GHSA-jc38-x7x8-2xc8: PHP JWT Framework: JWSVerifier uses algorithm from unprotected header, enabling algorithm confusion attacks

JWSVerifier::getAlgorithm() in src/Library/Signature/JWSVerifier.php (line 144) merges protected and unprotected headers using PHP’s spread operator:

$completeHeader = [...$signature->getProtectedHeader(), ...$signature->getHeader()];

In PHP, when spreading arrays with duplicate string keys, the last array’s values take precedence. Since the unprotected header (getHeader()) is spread second, an attacker can override the integrity-protected alg parameter by placing a different value in the unprotected header.

This creates a Time-of-Check/Time-of-Use (TOCTOU) vulnerability:

1. HeaderCheckerManager validates alg from the protected header
2. JWSVerifier uses alg from the unprotected header for actual verification

The same issue exists in JWEDecrypter.php (lines 120-124) where array_merge() exhibits the same last-wins behavior for alg and enc.