Integer Overflow in Aspnet with Basic Auth

In ASP.NET applications, an integer overflow can occur when arithmetic on user-supplied numeric values produces a result larger than the maximum value representable by the data type (for example, exceeding int.MaxValue or uint.MaxValue). When Basic Authentication is used, credentials are typically transmitted in the Authorization header as a base64-encoded string of username:password. Although the header itself is a string, developers sometimes parse embedded numeric metadata (such as iteration counts, timestamps, or sequence numbers) from credentials or from derived tokens, and those values may be deserialized and used in arithmetic without validation.

Consider an endpoint that accepts a custom header like X-Batch-Size or a JSON body field representing a count of operations to perform. If the endpoint uses Basic Auth to identify a tenant or a user, an attacker may control both the auth context and the numeric input. An attacker could send a large integer (for example, 999999999999) that overflows a 32-bit integer when combined with other values (such as in a loop counter, buffer size calculation, or array index computation). This can lead to unexpected behavior such as wrapping to a small positive number, which may bypass limits, trigger out-of-bounds reads/writes, or cause logic flaws that change access control checks, potentially allowing privilege escalation or unauthorized data access.

Because the scan is unauthenticated, middleBrick’s checks for input validation and property authorization can flag risky patterns where large numeric inputs are accepted in paths influenced by authentication context. For example, an OpenAPI spec may define a query parameter limit as type integer without explicit maximum constraints, and runtime probes may observe that the endpoint behaves differently when authentication headers are combined with large limit values. This combination highlights the importance of validating numeric fields independently of authentication mechanisms and of ensuring that arithmetic operations use safe libraries or checked contexts to prevent overflow.

Remediation focuses on avoiding integer overflow in any numeric processing that depends on values influenced by authentication, and on correctly handling Basic Auth credentials without introducing new risks. Use explicit range checks, prefer 64-bit types where appropriate, and avoid arithmetic on user-controlled numbers without validation.

Example: Safe parsing and arithmetic with Basic Auth in ASP.NET Core

using System;
using System.IdentityModel.Tokens.Jwt;
using System.Security.Claims;
using System.Text.Encodings.Web;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Authentication;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;

public class BasicAuthOptions : AuthenticationSchemeOptions { }

public class BasicAuthHandler : AuthenticationHandler<BasicAuthOptions>
{
    public BasicAuthHandler(
        IOptionsMonitor<BasicAuthOptions> options,
        ILoggerFactory logger,
        UrlEncoder encoder,
        ISystemClock clock) : base(options, logger, encoder, clock) { }

    protected override async Task<AuthenticateResult> HandleAuthenticateAsync()
    {
        if (!Request.Headers.ContainsKey("Authorization"))
        {
            return AuthenticateResult.NoResult();
        }

        var authHeader = Request.Headers["Authorization"].ToString();
        if (!authHeader.StartsWith("Basic ", StringComparison.OrdinalIgnoreCase))
        {
            return AuthenticateResult.NoResult();
        }

        var token = authHeader.Substring("Basic ".Length).Trim();
        var credentialBytes = Convert.FromBase64String(token);
        var credentials = System.Text.Encoding.UTF8.GetString(credentialBytes).Split(':', 2);
        var username = credentials[0];
        var password = credentials.Length > 1 ? credentials[1] : string.Empty;

        // Validate username/password against your user store here.
        // Avoid using numeric values derived from user-controlled strings in arithmetic without validation.
        // Example of safe usage: use parsed numeric input separately with explicit range checks.

        var identity = new ClaimsIdentity(new[] { new Claim(ClaimTypes.Name, username) }, "Basic");
        var principal = new ClaimsPrincipal(identity);
        var ticket = new AuthenticationTicket(principal, Scheme.Name);
        return AuthenticateResult.Success(ticket);
    }
}

When you must use numeric parameters influenced by authentication (e.g., tenant-specific limits), validate and sanitize them explicitly:

// Example: safe limit parsing with overflow checks
if (!int.TryParse(Request.Query["limit"], out int limit))
{
    return BadRequest("limit must be an integer");
}

// Use checked context to catch overflow during arithmetic
checked
{
    try
    {
        int adjusted = limit + 10; // example arithmetic
        if (adjusted < 0) // overflow would set adjusted negative on unchecked
        {
            return StatusCode(500, "Arithmetic overflow detected.");
        }
        // proceed with safe value
    }
    catch (OverflowException)
    {
        return StatusCode(500, "Arithmetic overflow detected.");
    }
}

Additionally, avoid constructing sizes or counts by concatenating user input with internal values. Use constants or server-side configuration for thresholds, and ensure that any iteration or buffer allocation uses values that are verified to stay within type bounds.