How severe is GHSA-jpw9-pfvf-9f58?

GHSA-jpw9-pfvf-9f58 has a CVSS score of 7.1/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by GHSA-jpw9-pfvf-9f58?

GHSA-jpw9-pfvf-9f58 affects the following packages: mcp (PyPI). Ecosystems affected: PyPI.

How do I fix GHSA-jpw9-pfvf-9f58?

Update mcp to 1.27.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-jpw9-pfvf-9f58 is resolved across your whole dependency graph.

How do I detect GHSA-jpw9-pfvf-9f58 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mcp. 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-jpw9-pfvf-9f58 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-jpw9-pfvf-9f58?

O3 pinpoints whether GHSA-jpw9-pfvf-9f58 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-jpw9-pfvf-9f58 actively exploited in the wild?

No public exploit code has been indexed for GHSA-jpw9-pfvf-9f58 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 type of vulnerability is GHSA-jpw9-pfvf-9f58?

GHSA-jpw9-pfvf-9f58 is classified as CWE-639 (CWE-639). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was GHSA-jpw9-pfvf-9f58 published, and has it been updated?

GHSA-jpw9-pfvf-9f58 was published on July 16, 2026 and was last updated on July 16, 2026. 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-jpw9-pfvf-9f58

HIGH

MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal

Also known asCVE-2026-52869

Published

Jul 16, 2026

Updated

Jul 16, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Blast Radius

1 pkg affected

🐍mcp

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

In affected versions, the SSE and Streamable HTTP server transports routed incoming requests to an existing session based only on the session identifier, without verifying that the request was authenticated as the same principal that created the session. Anyone who learned or guessed a session ID could send JSON-RPC messages on that session, regardless of which bearer token the request carried.

Am I affected?

Only if a developer's application server uses an HTTP transport (SSE, or Streamable HTTP in stateful mode) and authenticates requests. Servers on stdio, stateless Streamable HTTP, or with no authentication configured are not affected.

Details

Both transports look up the target session by its identifier alone — the session_id query parameter for SSE (mcp.server.sse.SseServerTransport) and the Mcp-Session-Id header for Streamable HTTP (mcp.server.streamable_http_manager.StreamableHTTPSessionManager). Once the lookup succeeded, the request was handled on that session without comparing its authentication context to the credentials presented when the session was created, so a request authenticated as a different OAuth client could inject messages into the session. On the SSE transport the response is delivered to the original client's event stream; on the Streamable HTTP transport it is returned on the injecting request, so the injecting client can also read the result. The SSE transport has been affected since the first release; the Streamable HTTP transport since version 1.8.0.

Impact

Servers using either HTTP transport together with the SDK's built-in bearer-token authentication are affected: the per-client isolation that authentication provides can be bypassed for any session whose ID is known. Session IDs are randomly generated UUIDs, so exploitation requires obtaining one out of band (logs, network observation). Servers that do not enable bearer-token authentication have no per-client isolation to bypass and are not addressed by this advisory, and stateless Streamable HTTP deployments do not maintain sessions and are unaffected.

Mitigation

Upgrade to version 1.27.2 or later, which records the authenticated principal that created each session — the OAuth client ID together with the token's issuer and subject when the token verifier supplies them — and answers requests presenting a different principal with the same 404 response as for an unknown session.

Deployments where many end users share a single OAuth client (hosted MCP clients, gateways) should ensure their token verifier populates AccessToken.subject (e.g. from the token's sub claim) so sessions are isolated per user rather than per client. Deployments using a custom authentication backend other than the built-in BearerAuthBackend should enforce an equivalent check themselves.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`mcp`	all versions	1.27.2

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mcp. 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 mcp to 1.27.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-jpw9-pfvf-9f58 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-jpw9-pfvf-9f58 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-jpw9-pfvf-9f58. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

### Summary In affected versions, the SSE and Streamable HTTP server transports routed incoming requests to an existing session based only on the session identifier, without verifying that the request was authenticated as the same principal that created the session. Anyone who learned or guessed a session ID could send JSON-RPC messages on that session, regardless of which bearer token the request carried. ### Am I affected? Only if a developer's application server uses an HTTP transport (SSE, or Streamable HTTP in stateful mode) **and** authenticates requests. Servers on stdio, stateless Str

O3 Security · Impact-Aware SCA

Is GHSA-jpw9-pfvf-9f58 in your dependencies?

O3 detects GHSA-jpw9-pfvf-9f58 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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