Heap Overflow in Aspnet with Hmac Signatures

A heap-based buffer overflow in an ASP.NET application that uses HMAC signatures can occur when unchecked input sizes are used in memory operations while cryptographic verification is performed on attacker-controlled data. Although .NET manages memory and provides bounds-checked primitives, unsafe code paths, interoperabilty (P/Invoke), or misuse of arrays and streams can introduce vulnerabilities.

Consider a scenario where an API accepts a message and an HMAC (Hash-based Message Authentication Code) signature, then processes the payload before verification. If the application deserializes or copies data into fixed-size buffers on the managed heap without validating length, an oversized payload can corrupt adjacent memory. This is especially risky when native interop is used, for example via System.Runtime.InteropServices.Marshal or custom C/C++ components exposed to ASP.NET.

HMAC verification itself does not introduce a heap overflow; it is the surrounding handling of the data that does. For instance, reading a large request body into a byte array sized by a value from the message, then computing HMAC over it, may trigger an out-of-memory or buffer corruption if the size is untrusted. Attackers can send crafted payloads that cause the runtime to allocate or copy data beyond intended boundaries, leading to crashes or potentially enabling code execution when unsafe blocks or native libraries are involved.

In the context of middleBrick’s checks, this pattern falls under Input Validation and Unsafe Consumption. The scanner looks for missing length checks before HMAC computation and unsafe memory practices. Because ASP.NET can host both managed and native components, the risk is elevated when the server processes untrusted input used to size buffers or streams prior to signature verification.

Real-world examples include CVE-2018-18845 (out-of-bounds read in JSON parsing affecting .NET-based services) and issues arising from improper use of ArraySegment<T> or MemoryStream with attacker-controlled length values. When HMAC verification is performed after unsafe deserialization, the combination increases the likelihood of accepting maliciously sized inputs that stress the heap.

Remediation focuses on validating input sizes before processing, using managed APIs that enforce bounds, and avoiding unsafe constructs unless strictly necessary and carefully audited. For HMAC-based authentication in ASP.NET, always verify length constraints and use cryptographic APIs that do not expose raw buffer manipulation.

Below are concrete, safe examples for ASP.NET Core using HMAC-SHA256 with proper input validation.

Example 1: Validated HMAC verification with length checks

using System;using System.Security.Cryptography;using System.Text;using Microsoft.AspNetCore.Mvc;
[ApiController]
[Route("api/[controller]")]
public class SignatureController : ControllerBase
{
    private const int MaxPayloadBytes = 1024 * 1024; // 1 MB limit
    private readonly byte[] _secretKey;

    public SignatureController()
    {
        // In practice, load key securely, e.g., from configuration or key vault
        _secretKey = Encoding.UTF8.GetBytes("super-secret-key-with-sufficient-length-32bytes!!");
    }

    [HttpPost("verify")]
    public IActionResult Verify([FromBody] MessageWithSignature model)
    {
        if (model?.Payload == null)
            return BadRequest("Payload is required.");

        // Validate payload size before any processing
        if (model.Payload.Length > MaxPayloadBytes)
            return BadRequest("Payload exceeds maximum allowed size.");

        using var hmac = new HMACSHA256(_secretKey);
        byte[] computedHash = hmac.ComputeHash(Encoding.UTF8.GetBytes(model.Payload));
        string computedSignature = Convert.ToBase64String(computedHash);

        if (!CryptographicOperations.FixedTimeEquals(computedHash, Convert.FromBase64String(model.Signature)))
        {
            return Unauthorized("Invalid signature.");
        }

        // Safe to process payload
        return Ok("Signature valid.");
    }
}

public class MessageWithSignature
{
    public string Payload { get; set; }
    public string Signature { get; set; }
}

Example 2: Using ArraySegment with bounds validation

using System;
using System.Security.Cryptography;
using System.Text;

public static class HmacValidator
{
    public static bool Validate(ArraySegment<byte> data, string signatureBase64, byte[] key)
    {
        // Ensure the segment is within bounds and size-limited
        if (data.Count <= 0 || data.Count > 1024 * 1024)
            return false;

        using var hmac = new HMACSHA256(key);
        byte[] hash = hmac.ComputeHash(data.Array, data.Offset, data.Count);
        string computed = Convert.ToBase64String(hash);
        return CryptographicOperations.FixedTimeEquals(hash, Convert.FromBase64String(signatureBase64));
    }
}

Key practices: enforce maximum sizes, prefer FixedTimeEquals to avoid timing attacks, avoid unsafe blocks unless necessary, and use managed collections instead of raw pointers. If native interop is required, validate buffer lengths rigorously before passing to unmanaged code.