CVE-2026-55040 — Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to…¶

Key facts

CVE-2026-55040 is a Critical (9.1) vulnerability: Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to…. This entry ships a detection-only PoC and a Docker test lab. It is on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2026-08-18), confirming active exploitation in the wild.

Actively exploited — on the CISA KEV catalog

CVE-2026-55040 is listed on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2026-08-18), confirming exploitation in the wild. Federal remediation deadline: 2026-08-21. Prioritise patching and deploy the detection and prevention rules below.


CVE	CVE-2026-55040
Severity	Critical (9.1)
CISA KEV	Actively exploited (added 2026-08-18)
Detection / prevention	—
PoC	poc.py
Source	2026/CVE-2026-55040/

Details¶

English | 한국어

Description¶

Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.

Vulnerability Details¶

Property	Value
CVE ID	CVE-2026-55040
CVSS Score	9.1

Proof of Concept¶

Download¶

# Download PoC script
curl -O https://raw.githubusercontent.com/fankh/vulnerability-poc/main/2026/CVE-2026-55040/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-2026-55040

# 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://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55040

Generated by PoC Generator on 2026-07-15T11:59:18.084445325

Frequently asked questions¶

What is CVE-2026-55040?¶

CVE-2026-55040 is a Critical (9.1) vulnerability: Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to…. This entry ships a detection-only PoC and a Docker test lab. It is on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2026-08-18), confirming active exploitation in the wild.

How do I detect CVE-2026-55040?¶

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-2026-55040?¶

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-2026-55040 actively exploited?¶

Yes. CVE-2026-55040 is on the CISA Known Exploited Vulnerabilities (KEV) catalog, added 2026-08-18, which means it has been exploited in the wild. CISA set a federal remediation deadline of 2026-08-21. Prioritise patching and deploy the detection and prevention rules above.

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