JupyterLab: Cross-site scripting (XSS) in JupyterLab via crafted language package (jupyterlab.json)

Summary

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.

Timeline

1. Sep 29, 2026, 6:53 PM UTCCVE published
2. Sep 29, 2026, 8:00 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: total

3. Sep 29, 2026, 8:00 PM UTCRecord last updated

Affected versions

• jupyterlab jupyterlab

  From 3.0.0 · fixed in 4.5.11; From 4.6.0 · fixed in 4.6.4

• jupyterlab jupyterlite-core

  fixed in 0.8.4

• jupyterlabPyPI

  From 4.6.0 · fixed in 4.6.4

• jupyterlabPyPI

  From 3.0.0 · fixed in 4.5.11

• jupyterlite-corePyPI

  fixed in 0.8.4

• jupyterlabPyPI

  From 3.0.0 · fixed in 4.5.11; From 4.6.0 · fixed in 4.6.4

References 6