How severe is GHSA-mxxr-jv3v-6pgc?

No CVSS score has been assigned to GHSA-mxxr-jv3v-6pgc yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by GHSA-mxxr-jv3v-6pgc?

GHSA-mxxr-jv3v-6pgc affects the following packages: fastmcp (PyPI). Ecosystems affected: PyPI.

How do I fix GHSA-mxxr-jv3v-6pgc?

Update fastmcp to 2.13.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-mxxr-jv3v-6pgc is resolved across your whole dependency graph.

How do I detect GHSA-mxxr-jv3v-6pgc in my PyPI dependencies?

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.

How do I mitigate GHSA-mxxr-jv3v-6pgc if there is no patch (or I can't update yet)?

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 does O3 Security protect against GHSA-mxxr-jv3v-6pgc?

O3 pinpoints whether GHSA-mxxr-jv3v-6pgc is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is GHSA-mxxr-jv3v-6pgc actively exploited in the wild?

No public exploit code has been indexed for GHSA-mxxr-jv3v-6pgc yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for GHSA-mxxr-jv3v-6pgc?

GHSA-mxxr-jv3v-6pgc has an EPSS (Exploit Prediction Scoring System) score of 0.2%, placing it in the 13th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is GHSA-mxxr-jv3v-6pgc?

GHSA-mxxr-jv3v-6pgc is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was GHSA-mxxr-jv3v-6pgc published, and has it been updated?

GHSA-mxxr-jv3v-6pgc was published on October 29, 2025 and was last updated on October 29, 2025. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍 PyPI

GHSA-mxxr-jv3v-6pgc

FastMCP vulnerable to reflected XSS in client's callback page

Also known asCVE-2025-62800

Published

Oct 29, 2025

Updated

Oct 29, 2025

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

EPSS Exploitation Probability

via FIRST.org ↗

0.2%probability of exploitation in next 30 days

Lower Risk13th percentile+0.17%

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.

Blast Radius

1 pkg affected

🐍fastmcp

Real-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

Summary

While setting up an oauth client, it was noticed that the callback page hosted by the client during the flow embeds user-controlled content without escaping or sanitizing it. This leads to a reflected Cross-Site-Scripting vulnerability.

Details

The affected code is located in https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/client/oauth_callback.py, which embeds all values passed to the create_callback_html function via the message parameter it into the callback page without escaping them. This can, for example, be abused by calling the callback server with an XSS payload inside the error GET parameter, the value of which will then be inserted into the callback page, causing the execution of attacker-controlled JavaScript code in the callback server's origin. Note that besides the error parameter, other parameters reaching this function are affected too.

PoC

Setup a simple fastmcp client such as this one (the callback server's port was fixated for simplicity):

url="http://127.0.0.1:8000/mcp"
oauth = OAuth(mcp_url=url,callback_port=1337)

async def main():
    async with Client(url, auth=oauth) as client:
        await client.ping()
        
        # List available operations
        tools = await client.list_tools()

        print(f"tools: {tools}")
       
asyncio.run(main())

Ensure that the MCP server located at http://127.0.0.1:8000/mcp supports oauth.
Start the client.
As soon as the callback server has been started, access http://localhost:1337/callback?error=<img/src/onerror=alert(window.origin)>

Note that the exploitation could also for example be initiated by a malicious authorization server by returning the exploitation URL mentioned before in the authorization_endpoint field. The client would then automatically open, causing the XSS to trigger immediatly.

Impact

The impact of this XSS vulnerability is the arbitrary JavaScript execution in the victim's browser in the callback server's origin.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`fastmcp`	all versions	2.13.0

Detection & mitigation playbook

Open-source dependency

Detect
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 2.13.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-mxxr-jv3v-6pgc 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-mxxr-jv3v-6pgc 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-mxxr-jv3v-6pgc. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

### Summary While setting up an oauth client, it was noticed that the callback page hosted by the client during the flow embeds user-controlled content without escaping or sanitizing it. This leads to a reflected Cross-Site-Scripting vulnerability. ### Details The affected code is located in *https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/client/oauth_callback.py*, which embeds all values passed to the `create_callback_html` function via the `message` parameter it into the callback page without escaping them. This can, for example, be abused by calling the callback server with an XSS

O3 Security · Impact-Aware SCA

Is GHSA-mxxr-jv3v-6pgc in your dependencies?

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