CVE-2025-64340 is a medium-severity (CVSS 6.7) OS Command Injection vulnerability in fastmcp. A fix is available for fastmcp — see the affected versions and patch details below.
FastMCP has a Command Injection vulnerability - Gemini CLI
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-64340.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2025-64340 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
fastmcpReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
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 | 3.2.0pip install --upgrade 'fastmcp==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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fastmcp to 3.2.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-64340 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2025-64340 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-64340. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2025-64340 in your dependencies?
O3 Security finds CVE-2025-64340 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.