GHSA-VJ7Q-GJH5-988W
Vulnerability from github – Published: 2026-07-16 20:14 – Updated: 2026-07-16 20:14Summary
In affected versions, the deprecated WebSocket server transport (mcp.server.websocket.websocket_server) accepted the WebSocket handshake without applying any Host or Origin header validation. The TransportSecuritySettings mechanism that the SSE and Streamable HTTP transports use for this purpose was not wired into the WebSocket transport, so there was no SDK-level way to restrict which origins could connect.
Am I affected?
Only if a developer's application server exposes mcp.server.websocket.websocket_server. This transport has never been part of the MCP specification, is marked deprecated, and is not reachable through FastMCP — a developer must have wired it into an ASGI application themselves. Servers using stdio, SSE, or Streamable HTTP are not affected by this advisory.
Details
websocket_server() constructed a Starlette WebSocket and called accept(subprotocol="mcp") immediately, with no inspection of the connection's headers. By contrast, SseServerTransport and StreamableHTTPServerTransport accept an optional security_settings: TransportSecuritySettings and run TransportSecurityMiddleware.validate_request() against the incoming Host and Origin headers before establishing a session. Because browsers attach an Origin header to cross-origin WebSocket upgrade requests but do not enforce a same-origin policy on the response, a web page served from any origin could open a WebSocket to a reachable MCP server on this transport, complete the initialize handshake, and issue JSON-RPC requests on the resulting session.
Impact
A user who runs an MCP server on this transport bound to localhost or a LAN address, without a separate authentication or origin gate in front of it, and visits a malicious web page, can have that page enumerate and invoke the server's tools and read its resources. The consequences depend entirely on what the server exposes. The transport itself requires no token or prior session. Some browsers prompt before allowing a public page to open a connection to a local-network address, which adds a user-interaction step but is not a substitute for server-side validation.
Mitigation
Upgrade to version 1.28.1 or later, in which websocket_server() accepts the same optional security_settings: TransportSecuritySettings argument as the other HTTP-based transports and validates the Host and Origin headers before accepting the handshake; a request that fails validation is rejected with HTTP 403 and ValueError("Request validation failed") is raised to the caller. As with the other transports the parameter defaults to None, which leaves validation disabled, so upgrading alone does not change behaviour: pass a TransportSecuritySettings with enable_dns_rebinding_protection=True and appropriate allowed_hosts / allowed_origins to receive the protection. The recommended path remains to migrate off this deprecated transport to Streamable HTTP, where FastMCP enables this protection automatically for localhost binds. The WebSocket transport has been removed entirely in v2.
{
"affected": [
{
"package": {
"ecosystem": "PyPI",
"name": "mcp"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "1.28.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-59950"
],
"database_specific": {
"cwe_ids": [
"CWE-1385",
"CWE-346"
],
"github_reviewed": true,
"github_reviewed_at": "2026-07-16T20:14:34Z",
"nvd_published_at": "2026-07-15T21:16:55Z",
"severity": "HIGH"
},
"details": "### Summary\nIn affected versions, the deprecated WebSocket server transport (`mcp.server.websocket.websocket_server`) accepted the WebSocket handshake without applying any `Host` or `Origin` header validation. The `TransportSecuritySettings` mechanism that the SSE and Streamable HTTP transports use for this purpose was not wired into the WebSocket transport, so there was no SDK-level way to restrict which origins could connect.\n\n### Am I affected?\nOnly if a developer\u0027s application server exposes `mcp.server.websocket.websocket_server`. This transport has never been part of the MCP specification, is marked deprecated, and is not reachable through `FastMCP` \u2014 a developer must have wired it into an ASGI application themselves. Servers using stdio, SSE, or Streamable HTTP are not affected by this advisory.\n\n### Details\n`websocket_server()` constructed a Starlette `WebSocket` and called `accept(subprotocol=\"mcp\")` immediately, with no inspection of the connection\u0027s headers. By contrast, `SseServerTransport` and `StreamableHTTPServerTransport` accept an optional `security_settings: TransportSecuritySettings` and run `TransportSecurityMiddleware.validate_request()` against the incoming `Host` and `Origin` headers before establishing a session. Because browsers attach an `Origin` header to cross-origin WebSocket upgrade requests but do not enforce a same-origin policy on the response, a web page served from any origin could open a WebSocket to a reachable MCP server on this transport, complete the `initialize` handshake, and issue JSON-RPC requests on the resulting session.\n\n### Impact\nA user who runs an MCP server on this transport bound to localhost or a LAN address, without a separate authentication or origin gate in front of it, and visits a malicious web page, can have that page enumerate and invoke the server\u0027s tools and read its resources. The consequences depend entirely on what the server exposes. The transport itself requires no token or prior session. Some browsers prompt before allowing a public page to open a connection to a local-network address, which adds a user-interaction step but is not a substitute for server-side validation.\n\n### Mitigation\nUpgrade to version 1.28.1 or later, in which `websocket_server()` accepts the same optional `security_settings: TransportSecuritySettings` argument as the other HTTP-based transports and validates the `Host` and `Origin` headers before accepting the handshake; a request that fails validation is rejected with HTTP 403 and `ValueError(\"Request validation failed\")` is raised to the caller. As with the other transports the parameter defaults to `None`, which leaves validation disabled, so upgrading alone does not change behaviour: pass a `TransportSecuritySettings` with `enable_dns_rebinding_protection=True` and appropriate `allowed_hosts` / `allowed_origins` to receive the protection. The recommended path remains to migrate off this deprecated transport to Streamable HTTP, where `FastMCP` enables this protection automatically for localhost binds. The WebSocket transport has been removed entirely in v2.",
"id": "GHSA-vj7q-gjh5-988w",
"modified": "2026-07-16T20:14:34Z",
"published": "2026-07-16T20:14:34Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-vj7q-gjh5-988w"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-59950"
},
{
"type": "WEB",
"url": "https://github.com/modelcontextprotocol/python-sdk/pull/2992"
},
{
"type": "WEB",
"url": "https://github.com/modelcontextprotocol/python-sdk/commit/777b8d06710c140e3606b0d4598e2aa48546c266"
},
{
"type": "PACKAGE",
"url": "https://github.com/modelcontextprotocol/python-sdk"
},
{
"type": "WEB",
"url": "https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.28.1"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "MCP Python SDK: WebSocket server transport does not support Host/Origin validation"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.