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CVE-2021-40438 — A crafted request uri-path can cause mod_proxy to forward the request to an origin server…

Key facts

CVE-2021-40438 is a Critical (9.0) vulnerability: A crafted request uri-path can cause mod_proxy to forward the request to an origin server…. This entry ships a detection-only PoC and a Docker test lab. It is on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2021-12-01), confirming active exploitation in the wild, and is linked to known ransomware campaigns.

Actively exploited — on the CISA KEV catalog

CVE-2021-40438 is listed on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2021-12-01), confirming exploitation in the wild. It is linked to known ransomware campaigns. Federal remediation deadline: 2021-12-15. Prioritise patching and deploy the detection and prevention rules below.
CVE CVE-2021-40438
Severity Critical (9.0)
CISA KEV Actively exploited (added 2021-12-01) · ransomware
Detection / prevention —
PoC poc.py
Source 2021/CVE-2021-40438/

Details

English | 한국어

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.

Vulnerability Details

Property Value
CVE ID CVE-2021-40438
Type SSRF
CVSS Score 9.0

Proof of Concept

Download

# Download PoC script
curl -O https://raw.githubusercontent.com/fankh/vulnerability-poc/main/2021/CVE-2021-40438/poc.py

Usage

# Run PoC against target
python3 poc.py --target <TARGET_URL>

# With authentication
python3 poc.py --target <TARGET_URL> --username <USER> --password <PASS>

⚠️ Warning: This PoC is for authorized security testing only. It uses detection-only techniques and does not exploit or damage target systems.

Test Lab

This repository includes a Docker-based test environment with both vulnerable and patched applications.

Quick Start

# Clone and navigate to CVE directory
cd CVE-2021-40438

# Start both applications
docker-compose up -d

# Run tests
./run-tests.sh    # Linux/Mac
./run-tests.ps1   # Windows PowerShell

Application Endpoints

Application URL Status
Vulnerable App http://localhost:8080 Shows vulnerability
Patched App http://localhost:8081 Demonstrates fix

Directory Structure

├── README.md           # English documentation
├── README_KO.md        # Korean documentation
├── poc.py              # PoC detection script
├── docker-compose.yml  # Container orchestration
├── run-tests.sh        # Linux/Mac test runner
├── run-tests.ps1       # Windows test runner
├── vulnerable-app/     # Vulnerable version
│   ├── app.py
│   ├── Dockerfile
│   └── requirements.txt
└── patched-app/        # Fixed version
    ├── app.py
    ├── Dockerfile
    └── requirements.txt

References

  • https://cert-portal.siemens.com/productcert/pdf/ssa-685781.pdf
  • https://httpd.apache.org/security/vulnerabilities_24.html
  • https://lists.apache.org/thread.html/r210807d0bb55f4aa6fbe1512be6bcc4dacd64e84940429fba329967a%40%3Cusers.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/r2eb200ac1340f69aa22af61ab34780c531d110437910cb9c0ece3b37%40%3Cbugs.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/r3925e167d5eb1c75def3750c155d753064e1d34a143028bb32910432%40%3Cusers.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/r61fdbfc26ab170f4e6492ef3bd5197c20b862ce156e9d5a54d4b899c%40%3Cusers.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/r82838efc5fa6fc4c73986399c9b71573589f78b31846aff5bd9b1697%40%3Cusers.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/r82c077663f9759c7df5a6656f925b3ee4f55fcd33c889ba7cd687029%40%3Cusers.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/rf6954e60b1c8e480678ce3d02f61b8a788997785652e9557a3265c00%40%3Cusers.httpd.apache.org%3E
  • https://lists.debian.org/debian-lts-announce/2021/10/msg00001.html
  • https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/SPBR6WUYBJNACHKE65SPL7TJOHX7RHWD/
  • https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZNCYSR3BXT36FFF4XTCPL3HDQK4VP45R/
  • https://security.gentoo.org/glsa/202208-20
  • https://security.netapp.com/advisory/ntap-20211008-0004/
  • https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-apache-httpd-2.4.49-VWL69sWQ
  • https://www.debian.org/security/2021/dsa-4982
  • https://www.oracle.com/security-alerts/cpuapr2022.html
  • https://www.oracle.com/security-alerts/cpujan2022.html
  • https://www.tenable.com/security/tns-2021-17
  • https://cert-portal.siemens.com/productcert/pdf/ssa-685781.pdf
  • https://httpd.apache.org/security/vulnerabilities_24.html
  • https://lists.apache.org/thread.html/r210807d0bb55f4aa6fbe1512be6bcc4dacd64e84940429fba329967a%40%3Cusers.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/r2eb200ac1340f69aa22af61ab34780c531d110437910cb9c0ece3b37%40%3Cbugs.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/r3925e167d5eb1c75def3750c155d753064e1d34a143028bb32910432%40%3Cusers.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/r61fdbfc26ab170f4e6492ef3bd5197c20b862ce156e9d5a54d4b899c%40%3Cusers.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/r82838efc5fa6fc4c73986399c9b71573589f78b31846aff5bd9b1697%40%3Cusers.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/r82c077663f9759c7df5a6656f925b3ee4f55fcd33c889ba7cd687029%40%3Cusers.httpd.apache.org%3E
  • https://lists.apache.org/thread.html/rf6954e60b1c8e480678ce3d02f61b8a788997785652e9557a3265c00%40%3Cusers.httpd.apache.org%3E
  • https://lists.debian.org/debian-lts-announce/2021/10/msg00001.html
  • https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/SPBR6WUYBJNACHKE65SPL7TJOHX7RHWD/
  • https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZNCYSR3BXT36FFF4XTCPL3HDQK4VP45R/
  • https://security.gentoo.org/glsa/202208-20
  • https://security.netapp.com/advisory/ntap-20211008-0004/
  • https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-apache-httpd-2.4.49-VWL69sWQ
  • https://www.debian.org/security/2021/dsa-4982
  • https://www.oracle.com/security-alerts/cpuapr2022.html
  • https://www.oracle.com/security-alerts/cpujan2022.html
  • https://www.tenable.com/security/tns-2021-17
  • https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-40438

Generated by PoC Generator on 2026-07-08T22:18:10.267444532

Frequently asked questions

What is CVE-2021-40438?

CVE-2021-40438 is a Critical (9.0) vulnerability: A crafted request uri-path can cause mod_proxy to forward the request to an origin server…. This entry ships a detection-only PoC and a Docker test lab. It is on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2021-12-01), confirming active exploitation in the wild, and is linked to known ransomware campaigns.

How do I detect CVE-2021-40438?

Run the detection-only PoC included with this entry against your Docker test lab to confirm whether a target is affected; pair it with your own IDS logging.

How do I block or mitigate CVE-2021-40438?

Upgrade to a fixed version per the vendor advisory; the NVD record lists the affected versions. Use the included PoC to verify your remediation.

Is CVE-2021-40438 actively exploited?

Yes. CVE-2021-40438 is on the CISA Known Exploited Vulnerabilities (KEV) catalog, added 2021-12-01, which means it has been exploited in the wild. CISA also links it to known ransomware campaigns. CISA set a federal remediation deadline of 2021-12-15. Prioritise patching and deploy the detection and prevention rules above.

For authorized security testing and research only. See the full source and NVD record.