Affected: Homebrew: cycode

### Summary In affected versions, the SDK's Streamable HTTP and SSE client transports followed HTTP redirects to any host. If the configured MCP endpoint answered with a redirect to a different origin:

- custom headers set on the client (for example an `X-API-Key` credential) were sent to that origin, and - on a `307` or `308` redirect the request body was re-sent there too.

The SDK's OAuth client providers followed redirects on their own discovery, registration and token requests in the same way.

A lone `Authorization` bearer header was not exposed; the HTTP library already drops it on cross-origin redirects.

Anyone able to make an endpoint you trust redirect elsewhere (a stale redirect to a lapsed domain, a misrouted proxy rule, redirect logic an attacker can influence) could capture that material. A server that is itself malicious gains nothing this way; it already receives your requests directly.

### Am I affected? Yes, if your code uses the SDK as an MCP client over HTTP through any of:

- `streamable_http_client` (on 2.x also `Client` given an `http://` or `https://` URL) - `streamablehttp_client` or `sse_client` - `ClientSessionGroup` with HTTP server parameters - the internal `create_mcp_http_client` helper

This includes an HTTP client you passed in yourself with `follow_redirects=True`.

Affected versions: 1.8.0 through 1.29.1, and 2.0.0 through 2.1.1.

Not affected:

- MCP servers - stdio clients - a client you supplied with `follow_redirects` left at its def…

The Model Context Protocol Python SDK is the official Python SDK for Model Context Protocol servers and clients. From 1.8.0 until 1.30.0 and 2.2.0, streamable_http_client, streamablehttp_client, sse_client, ClientSessionGroup, OAuth, and create_mcp_http_client client paths enable follow_redirects and follow redirects to a different origin, forwarding caller-supplied custom credential headers and, for 307 or 308 responses, the request body. An attacker who can influence a trusted endpoint's redirect destination can receive that material. This issue is fixed in versions 1.30.0 and 2.2.0.

Affected: PyPI: mcp

### Summary In affected versions, the SDK's Streamable HTTP and SSE client transports followed HTTP redirects to any host. If the configured MCP endpoint answered with a redirect to a different origin:

- custom headers set on the client (for example an `X-API-Key` credential) were sent to that origin, and - on a `307` or `308` redirect the request body was re-sent there too.

The SDK's OAuth client providers followed redirects on their own discovery, registration and token requests in the same way.

A lone `Authorization` bearer header was not exposed; the HTTP library already drops it on cross-origin redirects.

Anyone able to make an endpoint you trust redirect elsewhere (a stale redirect to a lapsed domain, a misrouted proxy rule, redirect logic an attacker can influence) could capture that material. A server that is itself malicious gains nothing this way; it already receives your requests directly.

### Am I affected? Yes, if your code uses the SDK as an MCP client over HTTP through any of:

- `streamable_http_client` (on 2.x also `Client` given an `http://` or `https://` URL) - `streamablehttp_client` or `sse_client` - `ClientSessionGroup` with HTTP server parameters - the internal `create_mcp_http_client` helper

This includes an HTTP client you passed in yourself with `follow_redirects=True`.

Affected versions: 1.8.0 through 1.29.1, and 2.0.0 through 2.1.1.

Not affected:

- MCP servers - stdio clients - a client you supplied with `follow_redirects` left at its def…