Type Coercion in Authentication Checks

TL;DR

Explanation

PHP's == operator uses type juggling. Classic attacks: (1) Magic hash collision: two MD5 hashes starting with 0e (like 0e462097431906509019562988736854) are both '0' in scientific notation, making them == equal. (2) 0 == 'anything' because PHP coerces 'anything' to 0. (3) True == 'anything'. The fix: always use === (strict equality) for password, token, and hash comparisons. Use hash_equals() for timing-safe string comparison. PHP's password_verify() and hash_equals() are safe. Never compare hashes with == or strcmp().

Common Misconception

Why It Matters

Common Mistakes

• Using == to compare password hashes — magic hash bypass.
• Using strcmp() for security comparisons — timing attack + type coercion.
• Not using hash_equals() for HMAC verification.

Code Examples

// Vulnerable: 0 == 'admin' is TRUE in PHP
if ($token == $expectedToken) { /* bypassable */ }

// Vulnerable: magic hash
$hash = md5($password); // e.g. '0e123...'
if ($hash == $storedHash) { /* 0e... == 0e... */ }

// Always strict comparison:
if ($token === $expectedToken) { /* correct */ }

// For timing-safe hash comparison:
if (hash_equals($storedHash, $computedHash)) { /* safe */ }

// For passwords, always use password_verify:
if (password_verify($input, $storedHash)) { /* safe */ }

References

• ↗ https://www.php.net/manual/en/function.hash-equals.php
• ↗ https://owasp.org/www-community/vulnerabilities/PHP_Object_Injection

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