JupyterHub has cross-origin form POSTs bypass XSRF (CWE-352)

Summary JupyterHub's XSRF protection updated in 4.1.0 inappropriately treated requests with Sec-Fetch-Mode: no-cors as same-origin requests, which they are not, bypassing XSRF checks. The JSON API is not affected, only HTTP form endpoints, such as /hub/spawn and /hub/accept-share, meaning attacke...