GHSA-m8x7-r2rg-vh5g is a medium-severity (CVSS 6.7) OS Command Injection vulnerability in fastmcp. O3 Security confirms whether GHSA-m8x7-r2rg-vh5g is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
FastMCP has a Command Injection vulnerability - Gemini CLI
Real-World Exposure
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Description
Server names containing shell metacharacters (e.g., &) can cause command injection on Windows when passed to fastmcp install claude-code or fastmcp install gemini-cli. These install paths use subprocess.run() with a list argument, but on Windows the target CLIs often resolve to .cmd wrappers that are executed through cmd.exe, which interprets metacharacters in the flattened command string.
PoC:
from fastmcp import FastMCP
mcp = FastMCP(name="test&calc")
@mcp.tool
def roll_dice(n_dice: int) -> list[int]:
"""Roll `n_dice` 6-sided dice and return the results."""
return [random.randint(1, 6) for _ in range(n_dice)]
fastmcp install claude-code server.py # or: fastmcp install gemini-cli server.py
On Windows, this opens Calculator via the &calc in the server name.
Impact:
Arbitrary command execution with the privileges of the user running fastmcp install. Affects Windows hosts where the target CLI (one of claude, gemini) is installed as a .cmd wrapper. Does not affect macOS/Linux, and does not affect config-file-based install targets (cursor, goose, mcp-json).
Patched in #3522 by validating server names to reject shell metacharacters.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | fastmcp | all versions | 3.2.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastmcp. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update fastmcp to 3.2.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-m8x7-r2rg-vh5g is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-m8x7-r2rg-vh5g is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-m8x7-r2rg-vh5g. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-m8x7-r2rg-vh5g in your dependencies?
O3 detects GHSA-m8x7-r2rg-vh5g across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.