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

> Attackers can inject JSON-RPC messages into existing sessions by guessing session IDs, bypassing authentication checks in the MCP Python SDK.

- **Severity:** High 7.1 (`CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Jul 2026
- **Updated:** 16 Jul 2026
- **Also known as:** ECHO-5da5-33db-f402, GHSA-jpw9-pfvf-9f58, MINI-24xq-v9j2-hcfp, MINI-3225-g9hp-39wm, MINI-34ph-9368-jwf6, MINI-53f5-8x3p-x6w4, MINI-7542-wp76-8r9f, MINI-87q4-r73x-fmqr, MINI-crwp-vp3w-pxfw, MINI-f853-6gp2-pvww, MINI-g4pj-f8mc-jvv6, MINI-gcwr-pqq5-p5xp, MINI-hjm6-823x-5xc6, MINI-j2mq-j8jv-586h, MINI-rwmf-jqhq-3j5f, MINI-vqv5-h76g-g34h, MINI-wpgw-9pv6-wf5v, MINI-xp8p-78px-gg6x, PYSEC-2026-3482, ROOT-APP-PYPI-CVE-2026-52869

## What is CVE-2026-52869?

CVE-2026-52869 is a high-severity missing authorization vulnerability in modelcontextprotocol python-sdk, published on 15 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official 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.

Weakness: CWE-639 (Authorization Bypass Through User-Controlled Key)

## Which versions are affected by CVE-2026-52869?

3 open source packages ship an affected version, including mcp. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mcp` | PyPI | All versions | 1.27.2 |
| `mcp` | PyPI | All versions | 1.27.2 |
| `mcp` | PyPI | All versions | 1.27.2 |

Products: Lfprojects Mcp Python Sdk.

## How do I fix CVE-2026-52869?

Upgrade mcp to 1.27.2 or later.

## Is CVE-2026-52869 being exploited?

There is no known exploitation of CVE-2026-52869 in the wild and no public exploit code.

## Other vulnerabilities in modelcontextprotocol python-sdk

- [CVE-2026-52870](https://insights.sbomify.com/CVE-2026-52870/): MCP Python SDK: Experimental task handlers allow any client to access and cancel other clients' tasks (high 7.6)
- [CVE-2026-59950](https://insights.sbomify.com/CVE-2026-59950/): MCP Python SDK: WebSocket server transport does not support Host/Origin validation (high 7.6)
- [CVE-2025-66416](https://insights.sbomify.com/CVE-2025-66416/): DNS Rebinding Protection Disabled by Default in Model Context Protocol Python SDK for Servers Running on Localhost (high 7.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-52869 and fix it if it is affected.

What it is: CVE-2026-52869 is a high-severity missing authorization vulnerability in modelcontextprotocol python-sdk, published on 15 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-52869 in the wild and no public exploit code.

Affected packages and fixed versions:
- mcp (PyPI): affected all versions; fixed in 1.27.2
- mcp (PyPI): affected all versions; fixed in 1.27.2
- mcp (PyPI): affected all versions; fixed in 1.27.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-52869/ (Markdown: https://insights.sbomify.com/CVE-2026-52869/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-52869
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-52869
- OSV: https://osv.dev/vulnerability/CVE-2026-52869

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-52869/
Summaries are generated and may contain mistakes; the official description above is the record.