CVE-2026-81091 — The proxy middleware in mcp-use's inspector forwards requests to a destination the caller…¶
Key facts
CVE-2026-81091 is a High (8.6) vulnerability: The proxy middleware in mcp-use's inspector forwards requests to a destination the caller…. This entry ships a detection-only PoC and a Docker test lab.
| CVE | CVE-2026-81091 |
| Severity | High (8.6) |
| Detection / prevention | — |
| PoC | poc.py |
| Source | 2026/CVE-2026-81091/ |
Details¶
English | 한국어
Description¶
The proxy middleware in mcp-use's inspector forwards requests to a destination the caller names. mountMcpProxy in libraries/typescript/packages/inspector/src/server/proxy/mcp-proxy.ts read the target from the X-Target-URL header or the __mcp_target parameter and proxied to it without inspecting the host, so loopback, link-local and private addresses were all accepted, as were names that resolve to them, and the validation was not reapplied to a redirect the destination returned. A caller could therefore make the server issue requests to addresses reachable only from the host it runs on and read the responses. The current code calls isSafeProxyTarget, which checks the resolved address against private, loopback and link-local ranges before proxying and bounds the number of redirects followed.
Vulnerability Details¶
| Property | Value |
|---|---|
| CVE ID | CVE-2026-81091 |
| Type | SSRF |
| CVSS Score | 8.6 |
Proof of Concept¶
Download¶
# Download PoC script
curl -O https://raw.githubusercontent.com/fankh/vulnerability-poc/main/2026/CVE-2026-81091/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-2026-81091
# 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/mcp-use/mcp-use
- https://github.com/mcp-use/mcp-use/security/advisories/GHSA-f2jg-rm2x-hc5p
- https://www.vulncheck.com/advisories/mcp-use-inspector-proxy-server-side-request-forgery-via-caller-supplied-target-url
Generated by PoC Generator on 2026-08-30T12:33:21.664498981
Frequently asked questions¶
What is CVE-2026-81091?¶
CVE-2026-81091 is a High (8.6) vulnerability: The proxy middleware in mcp-use's inspector forwards requests to a destination the caller…. This entry ships a detection-only PoC and a Docker test lab.
How do I detect CVE-2026-81091?¶
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-2026-81091?¶
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.