MaxCredentialLength = 1024; // reasonable upper bound
    private readonly RequestDelegate _next;

    public BasicAuthMiddleware(RequestDelegate next) => _next = next;

    public async Task Invoke(HttpContext context)
    {
        if (context.Request.Headers.TryGetValue("Authorization", out var authHeader))
        {
            var header = authHeader.ToString();
            if (header.StartsWith("Basic ", StringComparison.OrdinalIgnoreCase))
            {
                var token = header.Substring("Basic ".Length).Trim();
                // Reject if header is suspiciously long before base64 decode
                if (token.Length > MaxCredentialLength)
                {
                    context.Response.StatusCode = 400;
                    await context.Response.WriteAsync("Bad Request: Credential too long");
                    return;
                }

                try
                {
                    var data = Convert.FromBase64String(token);
                    var credentials = Encoding.UTF8.GetString(data);
                    var separator = credentials.IndexOf(':');
                    if (separator < 0) { context.Response.StatusCode = 400; return; }

                    var username = credentials.Substring(0, separator);
                    var password = credentials.Substring(separator + 1);

                    // Apply additional length checks on username/password
                    if (username.Length > 256 || password.Length > 256)
                    {
                        context.Response.StatusCode = 400;
                        await context.Response.WriteAsync("Bad Request: Username or password too long");
                        return;
                    }

                    // Perform authentication (example placeholder)
                    if (IsValidUser(username, password))
                    {
                        context.Items["User"] = username;
                        await _next(context);
                        return;
                    }
                }
                catch (FormatException)
                {
                    // Invalid base64
                }
            }
        }

        context.Response.StatusCode = 401;
        context.Response.Headers["WWW-Authenticate"] = "Basic realm=\"Restricted\"";
    }

    private static bool IsValidUser(string username, string password) => /* your logic */ false;
}

Example 2: Rejecting oversized content in controller action

using Microsoft.AspNetCore.Mvc;
using System;

[ApiController]
[Route("api/[controller]")]
public class ProfileController : ControllerBase
{
    [HttpGet]
    public IActionResult Get()
    {
        if (!Request.Headers.TryGetValue("Authorization", out var authHeader))
            return Unauthorized();

        var token = authHeader.ToString();
        if (token.StartsWith("Basic ", StringComparison.OrdinalIgnoreCase))
        {
            var credentials = token.Substring("Basic ".Length);
            if (credentials.Length > 2048)
                return BadRequest("Authorization header too large");

            try
            {
                var bytes = Convert.FromBase64String(credentials);
                var decoded = Encoding.UTF8.GetString(bytes);
                // Further validation and authentication...
            }
            catch
            {
                return BadRequest("Invalid credentials");
            }
        }

        return Unauthorized();
    }
}

Additionally, consider avoiding Basic Auth where possible in favor of token-based mechanisms with short lifetimes. If you must use Basic Auth, enforce HTTPS, apply strict length limits, and monitor for repeated malformed requests that could indicate probing. middleBrick scans can validate these mitigations by checking that endpoints with authentication include appropriate security schemes in the OpenAPI spec and that runtime tests do not trigger input validation warnings.