Skip to content

CVE-2021-26855 — CVE ID: CVE-2021-26855, CWE: CWE-918, Type: SSRF, Attack Vector: NETWORK, CVSS: 9.1

Key facts

CVE-2021-26855 is a Critical (9.1) vulnerability: CVE ID: CVE-2021-26855, CWE: CWE-918, Type: SSRF, Attack Vector: NETWORK, CVSS: 9.1. This entry ships a detection-only PoC and a Docker test lab. It is on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2021-11-03), confirming active exploitation in the wild, and is linked to known ransomware campaigns.

Actively exploited — on the CISA KEV catalog

CVE-2021-26855 is listed on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2021-11-03), confirming exploitation in the wild. It is linked to known ransomware campaigns. Federal remediation deadline: 2022-05-03. Prioritise patching and deploy the detection and prevention rules below.
CVE CVE-2021-26855
Severity Critical (9.1)
CISA KEV Actively exploited (added 2021-11-03) · ransomware
Detection / prevention —
PoC poc.py
Source 2021/CVE-2021-26855/

Details

English | 한국어

Description

CVE ID: CVE-2021-26855, CWE: CWE-918, Type: SSRF, Attack Vector: NETWORK, CVSS: 9.1

Vulnerability Details

Property Value
CVE ID CVE-2021-26855
Type SSRF
CVSS Score 9.1

Proof of Concept

Download

# Download PoC script
curl -O https://raw.githubusercontent.com/fankh/vulnerability-poc/main/2021/CVE-2021-26855/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-2021-26855

# 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

  • http://packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.html
  • http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html
  • http://packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.html
  • http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.html
  • https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855
  • http://packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.html
  • http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html
  • http://packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.html
  • http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.html
  • https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855
  • https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855

Generated by PoC Generator on 2026-07-08T15:39:37.021444254

Frequently asked questions

What is CVE-2021-26855?

CVE-2021-26855 is a Critical (9.1) vulnerability: CVE ID: CVE-2021-26855, CWE: CWE-918, Type: SSRF, Attack Vector: NETWORK, CVSS: 9.1. This entry ships a detection-only PoC and a Docker test lab. It is on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2021-11-03), confirming active exploitation in the wild, and is linked to known ransomware campaigns.

How do I detect CVE-2021-26855?

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-2021-26855?

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-2021-26855 actively exploited?

Yes. CVE-2021-26855 is on the CISA Known Exploited Vulnerabilities (KEV) catalog, added 2021-11-03, which means it has been exploited in the wild. CISA also links it to known ransomware campaigns. CISA set a federal remediation deadline of 2022-05-03. Prioritise patching and deploy the detection and prevention rules above.

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