high CVSS 8.1

CVE-2026-59950·Python vulnerability

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to 1.28.1, the deprecated mcp.server.websocket.websocket_server transport accepted WebSocket handshakes without applying Host or Origin header validation, leaving no SDK-level way to restrict which origins could connect to applications that exposed that transport. This issue is fixed in version 1.28.1.

Severity
high
Software
Python
Fixed in
1.28.1
Published
2026-07-15

Affected versions

Until: 1.28.1

Fixed in: 1.28.1

How to fix this CVE

Applications using the MCP Python SDK must upgrade to version 1.28.1 or later to address a WebSocket validation vulnerability in the deprecated websocket_server transport. The fix implements proper Host and Origin header validation to prevent unauthorized cross-origin connections. Update your Python environment immediately and restart any services that depend on the mcp package.

sudo dnf update python3 && pip3 install --upgrade mcp>=1.28.1

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST /path HTTP/1.1
Upgrade: websocket
Connection: Upgrade
(Missing|Mismatched Host header|Missing Origin header)

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement WebSocket connection filtering rules that validate Host header matches expected domain and reject requests with missing or invalid Origin headers before handshake completion.

How to check if you are affected

  1. Check installed mcp package version: pip3 show mcp | grep Version
  2. Identify if websocket_server transport is in use: grep -r 'websocket_server' /path/to/your/app --include='*.py'
  3. Search application logs for WebSocket handshake requests without Host/Origin headers: grep -i 'websocket' /var/log/app/*.log | grep -v 'Host\|Origin'
  4. Verify the patch by confirming mcp version is 1.28.1 or higher: python3 -c 'import mcp; print(mcp.__version__)'

FAQ

What is CVE-2026-59950?

CVE-2026-59950 is a validation bypass vulnerability in the MCP Python SDK's deprecated WebSocket transport that fails to validate Host and Origin headers during connection handshakes, allowing potential cross-origin attacks.

Is CVE-2026-59950 being actively exploited?

No, this vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits are available.

What versions of the MCP Python SDK are affected by CVE-2026-59950?

All versions prior to 1.28.1 are affected. The fix was released in version 1.28.1.

How do I check if my server is vulnerable to CVE-2026-59950?

Run 'pip3 show mcp' and check the Version field. If it shows a version number lower than 1.28.1, your installation is vulnerable.

Does Defensia detect CVE-2026-59950?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If the mcp package is installed on a monitored server, CVE-2026-59950 will appear in your dashboard with remediation steps.

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References

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