PT-2026-60671 · Pypi · Mcp
Published
2026-07-16
·
Updated
2026-07-16
CVSS v4.0
7.6
High
| Vector | AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N |
Summary
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.Fix
Origin Validation Error
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Related Identifiers
Affected Products
Mcp