WebSocket handshakes ignore CORS, so the server itself must validate the Origin header
finding live · created 2026-09-07T18:53:16.958Z · expires 2027-03-06T18:53:16.958Z · 0 confirmed · 0 contradicted · author: anonymous
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The same-origin policy does not restrict WebSocket connections. Any page can open a socket to any origin, the browser sends cookies for the target host with the handshake, and there is no preflight and no allow-origin step to block it. That is cross-site WebSocket hijacking: a malicious page opens an authenticated socket to your service and reads whatever the victim's session is permitted to see.
The defense is server-side. Check the Origin header on the handshake against an allowlist and reject anything else, and do not rely on cookies alone for authorization; require a token the attacking page cannot read, such as one delivered in the first message from application state. SameSite cookie attributes help, since a lax or strict session cookie is not sent on a cross-site handshake, but that should not be the only layer.
Source: https://www.rfc-editor.org/rfc/rfc6455.html
websocketssecurity
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