CVE-2026-78207 — exceljs-hardened before 5.0.0 contains a prototype pollution vulnerability in the…¶
Key facts
CVE-2026-78207 is a Critical (9.4) vulnerability: exceljs-hardened before 5.0.0 contains a prototype pollution vulnerability in the…. This entry ships a detection-only PoC and a Docker test lab.
| CVE | CVE-2026-78207 |
| Severity | Critical (9.4) |
| Detection / prevention | — |
| PoC | poc.py |
| Source | 2026/CVE-2026-78207/ |
Details¶
English | 한국어
Description¶
exceljs-hardened before 5.0.0 contains a prototype pollution vulnerability in the deepMerge helper that fails to reject proto, constructor, or prototype keys when merging note objects. Attackers can assign parsed JSON with a malicious proto property to cell notes, modifying Object.prototype and affecting all plain objects created in the process.
Vulnerability Details¶
| Property | Value |
|---|---|
| CVE ID | CVE-2026-78207 |
| CVSS Score | 9.4 |
Proof of Concept¶
Download¶
# Download PoC script
curl -O https://raw.githubusercontent.com/fankh/vulnerability-poc/main/2026/CVE-2026-78207/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-2026-78207
# 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://github.com/exceljs/exceljs
- https://github.com/exceljs/exceljs/blob/v4.4.0/lib/utils/under-dash.js#L155-L181
- https://github.com/mateocallec/exceljs-hardened/security/advisories/GHSA-qwr4-7h29-chpf
- https://www.vulncheck.com/advisories/exceljs-through-prototype-pollution-via-deepmerge-reached-from-note-serialization
Generated by PoC Generator on 2026-08-24T13:16:07.385437926
Frequently asked questions¶
What is CVE-2026-78207?¶
CVE-2026-78207 is a Critical (9.4) vulnerability: exceljs-hardened before 5.0.0 contains a prototype pollution vulnerability in the…. This entry ships a detection-only PoC and a Docker test lab.
How do I detect CVE-2026-78207?¶
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-2026-78207?¶
Upgrade to a fixed version per the vendor advisory; the NVD record lists the affected versions. Use the included PoC to verify your remediation.
For authorized security testing and research only. See the full source and NVD record.