CVE-2026-40110: jupyter-server CORS origin validation bypass via unanchored regex in allow_origin_pat
Jupyter Server is the backend for Jupyter web applications. In versions 2.17.0 and earlier, the Origin header validation uses Python's re.match() to check incoming origins against the alloworiginpat configuration value. Because re.match() only anchors at the start of the string and does not require a full match, a pattern intended to match only a trusted domain (e.g., trusted.example.com) will also match any origin that begins with that domain followed by additional characters (e.g., trusted.example.com.evil.com). An attacker who controls such a domain can bypass the CORS origin restriction and make cross-origin requests to the Jupyter Server API from an untrusted site. This issue has been fixed in version 2.18.0.
Other sources
Jupyter Server uses re.match() to validate the Origin header against the alloworiginpat configuration.
Since re.match() only anchors at the start of the string, an attacker who controls a domain like http://trusted.example.com.evil.com/ passes validation against a pattern intended to match only trusted.example.com.
Impact
<=2.17.0
Patches
057869a327c46730afede3eab0ca2d2e3e74acea, 49b34392feaa97735b3b777e3baf8f22f2a14ed8
Workarounds
Wrap your alloworiginpat value with ^ and $
References
https://github.com/jupyter-server/jupyterserver/pull/603 https://docs.python.org/3/library/re.html#re.fullmatch https://docs.python.org/3/library/re.html#re.match
— GitHub
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/jupyter-serverto a version that resolves this vulnerability.Fixed in 2.18.0 - Upgrade
Upgrade
jupyter-serverto a version that resolves this vulnerability.Fixed in 2.18.0 - Configuration
Update the Origin allowlist regex by anchoring it to the start and end: wrap your allow_origin_pat value with ^ and $ to prevent bypass from origins that begin with the trusted domain but include additional characters.
Jupyter Server allow_origin_pat = Wrap the allow_origin_pat pattern with ^ and $ (e.g., use ^trusted\.example\.com$)
Event History
Frequently Asked Questions
What is the severity of CVE-2026-40110?
CVE-2026-40110 is classified as a high severity vulnerability due to potential unauthorized resource access.
How do I fix CVE-2026-40110?
To fix CVE-2026-40110, upgrade jupyter-server to version 2.18.0 or later.
What software versions are affected by CVE-2026-40110?
CVE-2026-40110 affects jupyter-server versions 2.17.0 and earlier.
What type of vulnerability is CVE-2026-40110?
CVE-2026-40110 is a CORS origin validation bypass vulnerability.
What does the CVE-2026-40110 vulnerability impact?
CVE-2026-40110 impacts the validation of the Origin header in requests to the Jupyter Server backend, leading to possible security bypass.