⚠ Known Exploited Vulnerability (KEV) Tracker

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.

Lifecycle Timeline

• 2021-09-16
  CVE published

  Entered the public CVE record.

  CVE.org →
• 2021-10-18
  Public exploit code 32 days later

  Public GitHub repository xiaojiangxl/CVE-2021-40438 — unvetted, may contain malware

  GitHub PoC →
• 2021-12-01
  Added to CISA KEV 44 days later

  Listed as known exploited, which starts the federal remediation clock.

  CISA KEV →
• 2021-12-15
  Federal remediation due 14 days later

  Deadline for federal civilian agencies under BOD 22-01 — now passed.

  CISA KEV
• not recorded
  EPSS flags exploitation risk

  Already at 36.6% when EPSS history begins (2021-12-11), so the moment it was first flagged predates the record.
• not recorded
  First national advisory

  No tracked government advisory analyses this CVE. Routine KEV catalog-addition alerts do not count here.
• not recorded
  Vendor fix published

  No dated vendor fix. Only Microsoft publishes a machine-readable release date; other vendors state a fixed version without one.

Each milestone comes from a different source, so gaps between them are gaps in what those sources publish, not evidence that nothing happened. EPSS history for this CVE holds 262 points from 2021-12-11, tracked daily from 2026-09-09; earlier points are weekly, which dates a spike to within a week. Peak probability 100.0% on 2026-06-20.

Published Applicability

Check whether a specific version is affected →

What each source publishes about which product versions carry this vulnerability, kept as written rather than merged. Absence here means no source has published applicability data, not that nothing is affected.

Research & News Coverage

No mentions found in monitored research blogs or news feeds.

Detection & Exploitation

Coverage across public collections only: Nuclei and OpenVAS NVT (scan); Snort and Suricata ET Open (network); Sigma and YARA (host/SIEM); Metasploit, Exploit-DB and public GitHub repositories (exploit). Absence here means nothing public was found, not that detection is impossible.
GitHub PoC repositories are collected automatically by PoC-in-GitHub (CC0) and are not vetted: some are fakes and some are malware. Treat them as evidence that code was published, not as code to run.
OpenVAS coverage derives from the Greenbone Community Feed, made available under the Open Database License (ODbL) v1.0; its individual Vulnerability Tests are licensed GNU GPL v2.

Distribution Status

Where the distribution has shipped a fix, and in which version. Green means the release has resolved it, amber that it is still open. Data from the Debian Security Tracker, used under Debian's MIT / GPLv2+ terms. Debian is upstream of several other distributions, so a fix here often precedes theirs.

CISA Assessment (Vulnrichment / SSVC)

D3FEND Countermeasures

Defensive techniques inferred by MITRE D3FEND from the underlying weakness (CWE-918) — general hardening and detection guidance for this weakness class, not a vendor patch for this specific CVE.

KEV Catalog Status

Listed By

Weaponization Timeline — Offensive & Testing Tools

No exploit-tool sightings recorded for this CVE.

Evidence tiers are explained on the About page — a merged Metasploit module is far stronger evidence than a distro package mention.

External References

CVE.org record → NVD entry → CISA KEV catalog → FortiGuard Labs → Rapid7 AttackerKB →