7.6

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2026-40110

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 allow_origin_pat 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.

(GitHub Advisory)

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CVE-2026-40110

CVE-2026-40110:

7.6

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

SSVC / BOD 26-04 ↗

Exploitation NoneAutomatable NoTech Impact Total

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Package Name	Ecosystem	Vulnerable Versions	First Patched Version
jupyter-server	pip	<= 2.17.0	2.18.0

Technical Details

Vulnerability Intelligence
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Root Cause Analysis

The vulnerability is a bypass of the Cross-Origin Resource Sharing (CORS) origin validation in Jupyter Server. The allow_origin_pat setting is intended to be a regex pattern to validate the Origin header of incoming requests. However, the server was using Python's re.match() function for this validation. re.match() only checks for a match at the beginning of the string, not the entire string.

This means if allow_origin_pat was set to, for example, https://trusted.example.com, an attacker could register a domain like https://trusted.example.com.evil.com and it would pass the validation, as re.match() would find a match at the start of the string. This would allow the attacker's malicious site to make cross-origin requests to the Jupyter server, potentially leading to the execution of unauthorized code or theft of sensitive data.

The patch replaces re.match() with re.fullmatch() (wrapped in a helper function _origin_matches_pat and later origin_matches_pat). re.fullmatch() ensures that the entire Origin header string matches the pattern, thus preventing this bypass.

The vulnerable functions are those that were performing this flawed origin check. The analysis of the patch commits reveals that the check was performed in multiple handlers:

LoginHandler._redirect_safe: when handling login redirects.
JupyterHandler.set_cors_headers: when setting CORS headers for a request.
JupyterHandler.check_origin: when checking the origin for a standard request.
JupyterHandler.check_referer: when checking the referer header.
WebSocketMixin.check_origin: when checking the origin for a WebSocket connection.

An exploit would involve a user visiting a malicious website which then makes cross-origin requests to a vulnerable Jupyter Server instance. The profiler on the server would show calls to one of the identified vulnerable functions, which would incorrectly grant access to the malicious origin.

Vulnerable functions

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WAF Rule

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Reasoning

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