An issue was discovered in PHP 5.x and 7.x, when the configuration uses
apache2handler/mod_php or php-fpm with OpCache enabled. With 5.x after
5.6.28 or 7.x after 7.0.13, the issue is resolved in a non-default
configuration with the opcache.validate_permission=1 setting. The
vulnerability details are as follows. In PHP SAPIs where PHP interpreters
share a common parent process, Zend OpCache creates a shared memory object
owned by the common parent during initialization. Child PHP processes
inherit the SHM descriptor, using it to cache and retrieve compiled script
bytecode ("opcode" in PHP jargon). Cache keys vary depending on
configuration, but filename is a central key component, and compiled opcode
can generally be run if a script's filename is known or can be guessed.
Many common shared-hosting configurations change EUID in child processes to
enforce privilege separation among hosted users (for example using
mod_ruid2 for the Apache HTTP Server, or php-fpm user settings). In these
scenarios, the default Zend OpCache behavior defeats script file
permissions by sharing a single SHM cache among all child PHP processes.
PHP scripts often contain sensitive information: Think of CMS
configurations where reading or running another user's script usually means
gaining privileges to the CMS database.