Zend Engine

Mitigating PHP2Shell RCE Vulnerabilities CVE-2026-109618 and CVE-2026-109619 on Legacy PHP 5.6 Stacks with Varnish, Docker and Apache mod_php

Two critical remote code execution vulnerabilities dubbed PHP2Shell (CVE-2026-109618 and CVE-2026-109619, CVSS 9.2) affect PHP before any application code executes, targeting the Zend Engine's longjmp() mechanism during OOM conditions and use-after-free issues with session upload progress. Official patches exist only for supported branches 8.2.35 and newer, leaving PHP 5.6 installations exposed, especially those using Apache with mod_php. An administrator of a high-traffic legacy enterprise application running PHP 5.6 inside Docker containers behind Varnish implemented a four-layer defense without rewriting the monolith. The approach disables session.upload_progress.enabled and sets memory_limit to -1 inside PHP, enforces strict cgroups memory limits and MaxConnectionsPerChild 1 in Apache, adds request-size and cookie-based blocks in Varnish VCL, and forces Connection: close on synthetic error responses. Testing revealed HTTP keep-alive drain issues and differing behavior between direct Apache requests and Varnish-routed traffic. The layered controls successfully neutralize both attack vectors while preserving production stability.