CVE-2026-52869 json
MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal
published 5mo ago modified 18d ago also EUVD-2026-44795 , GHSA-jpw9-pfvf-9f58 , PYSEC-2026-3482
- cvss
- 7.1
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.5%
- EPSS 0.5% chance of exploitation in 30 days, percentile 45%
- public exploits
- 0
- none known
Summary
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.
Patch / fix
- pypi mcp 1.27.2
- lfprojects mcp_python_sdk 1.27.2
- github.com/modelcontextprotocol/python-sdk@1abcca2 commit
- github.com/modelcontextprotocol/python-sdk@ce267b6 commit
- github.com/modelcontextprotocol/python-sdk#2690 pull request
- github.com/modelcontextprotocol/python-sdk#2719 pull request
- github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2 release
Is CVE-2026-52869 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.
Check with FencerCISA SSVC
- Exploitation
- none
- Automatable
- no
- Technical impact
- total
- Assessed
- 3mo ago
Exploit prediction
0.54% chance of exploitation in the next 30 days, higher than 45% of all scored CVEs (FIRST EPSS, 2026-11-11).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 3.1 | 7.1 | CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L |
| GitHub | 3.1 | 7.1 | CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L |
| OSV | 3.1 | 7.1 | CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L |
Affected packages
| ecosystem | package | affected | fixed in |
|---|---|---|---|
| pypi | mcp | <1.27.2 |
1.27.2 |
Affected products
- modelcontextprotocol python-sdk
Classification
- Weakness
-
- CWE-639 Authorization Bypass Through User-Controlled Key
References
-
github.com
/modelcontextprotocol/python-sdk/pull/2690
web, issue-tracking, patch
-
github.com
/modelcontextprotocol/python-sdk/pull/2719
web, issue-tracking, patch
-
github.com
/modelcontextprotocol/python-sdk/commit/1abcca2408a6b50e10ec601181f63f9978705c00
web, patch
-
github.com
/modelcontextprotocol/python-sdk/commit/ce267b6fc515dc4efc1dc70b6975b16ff0feef0a
web, patch
-
github.com
/modelcontextprotocol/python-sdk/security/advisories/GHSA-jpw9-pfvf-9f58
web, vendor-advisory, advisory
-
github.com
/modelcontextprotocol/python-sdk/releases/tag/v1.27.2
web, release-notes
-
nvd.nist.gov
/vuln/detail/CVE-2026-52869
advisory
-
github.com
/modelcontextprotocol/python-sdk
package
-
pypi.org
/project/mcp
package
-
github.com
/advisories/GHSA-jpw9-pfvf-9f58
advisory
Sources
- CVE List CVE-2026-52869
- GitHub Advisory GHSA-jpw9-pfvf-9f58
- NVD CVE-2026-52869
- OSV PYSEC-2026-3482
- Maintainer advisory GHSA-jpw9-pfvf-9f58
Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.
The maintainers of modelcontextprotocol/python-sdk published their own advisory on GitHub. Reviewed sources take precedence over it.