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 release is recorded for CVE-2021-40438 yet. Track the upstream advisory, and apply the exposure-reduction steps below in the meantime.

How do I detect CVE-2021-40438 in my code and dependencies?

Identify every host running the affected component and compare the installed build against the fixed version below — for source-built or distro-packaged software the version string, not a lockfile, is the source of truth (`dpkg -l`, `rpm -q`, or the binary's own `--version`).

How do I mitigate CVE-2021-40438 if there is no patch (or I can't update yet)?

Restrict outbound requests from the affected component to an allowlist of hosts, block access to link-local and internal address ranges at the network layer, and require authentication on internal services so a forged request cannot reach them unauthenticated.

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

Yes. There are 13 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 100.0%, 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. This CVE has been used in known ransomware campaigns. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

What type of vulnerability is CVE-2021-40438?

CVE-2021-40438 is classified as Server-Side Request Forgery (SSRF) (CWE-918). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

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 September 23, 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 · ACTIVELY EXPLOITED

CRITICAL severity

CVE-2021-40438 — httpd

CRITICAL

CVE-2021-40438 is a critical-severity (CVSS 9) Server-Side Request Forgery (SSRF) vulnerability. It is in CISA's Known Exploited Vulnerabilities catalog (added 2021-12-01) and has been used in ransomware campaigns — treat it as actively exploited and patch now. No vendor fix is recorded yet; mitigation options are listed below.

Also known asBIT-apache-2021-40438

Published

Sep 16, 2021

Updated

Sep 23, 2026

Affected

39 products

Patched

See advisory

Exploits

13 known

Exploitation data as of Oct 5, 2026 · OSV.dev, NVD, CISA KEV, FIRST.org (EPSS)

Exploitation Status

Actively exploited in the wild

Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2021-12-01 — including use in ransomware campaigns. Federal agencies were required to remediate by 2021-12-15.
CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
A successful exploit gives an attacker total control of the affected component, not partial access.

Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2021-40438.

EPSS Exploitation Probability

via FIRST.org ↗

100.0%probability of exploitation in next 30 days

Very High Risk0.00%

Lower risk than most CVEs100th percentile — riskier than 100% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-40438 by exploitation likelihood (EPSS) against impact (CVSS). In the shaded patch-first corner (EPSS 50%+, CVSS 7.0+).

Where this sits among everything scored

Of 383,485 CVEs with a current EPSS score, this one falls in the ≥ 90% band (highlighted). Counts from FIRST.org, log-scaled.

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.

Affected Products

39 products · 83 configurations

Application

http serverapache

≤ 2.4.48

range

brocade fabric operating system firmwarebroadcom

all

debian linuxdebian

3 versions

9.010.011.0

f5osf5

≥ 1.2.0 && ≤ 1.2.1

range

fedorafedoraproject

2 versions

3435

Application

cloud backupnetapp

all

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.