Heap Overflow in Aspnet with Bearer Tokens

A heap-based buffer overflow in an ASP.NET application typically stems from unsafe handling of byte or character buffers combined with unchecked input length. When bearer tokens are involved, the token string is often copied into fixed-size buffers during parsing, validation, or custom authentication logic. If the token length is not validated and a maliciously long token is supplied, writing beyond the allocated buffer can corrupt adjacent memory. This can lead to arbitrary code execution or process crashes, and in the context of API security scanning, such a vulnerability would be surfaced as a high-severity finding under the BFLA/Privilege Escalation and Input Validation checks.

In practice, an attacker might send an HTTP request with an excessively long Authorization header containing a bearer token. For example:

GET /api/resource HTTP/1.1
Host: example.com
Authorization: Bearer AAAA... (very long string)

During token processing, if the application uses fixed buffers (e.g., stack-allocated character arrays) without proper length checks, the copy operation can overflow. Because ASP.NET may host the runtime on platforms where such overflows are exploitable, this becomes a severe concern. The scanner’s input validation and BFLA checks are designed to detect endpoints that accept large, uncontrolled inputs without adequate safeguards, which includes bearer token handling. Findings include evidence that long tokens can reach parsing routines and the absence of mitigations such as length validation or bounded buffers.

Additionally, if the token is base64-encoded or contains nested structures that are deserialized, heap overflows may arise during deserialization logic that does not bound allocations. OWASP API Top 10 items like Improper Input Validation and Security Misconfiguration intersect here. The scanner’s runtime tests include sending oversized tokens to observe whether parsing or authorization logic triggers faults, and results appear in the per-category breakdown with remediation steps that emphasize input validation and secure coding practices.

Remediation focuses on validating token length, avoiding fixed-size buffers, and using safe string handling APIs. Below are concrete examples for an ASP.NET Core application using Bearer tokens with JWT validation and a custom authorization handler.

1. Configure JWT Bearer authentication with length and format checks

using Microsoft.AspNetCore.Authentication.JwtBearer;
using Microsoft.IdentityModel.Tokens;
using System.Text;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidIssuer = "https://auth.example.com",
            ValidateAudience = true,
            ValidAudience = "api.example.com",
            ValidateLifetime = true,
            ValidateIssuerSigningKey = true,
            IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("very_long_secret_at_least_32_bytes_long!")),
            // Limit token size to mitigate heap overflow risks
            RequireSignedTokens = true
        };
        // Optional: enforce maximum token size at the handler level
        options.Events = new JwtBearerEvents
        {
            OnMessageReceived = context =>
            {
                const int MaxTokenLength = 4096;
                if (context.Request.Headers.TryGetValue("Authorization", out var values))
                {
                    var token = values.ToString()?.Replace("Bearer ", "", StringComparison.OrdinalIgnoreCase);
                    if (!string.IsNullOrEmpty(token) && token.Length > MaxTokenLength)
                    {
                        context.Fail("Token too long");
                        return;;
                    }
                }
                return Task.CompletedTask;
            }
        };
    });

builder.Services.AddAuthorization();
var app = builder.Build();
app.UseAuthentication();
app.UseAuthorization();
app.MapGet("/api/resource", () => "Secure data");
app.Run();

2. Custom policy with explicit length validation in authorization handlers

using System.Security.Claims;
using Microsoft.AspNetCore.Authorization;
using System.Threading.Tasks;

public class TokenLengthRequirement : IAuthorizationRequirement
{
    public int MaxTokenLength { get; }
    public TokenLengthRequirement(int maxTokenLength) => MaxTokenLength = maxTokenLength;
}

public class TokenLengthHandler : AuthorizationHandler<TokenLengthRequirement>
{
    protected override Task HandleRequirementAsync(AuthorizationHandlerContext context, TokenLengthRequirement requirement)
    {
        if (context.User.FindFirst("token_length")?.Value is string lengthStr && int.TryParse(lengthStr, out int length))
        {
            if (length <= requirement.MaxTokenLength)
            {
                context.Succeed(requirement);
            }
        }
        return Task.CompletedTask;
    }
}

// In Program.cs or Startup:
builder.Services.AddAuthorization(options =>
{
    options.AddPolicy("TokenLengthPolicy", policy =>
        policy.Requirements.Add(new TokenLengthRequirement(4096));
});
builder.Services.AddSingleton<IAuthorizationHandler, TokenLengthHandler>();

// Example endpoint enforcing the policy:
app.MapGet("/api/secure", () => "Authorized and token length checked")
    .RequireAuthorization("TokenLengthPolicy");
app.Run();

3. Avoid unsafe buffers; prefer string and span APIs

Never use fixed-size buffers for token storage. Instead, use string and ReadOnlySpan<char> to avoid heap corruption. For example, if you must inspect the token, do so with safe slicing:

string token = "your.jwt.token.here";
if (token.Length <= 4096)
{
    ReadOnlySpan<char> slice = token.AsSpan(0, Math.Min(token.Length, 100));
    // Process slice safely
}
else
{
    // Reject token