How severe is CVE-2021-40438?

CVE-2021-40438 has a CVSS score of 9/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-40438?

Affected package information for CVE-2021-40438 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2021-40438?

No fixed version has been published for CVE-2021-40438 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2021-40438 actively exploited in the wild?

Yes. There are 12 known exploit references for CVE-2021-40438, including 9 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-40438?

CVE-2021-40438 has an EPSS (Exploit Prediction Scoring System) score of 94.4%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

Is CVE-2021-40438 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2021-40438 (Apache HTTP Server-Side Request Forgery (SSRF)) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2021-12-01. CISA required all US federal civilian agencies to remediate this vulnerability by 2021-12-15. The required action was: Apply updates per vendor instructions. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

Is there exploit tooling available for CVE-2021-40438?

A Nuclei detection template exists for CVE-2021-40438, meaning automated scanners can fingerprint it. Public exploit tooling significantly increases real-world risk — prioritize patching and ensure detection coverage. Exploit tooling should only ever be used in isolated, authorized testing environments.

When was CVE-2021-40438 published, and has it been updated?

CVE-2021-40438 was published on September 16, 2021 and was last updated on April 16, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CISA KEV·Added 2021-12-01 — agencies required to remediate by 2021-12-15

CVE-2021-40438

CRITICAL

Also known asBIT-apache-2021-40438

Published

Sep 16, 2021

Updated

Apr 16, 2026

Affected

0 pkgs

Patched

None yet

Exploits

12 known

EPSS Exploitation Probability

via FIRST.org ↗

94.4%probability of exploitation in next 30 days

Very High Risk100th percentile0.00%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Description

A crafted request uri-path can cause mod_proxy to forward the request to an origin server choosen by the remote user. This issue affects Apache HTTP Server 2.4.48 and earlier.

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.