CVE-2026-73256 — Mongoose is an embedded web server and network library.¶

Key facts

CVE-2026-73256 is a Critical (9.1) vulnerability: Mongoose is an embedded web server and network library.. This entry ships a detection-only PoC and a Docker test lab.


CVE	CVE-2026-73256
Severity	Critical (9.1)
Detection / prevention	—
PoC	poc.py
Source	2026/CVE-2026-73256/

Details¶

English | 한국어

Description¶

Mongoose is an embedded web server and network library. Prior to 7.22, a remote unauthenticated attacker can exploit an HTTP/1.0 reverse-proxy deployment by sending a request with Transfer-Encoding: chunked and conflicting framing. The http_cb() function in src/http.c tests hm.proto.len with an impossible greater-than-eight condition even though mg_http_parse() requires an eight-byte protocol string, so is_http_1_0 is never set. Mongoose consequently processes chunked encoding that an HTTP/1.0 proxy can ignore, enabling request smuggling and unauthorized access or state changes. This issue is fixed in version 7.22.

Vulnerability Details¶

Property	Value
CVE ID	CVE-2026-73256
CVSS Score	9.1

Proof of Concept¶

Download¶

# Download PoC script
curl -O https://raw.githubusercontent.com/fankh/vulnerability-poc/main/2026/CVE-2026-73256/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-2026-73256

# 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/cesanta/mongoose/commit/a9df523f76f43a38bd53b4232b9cfd4c16869e71
https://github.com/cesanta/mongoose/pull/3611
https://github.com/cesanta/mongoose/releases/tag/7.22
https://github.com/cesanta/mongoose/security/advisories/GHSA-mgp5-rjrv-h5j3

Generated by PoC Generator on 2026-08-23T00:38:01.762173142

Frequently asked questions¶

What is CVE-2026-73256?¶

CVE-2026-73256 is a Critical (9.1) vulnerability: Mongoose is an embedded web server and network library.. This entry ships a detection-only PoC and a Docker test lab.

How do I detect CVE-2026-73256?¶

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-2026-73256?¶

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.