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MCP Python SDK: WebSocket server transport does not support Host/Origin validation

High severity GitHub Reviewed Published Jul 7, 2026 in modelcontextprotocol/python-sdk • Updated Jul 16, 2026

Package

pip mcp (pip)

Affected versions

< 1.28.1

Patched versions

1.28.1

Description

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.

References

@maxisbey maxisbey published to modelcontextprotocol/python-sdk Jul 7, 2026
Published by the National Vulnerability Database Jul 15, 2026
Published to the GitHub Advisory Database Jul 16, 2026
Reviewed Jul 16, 2026
Last updated Jul 16, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(8th percentile)

Weaknesses

Origin Validation Error

The product does not properly verify that the source of data or communication is valid. Learn more on MITRE.

Missing Origin Validation in WebSockets

The product uses a WebSocket, but it does not properly verify that the source of data or communication is valid. Learn more on MITRE.

CVE ID

CVE-2026-59950

GHSA ID

GHSA-vj7q-gjh5-988w

Source code

Credits

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