In a new post to his site Mattias Geniar looks at an old security issue in PHP, opcache information leakage and how to mitigating the issue.
A very old security vulnerability has been fixed in PHP regarding the way it handles its OPCaches in environments where a single master process shares multiple PHP-FPM pools. This is the most common way to run PHP nowadays and might affect you, too.
He starts by talking about the vulnerability itself, that the PHP process doesn't validate the userid when fetching cached bytecode. This could result in information from other operations/scripts being exposed to other processes in a PHP-FPM pool. His solution? Upgrade PHP (the bug is fixed back in PHP 5.6.5) and set a few additional
opcache ini settings to enforce the validation. Besides 5.6.29, it was also corrected in the PHP 7 releases (7.0.14 and 7.1.0). The post then talks about the potential exploit - an indirect local privilege escalation to root where the shared memory is read and access to outside information is possible.