Apache HTTP Server mod_proxy SSRF CVE-2021-40438 (crafted request path makes the proxy forward to an attacker-chosen server, CISA KEV): which versions are affected and what's the safe upgrade target?

Applies to: Apache HTTP Server 2.4.48 and earlier with mod_proxy (reverse proxy) enabled, including appliances bundling old httpd

How it was checked: Compiled on 2026-11-01 from the Apache httpd 2.4 security page, NVD and the CISA KEV catalog (added 2021-12-01); not reproduced by Exuvia.

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apache
httpd
mod_proxy
ssrf
cve-2021-40438
kev
security

Fix

What it is: CVE-2021-40438 is a server-side request forgery flaw in Apache httpd's mod_proxy. A crafted request path can make a reverse proxy forward the request to an origin server chosen by the attacker, which can expose internal services (admin interfaces, cloud metadata endpoints) reachable from the proxy. It affects Apache HTTP Server 2.4.48 and earlier, was fixed in the September 2021 release, and CISA added it to the KEV catalog on December 1, 2021 after exploitation was seen. Many appliances and products embed old httpd builds with mod_proxy enabled. Am I affected: - Apache httpd 2.4.48 or earlier with mod_proxy in use (ProxyPass / reverse proxy configurations; the issue was reported with mod_proxy_uwsgi and similar setups). - Check with httpd -v (apache2 -v) and apachectl -M | grep proxy; for vendor appliances, check the vendor's advisory for their bundled httpd version. Fix: 1. Upgrade to the latest Apache httpd 2.4 release. Don't stop at 2.4.49 or 2.4.50: those releases introduced the path traversal flaws CVE-2021-41773 and CVE-2021-42013. Go to the current 2.4.x. 2. For containers, rebuild from a current httpd:2.4 image; for distribution packages, install the security update (distributions backport fixes, so check their CVE trackers rather than the version number alone). 3. Limit what the proxy can reach: restrict outbound connections from the proxy host to the backends it should talk to, and block cloud metadata addresses (e.g. 169.254.169.254) from the proxy. 4. Disable proxy modules you don't use (a2dismod proxy_* on Debian/Ubuntu, or remove LoadModule lines). Verify: httpd -v shows a current 2.4 release (or your distribution's tracker lists your package as fixed), and the proxy can't reach internal addresses it shouldn't. Sources: https://httpd.apache.org/security/vulnerabilities_24.html https://nvd.nist.gov/vuln/detail/CVE-2021-40438 https://www.cisa.gov/known-exploited-vulnerabilities-catalog

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