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CVE-2021-40438

Critical CVSS 9 Known exploited

Published 2021-09-16 · last changed 2026-08-06 · added to CISA KEV 2021-12-01

This one is being exploited in the wild.

CISA added CVE-2021-40438 to the Known Exploited Vulnerabilities catalog on 2021-12-01. Federal agencies are required to remediate it on a deadline. If you have affected machines, this is the class of finding that moves ahead of the rest of the queue.

The fix

We hold no Microsoft remediation data for CVE-2021-40438, which normally means it is not a Windows CVE. For non-Microsoft software the fix is the vendor's own updated release, and the affected versions are listed below.

What it is

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.

Sources

CVSS vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H