Even at the time of 5.6.40’s release, several known vulnerabilities remained unpatched or were backported incompletely.
Welcome, Maintainer. You are running PHP 5.6.40. This is the final boss of the PHP 5 era. It is the last stable release before the great migration to PHP 7.
If you are reading this, you are likely maintaining a legacy application that cannot be upgraded easily. While 5.6.40 is the most secure version of the PHP 5 branch, it is a castle built on a foundation that is no longer inspected by the kingdom's engineers (the PHP Group). php version 5640 vulnerabilities verified
This guide covers the verified architectural vulnerabilities inherent to the PHP 5.x series and how to defend your fortress.
This is arguably the most dangerous function in PHP 5. The unserialize function takes a stashed string and turns it back into a PHP object. In PHP 5, if a hacker can manipulate that string, they can force your application to instantiate objects that execute malicious code (Object Injection). Even at the time of 5
The Vulnerability:
User input feeds directly into unserialize().
// DANGEROUS
$user_object = unserialize($_COOKIE['user_data']);
The Fix:
Use json_encode and json_decode instead. If you absolutely must use unserialize, use the allowed classes option (though this is less reliable in older PHP versions). Welcome, Maintainer
Verification source: NVD (nvd.nist.gov), PHP ChangeLog for 5.6.40 (php.net/ChangeLog-5.php), and Debian/Red Hat security trackers.
Many developers cling to PHP 5.6.40 because "it works." Here is why that logic fails security verification:
PHP 5.6.40 is significant because it was the last release before the PHP team ceased all active support and security patching for the 5.x branch.
PHP 5.6.40 was released before modern cryptography became the standard. It relies on libraries that are now considered weak.