Integer Overflow in Express with Basic Auth

An integer overflow in an Express application that uses HTTP Basic Authentication can arise when numeric values derived from authentication or request metadata are handled with fixed-size integer types. For example, if the server parses the Authorization header, decodes the Base64 credential string, and then performs arithmetic on parsed components—such as combining a user identifier with a nonce or a timestamp—an unchecked addition or multiplication may exceed the maximum safe integer value for the language runtime (for instance, exceeding 2^32 - 1 for 32-bit integers or 2^53 - 1 for JavaScript numbers used in Node.js). This overflow can cause the resulting integer to wrap around, producing a small or zero value where a large value was expected.

When combined with Basic Auth, this becomes relevant if the application derives numeric values from the username or password, or uses numeric IDs extracted from the decoded credentials to index arrays, compute memory offsets, or construct resource keys. An attacker may supply specially crafted credentials that, when parsed, produce values which, when combined, overflow integer boundaries. The resulting wrap-around can lead to incorrect type checks, out-of-bounds memory access patterns in downstream systems, or authorization logic that incorrectly evaluates a user’s privileges. Because the authentication step occurs early in the request lifecycle, an overflow in these numeric operations may bypass intended access controls or cause unstable routing or data retrieval, which may be surfaced as 500-series errors or inconsistent behavior detectable through repeated requests with varying credentials.

In the context of middleBrick’s checks, this scenario would be surfaced under the Authentication and BOLA/IDOR categories if the overflow leads to authorization confusion, and under Input Validation if malformed numeric inputs derived from credentials are not properly bounded. The scanner tests unauthenticated attack surfaces, so it can send malformed Basic Auth credentials designed to trigger unusual numeric behavior in the endpoint and observe responses that indicate computation anomalies. Because the vulnerability stems from unsafe arithmetic on values extracted from authentication data rather than the authentication mechanism itself, remediation focuses on validating and sanitizing numeric inputs, using safe integer types, and avoiding arithmetic on user-controlled identifiers without range checks.

To mitigate integer overflow risks while using HTTP Basic Authentication in Express, ensure that any numeric values derived from credentials are validated, bounded, and handled with safe arithmetic. Avoid using parsed numeric components directly in memory offsets, array indices, or cryptographic nonces. Instead, treat values as opaque identifiers or convert them to safe integer representations with explicit range checks.

Example of unsafe code that decodes Basic Auth and uses a numeric user ID directly in arithmetic:

const authHeader = req.headers['authorization'];
if (authHeader && authHeader.startsWith('Basic ')) {
  const base64 = authHeader.split(' ')[1];
  const decoded = Buffer.from(base64, 'base64').toString('utf8');
  const [user, pass] = decoded.split(':');
  // user may contain a numeric string derived from credentials
  const userId = parseInt(user, 10);
  // Unsafe arithmetic: overflow risk if userId is large
  const combined = userId + 10000000000; // may overflow
  const buffer = new ArrayBuffer(combined); // could allocate incorrectly
  res.send({ combined });
}

Safer approach with validation and bounded arithmetic:

const authHeader = req.headers['authorization'];
if (authHeader && authHeader.startsWith('Basic ')) {
  const base64 = authHeader.split(' ')[1];
  const decoded = Buffer.from(base64, 'base64').toString('utf8');
  const [user, pass] = decoded.split(':');
  const userId = parseInt(user, 10);
  // Validate input is a finite integer within expected range
  if (!Number.isFinite(userId) || userId < 1 || userId > 1000000) {
    return res.status(400).send({ error: 'Invalid user identifier' });
  }
  // Use safe arithmetic with BigInt if large values are required
  const combined = BigInt(userId) + 10000000000n;
  // Ensure combined fits within safe bounds before using
  if (combined > Number.MAX_SAFE_INTEGER) {
    return res.status(400).send({ error: 'Combined value out of range' });
  }
  const buffer = new ArrayBuffer(Number(combined));
  res.send({ combined: combined.toString() });
}

Additional measures include rate limiting authentication endpoints, avoiding direct use of decoded credentials in low-level operations, and applying the principle of least privilege to endpoints that rely on Basic Auth. middleBrick’s scans can validate these practices by checking for missing input validation and improper handling of numeric values derived from authentication data.