CVE-2025-61669: jupyter_server next parameter open redirect can redirect users to external domains

### Summary The `?next=...` URL query parameter has an open redirection vulnerability. In `jupyter_server<=2.17.0`, this URL query parameter allows redirection to arbitrary external domains, which can be exploited to facilitate phishing attacks on server users. ### Details The vulnerability is caused by insufficient validation in the `LoginFormHandler._redirect_safe()` method. - Source code reference: https://github.com/jupyter-server/jupyter_server/blob/987ebdd5e188cdc49751b01a0d6782d686492a53/jupyter_server/auth/login.py#L33-L76 This vulnerability was originally reported by Noriaki Iwasaki. All discovery credit goes to them. ### PoC 1. Navigate to `http://localhost:8888/login?next=///google.com` 2. Observe that the user is redirected to `google.com` despite it being an external domain. The external domain passed in the `?next` parameter may be replaced with a malicious lookalike to facilitate phishing attacks. Jupyter Server deployments served on a public domain are especially vulnerable, as `prod.company.com` may be redirected to a look-alike URL such as `prod.company.dev`. ### Impact This vulnerability affects all users, especially enterprise users who work with sensitive/confidential data. ### Patches Jupyter Server 2.18+ ### Workaround None.