CVE-2026-102830: JupyterLab: Cross-site scripting (XSS) in JupyterLab via crafted language package (jupyterlab.json)
JupyterLab is an extensible environment for interactive and reproducible computing, based on the Jupyter Notebook Architecture. From JupyterLab 3.0.0 until 4.5.11 and 4.6.4, and in JupyterLite Core 0.8.3 and earlier, the Plural-Forms header in a selected third-party language pack can append JavaScript after a valid plural rule because prefix-only regular-expression validation accepts a matching prefix without requiring the entire header to match. JupyterLab passes the accepted expression to new Function, so loading the catalogue and translating a plural string executes the appended code in the authenticated JupyterLab origin. Where Jupyter Server kernels, terminals, and APIs are exposed, the code can use authenticated server APIs to read or modify files and run code. Impact is much more limited in JupyterLite because it typically lacks most exposed Jupyter Server surfaces. The default English locale is unaffected because it does not load a translation catalogue. This issue is fixed in JupyterLab 4.5.11 and 4.6.4 and JupyterLite Core 0.8.4.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
JupyterLabto a version that resolves this vulnerability.Fixed in 4.5.11 - Upgrade
Upgrade
JupyterLabto a version that resolves this vulnerability.Fixed in 4.6.4 - Upgrade
Upgrade
JupyterLite Coreto a version that resolves this vulnerability.Fixed in 0.8.4
Event History
Frequently Asked Questions
Which deployments have the greatest potential impact?
JupyterLab deployments where Jupyter Server kernels, terminals, or APIs are exposed have the greatest impact. Code executed through the issue runs in the authenticated JupyterLab origin and can use those authenticated server APIs to read or modify files and run code.
What conditions are needed for exploitation?
A crafted third-party language pack must be selected, its catalogue must be loaded, and a plural string must be translated. The malicious code is appended after an otherwise valid Plural-Forms rule.
Are default installations affected?
The default English locale is unaffected because it does not load a translation catalogue. Exposure depends on selecting a third-party language pack.
What can be done before an update is available?
Use the default English locale and avoid selecting third-party language packs, particularly untrusted ones. This prevents the affected translation catalogue path from being used.
Which releases contain the fix?
The issue is fixed in JupyterLab 4.5.11 and 4.6.4, and in JupyterLite Core 0.8.4.