CVE-2019-0211

What is Apache HTTP Server?

Apache HTTP Server (httpd) is one of the most widely deployed web server platforms in the world, running everything from small personal sites to large enterprise infrastructure. Its Multi-Processing Modules (MPMs) — prefork, worker, and event — control how the server manages concurrent connections using separate processes or threads, typically starting as root to bind privileged ports and then dropping privileges in child worker processes that handle actual client requests, including any scripting interpreters (like PHP or CGI) running in-process.

Overview

CVE-2019-0211 is a local privilege escalation vulnerability affecting Apache HTTP Server when running under the prefork, worker, or event MPMs. Normally, the unprivileged child processes or threads that handle requests — including any embedded scripting interpreter executing web application code — are supposed to be fully isolated from the privileges of the parent process, which typically still runs as root. This vulnerability broke that isolation: a local attacker capable of running code in a child worker process (for example, through a web application vulnerability that allows arbitrary PHP/CGI script execution) could manipulate Apache's internal "scoreboard" — a shared memory structure used for inter-process status tracking — to escalate to the privileges of the root parent process.

Technical Details

The flaw (CWE-416, Use After Free — the scoreboard manipulation exploited a use-after-free/type-confusion issue in how child processes could influence the shared scoreboard structure) allowed a lower-privileged worker process to corrupt scoreboard data such that the privileged parent process would execute attacker-controlled code or configuration on its behalf. Because Apache's threat model assumed the parent-child privilege boundary was solid, any application capable of arbitrary code execution within a worker process (a "post-compromise" scenario for a hosted web application) could pivot from that limited foothold to root on the entire server — turning what should have been a contained web-application compromise into full host takeover.

Discovery

Publicly disclosed by security researcher Charles Fol of Ambionics Security, who published detailed technical research demonstrating the scoreboard manipulation technique used to escalate privileges from an Apache child process to root.

Exploitation Context

CISA's KEV catalog listing confirms this vulnerability has been exploited in the wild. This bug is particularly significant in shared-hosting environments, where multiple customers' web applications run on the same Apache server: a vulnerability in any one hosted application that permits code execution (e.g., an insecure PHP script) could be escalated via CVE-2019-0211 into full root compromise of the shared server, impacting every other tenant hosted there.

Remediation

1. Upgrade to Apache HTTP Server 2.4.39 or later, which contains the fix.
2. In shared-hosting environments, treat this as especially urgent, since any compromised hosted application could otherwise be leveraged for full server takeover.
3. Harden web applications running under Apache to minimize the risk of an initial code-execution foothold in a worker process (e.g., via CGI/PHP misconfigurations).
4. Apply additional isolation technologies (containers, chroot, SELinux/AppArmor policies) to limit the blast radius even if the underlying Apache bug were somehow re-triggered by a related issue.
5. Review server logs and process history for anomalous root-level activity correlated with web application requests predating the patch.