Out Of Bounds Write in Aspnet with Hmac Signatures

An Out Of Bounds Write occurs when application logic writes data outside the intended memory boundaries, often corrupting adjacent memory. In ASP.NET applications, combining HMAC-based integrity checks with unsafe buffer handling or unchecked input lengths can create or expose such vulnerabilities.

Consider an API endpoint that accepts a payload and an HMAC signature for integrity verification. If the server computes the HMAC over a fixed-size buffer but does not validate the actual payload length before copying data into that buffer, an attacker can supply an oversized payload. The HMAC verification may still pass if the signature covers only the payload (not the buffer boundaries), but the unchecked copy operation writes past the allocated buffer, leading to memory corruption.

This pattern is risky when developers assume HMAC guarantees message safety and neglect input validation. For example, deserializing an object with an HMAC and then writing its contents into a fixed-size byte array without length checks can result in writes beyond the array’s bounds. Attackers can craft payloads that exploit this to corrupt stack or heap memory, potentially leading to arbitrary code execution or denial of service.

In the context of OWASP API Top 10, this maps closely to the "Broken Function Level Authorization" and "Injection" classes when malformed data triggers unexpected behavior. Because HMACs ensure integrity but not input safety, developers must still enforce strict size and type constraints. middleBrick’s checks for Input Validation and BOLA/IDOR can surface such insecure patterns by correlating runtime behavior with OpenAPI specifications that omit proper schema constraints.

Real-world examples include scenarios where an ASP.NET Core app uses HMACSHA256 to verify a token but then deserializes JSON into a fixed-size structure without validating array lengths. The signature may verify correctly, but the subsequent buffer operations perform an Out Of Bounds Write. middleBrick’s LLM/AI Security checks do not directly test this class of memory safety issue, but its inventory and input validation scans help identify missing length constraints in spec-defined schemas that enable such bugs.

To remediate Out Of Bounds Write risks when using HMAC Signatures in ASP.NET, enforce strict input validation and avoid unsafe buffer operations. Always validate payload sizes before processing and use safe abstractions that respect array bounds.

Below is a secure example in ASP.NET Core that verifies an HMAC signature and safely processes the payload without exposing out-of-bounds writes:

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;
using System.Text.Json;
using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("api/[controller]")]
public class DataController : ControllerBase
{
    private const int MaxPayloadSize = 4096; // Enforce a reasonable upper bound
    private static readonly byte[] SecretKey = Convert.FromBase64String(Environment.GetEnvironmentVariable("HMAC_SECRET") ?? throw new InvalidOperationException("Key missing"));

    [HttpPost("submit")]
    public IActionResult Submit([FromBody] RequestModel request)
    {
        if (request?.Payload == null || request.Payload.Length == 0)
            return BadRequest("Payload is required");

        // Enforce size limit before any processing
        if (request.Payload.Length > MaxPayloadSize)
            return RequestSizeTooLarge();

        // Verify HMAC
        if (!VerifyHmac(request.Payload, request.Signature, SecretKey))
            return Unauthorized("Invalid signature");

        // Safe deserialization with size-aware options
        var options = new JsonSerializerOptions { PropertyNameCaseInsensitive = true };
        try
        {
            var data = JsonSerializer.Deserialize<PayloadData>(Convert.ToBase64String(request.Payload), options);
            // Process data safely; no unchecked buffer copies
            return Ok(new { Message = "Processed", Data = data });
        }
        catch (JsonException)
        {
            return BadRequest("Invalid payload format");
        }
    }

    private static bool VerifyHmac(byte[] payload, string receivedSignature, byte key)
    {
        using var hmac = new HMACSHA256(key);
        byte[] computedHash = hmac.ComputeHash(payload);
        string computedSignature = Convert.ToBase64String(computedHash);
        return CryptographicOperations.FixedTimeEquals(Convert.FromBase64String(receivedSignature), computedHash);
    }

    private static IBadRequestObjectResult RequestSizeTooLarge() => new BadRequestObjectResult("Payload exceeds maximum allowed size");
}

public class RequestModel
{
    public required byte[] Payload { get; set; }
    public required string Signature { get; set; }
}

public class PayloadData
{
    public string Content { get; set; } = string.Empty;
    // Add other fields as needed
}

Key remediation points:

• Enforce a maximum payload size before deserialization or copying.
• Use CryptographicOperations.FixedTimeEquals to prevent timing attacks during HMAC comparison.
• Avoid fixed-size buffers for incoming data; prefer dynamic collections or streams with explicit length checks.
• Validate schema constraints in your OpenAPI spec so tools like middleBrick can detect missing bounds that enable out-of-bounds writes.

For continuous protection, enable the middleBrick Pro plan’s continuous monitoring and CI/CD integration. This ensures that any future changes to payload handling or HMAC usage are automatically scanned against your defined risk thresholds, helping prevent regressions that could reintroduce unsafe memory operations.