---
id: "CVE-2021-40438"
kind: "vulnerability"
aliases: ["CVE-2021-40438"]
title: "Apache Apache: Apache HTTP Server-Side Request Forgery (SSRF)"
severity: "critical"
cvss: 9.0
epss: 0.99999
exploited: true
status: "open"
category: "runtime"
products: [{"name": "Apache Apache", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["apache-httpd", "exploited", "ransomware", "reverse-proxy", "runtime", "self-hosted"]
not_affected_if: ["mod_proxy-not-loaded", "version-above-2.4.48"]
published_at: "2021-09-16T15:15:07+00:00"
first_seen_at: "2026-10-23T19:11:53+00:00"
updated_at: "2026-10-23T21:57:35+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-40438", "first_seen_at": "2026-10-23T19:11:53+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-40438", "first_seen_at": "2026-10-23T19:13:44+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-40438", "first_seen_at": "2026-10-23T21:57:35+00:00"}]
---

# Apache Apache: Apache HTTP Server-Side Request Forgery (SSRF)

Apache HTTP Server's mod_proxy can be tricked by a crafted request URI into forwarding the request to a backend server the remote user picks. An attacker can use the server as a proxy to reach internal services that the server itself can access.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:Apache Apache affected ? fixed ?

## How to check

Run httpd -v and check whether the version is 2.4.48 or earlier.
Check whether mod_proxy is loaded (httpd -M | grep proxy) and whether ProxyPass or ProxyPassMatch directives are present in the config.

## Fix

Upgrade Apache HTTP Server to a release later than 2.4.48 per the vendor instructions.

## Timeline

- 2026-10-23T19:11:53+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-40438
- 2026-10-23T19:13:44+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-40438
- 2026-10-23T21:57:35+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-40438

## From the sources

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.

CISA required action: Apply updates per vendor instructions.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_