MCP SDKs: improper authorization

### Summary In affected versions, the SDK's server-side bearer authentication had no way to check that a token was issued for your server (its audience). Where one authorization server issues tokens for several services, a valid token for another service was also accepted by your MCP server. ### Am I affected? Yes, if your HTTP server uses the SDK's bearer authentication (`auth=AuthSettings(...)` with `token_verifier=`), its authorization server also issues tokens for other services, and your `TokenVerifier` does not check the token's audience (`aud`) itself. Affected versions: - `mcp` 1.10.0 through 1.29.1 - `mcp` 2.0.0 through 2.1.1 ### Fix Upgrade to: - 1.x: `mcp` 1.30.0 or later - 2.x: `mcp` 2.2.0 or later **Upgrading alone changes nothing.** The patched versions add a `validate_token_resource` setting that is off unless you set it. To turn the check on, make both of these changes together: 1. Set `validate_token_resource=True` in `AuthSettings`, with `resource_server_url` set to the audience your authorization server puts in tokens for your server. 2. Return the token's audience from `verify_token()` as `AccessToken.resource`. A token for another service then gets `401`. With only the first change, every request gets `401`. If your `TokenVerifier` already checks the audience, set `validate_token_resource=False` to silence the deprecation warning. The setting will default to `True` in 3.0. If you cannot upgrade yet, reject any token whose `aud` does not include your server inside `verify_token()`. That closes the issue on every version.

CVSS 3.1

CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N

Severity from

GitHub (reviewed advisory)

Weakness

CWE-863

Also known as

CVE-2026-116684

• github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-w4fh-qvv9-3v23
• github.com/modelcontextprotocol/python-sdk/pull/3447
• github.com/modelcontextprotocol/python-sdk/pull/3451
• github.com/modelcontextprotocol/python-sdk/commit/0c9136841ff092987a20700ff9c2c9789bda2f40
• github.com/modelcontextprotocol/python-sdk/commit/8c2fa6eabacde869d75568e33b9d466b58c2f512
• github.com/modelcontextprotocol/python-sdk
• github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0
• github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0