Jupyter Server has an open redirection vulnerability in `next` query parameter
MediumCVE-2025-61669 · Published May 5, 2026 · updated Sep 10, 2026
### 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 sensi...
Affected versions
| Package | Affected | Fixed in |
|---|---|---|
| jupyter-server PyPI | < 2.18.0 | 2.18.0 |
Details and references
### 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.
- CVSS 4.0
- CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:N/SA:N
- Severity from
- GitHub (reviewed advisory)
- Weakness
- CWE-601
- Also known as
- CVE-2025-61669, PYSEC-2026-67
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