Integer Overflow in Aspnet with Bearer Tokens

In ASP.NET APIs that rely on Bearer Token authentication, an integer overflow can occur when numeric claims or token-related metadata (such as expiration timestamps or issued‑at values) are parsed into fixed‑size integer types without proper validation. This is particularly relevant when the token payload includes numeric fields like exp, nbf, or custom numeric scopes that are deserialized into int or long variables.

An attacker can supply a token with a crafted numeric value that appears valid at first glance but is close to the limits of the target type. For example, setting exp to 2147483647 (the maximum value for a signed 32‑bit integer) or slightly above it can cause an overflow when the value is added to an offset or compared against current time. If the application performs arithmetic such as DateTimeOffset.FromUnixTimeSeconds(exp).ToUnixTimeSeconds() or computes remaining lifetime as exp - currentEpoch, the overflow may produce a negative number or an unexpectedly large value, bypassing intended validity checks.

This overflow can lead to logic flaws such as accepting expired tokens as valid, granting extended sessions, or triggering incorrect authorization decisions. Because Bearer Token validation often occurs early in the request pipeline, the compromised check may affect multiple endpoints, especially when centralized authentication middleware is used. The vulnerability aligns with authorization flaws such as BOLA/IDOR when token claims are trusted without range checks.

Consider an endpoint that parses a custom session_duration claim from a Bearer Token and uses it to extend a cache entry:

var token = HttpContext.User.FindFirst("session_duration")?.Value;
if (long.TryParse(token, out long duration)) {
    cache.Set(entryKey, data, TimeSpan.FromSeconds(duration));
}

If token contains a value like 9223372036854775807, the TimeSpan.FromSeconds conversion can overflow, resulting in an invalid or unintended cache lifetime. An attacker could leverage this to maintain access beyond policy or to cause denial of service through resource exhaustion.

The 12 parallel security checks in middleBrick exercise such scenarios by analyzing OpenAPI/Swagger specifications with full $ref resolution and correlating them with runtime behavior. This helps detect unsafe numeric handling in Bearer Token flows, including missing range validation and unsafe consumption patterns that may amplify integer overflow risks.

Remediation focuses on validating and constraining numeric values derived from Bearer Token claims before using them in arithmetic or time‑based operations. Always treat token claims as untrusted input and apply explicit range checks and type conversions that guard against overflow.

Instead of using long.TryParse directly, validate the value against realistic bounds for your application. For example, if a session duration claim should not exceed one week (604800 seconds), enforce this limit explicitly:

var token = HttpContext.User.FindFirst("session_duration")?.Value;
if (long.TryParse(token, out long duration)) {
    const long MaxDurationSeconds = 604800; // 7 days
    if (duration > 0 && duration <= MaxDurationSeconds) {
        cache.Set(entryKey, data, TimeSpan.FromSeconds(duration));
    } else {
        // Reject invalid duration
        throw new SecurityException("Invalid session duration");
    }
}

When working with standard JWT numeric claims like exp and nbf, use the built-in token validation mechanisms provided by ASP.NET Core, which handle overflow and range checks internally:

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options => {
        options.TokenValidationParameters = new TokenValidationParameters {
            ValidateIssuer = true,
            ValidateAudience = true,
            ValidateLifetime = true,
            ValidateIssuerSigningKey = true,
            ValidIssuer = "https://auth.example.com",
            ValidAudience = "https://api.example.com",
            IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(Environment.GetEnvironmentVariable("JWT_SECRET")))
        };
    });

If custom numeric claims must be processed, perform arithmetic using checked contexts or use BigInteger for unbounded values, then compare against safe thresholds:

using System.Numerics;

var expClaim = HttpContext.User.FindFirst("exp")?.Value;
if (long.TryParse(expClaim, out long expTimestamp)) {
    BigInteger expiration = new BigInteger(expTimestamp);
    BigInteger current = DateTimeOffset.UtcNow.ToUnixTimeSeconds();
    if (expiration > current && (expiration - current) <= 86400) { // within 24 hours
        // Apply limited extension safely
    }
}

Additionally, enforce rate limiting and monitor for anomalous token usage patterns through the dashboard and alerts. The Pro plan’s continuous monitoring and configurable scanning schedule can help detect abnormal claim behavior across deployments, while the GitHub Action can enforce security thresholds in CI/CD pipelines.

For development teams leveraging AI coding assistants, the MCP Server allows scanning API endpoints directly from the IDE, ensuring that token handling logic is reviewed early. Combine this with the Web Dashboard to track security scores over time and integrate findings into compliance workflows mapped to OWASP API Top 10 and SOC2 controls.