Permissive Regular Expression Affecting jupyter-server package, versions [1.12.0,2.18.0)
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-PYTHON-JUPYTERSERVER-17220130
- published7 Jun 2026
- disclosed3 Jun 2026
- creditVinayak Mishra
Introduced: 3 Jun 2026
CVE-2026-6657 (opens in a new tab)How to fix?
Upgrade jupyter-server to version 2.18.0 or higher.
Overview
Affected versions of this package are vulnerable to Permissive Regular Expression in the use of re.match() to verify allow_origin_pat values. This allows an attacker to bypass intended CORS restrictions and gain unauthorized access to sensitive API responses. Code execution is possible by convincing an authenticated user to visiting a malicious origin while the non-default CORS regex allowlist is enabled. re.match() is based on prefix matching, so a malicious domain whose lefthand segments are a trusted domain passes.
PoC
https://trusted.example.com/login?next=https://trusted.example.com.evil.com/capture