Timing Attack

Also Known As

TL;DR

Explanation

A timing attack exploits the fact that string comparison operators (===, strcmp) return early on the first mismatched character, so comparing a correct token takes slightly longer than an incorrect one. By making thousands of requests and measuring response times, an attacker can determine the correct token one character at a time. hash_equals() prevents this by always taking the same time regardless of where the strings differ. Always use it when comparing tokens, HMAC signatures, and CSRF values.

How It's Exploited

Common Misconception

Why It Matters

Common Mistakes

• Using === to compare password hashes, HMACs, or tokens — exits early on first byte mismatch.
• Using strcmp() which also short-circuits and has additional issues with null bytes.
• Not using hash_equals() in PHP for all secret comparisons.
• Applying timing-safe comparison only to the final comparison but not to intermediate validation steps.

Avoid When

• Never use === or == to compare HMAC signatures, API tokens, or password hashes — both short-circuit on the first differing byte.
• Do not compare secrets with strcmp() or strncmp() — they are not constant-time.

When To Use

• Use hash_equals() for all security-sensitive string comparisons — it runs in constant time regardless of where strings differ.
• Use password_verify() for password checks — it uses constant-time comparison internally.

Code Examples

// === exits on first mismatch — reveals partial token via timing
if (\$_GET['token'] === \$secretToken) { grantAccess(); }

// hash_equals() — constant-time comparison regardless of where mismatch occurs
if (hash_equals(\$secretToken, \$_GET['token'] ?? '')) {
    grantAccess();
}

// For password verification — always use password_verify (also constant-time)
if (password_verify(\$input, \$storedHash)) { /* authenticated */ }

// Timing-safe HMAC comparison for API signatures
\$expected = hash_hmac('sha256', \$requestBody, \$secret);
\$provided = \$_SERVER['HTTP_X_SIGNATURE'] ?? '';
if (!hash_equals(\$expected, \$provided)) { abort(401); }

References

• ↗ https://cwe.mitre.org/data/definitions/208.html
• ↗ https://www.php.net/manual/en/function.hash-equals.php

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