Description
fsspec is a specification and Python implementation framework for filesystem interfaces. From 0.9.0 until 2026.6.0, fsspec.implementations.reference.ReferenceFileSystem evaluates fields from Kerchunk reference JSON documents through unrestricted jinja2.Template(...).render(...) calls in _process_references1._render_jinja, _process_templates, and _process_gen in fsspec/implementations/reference.py. A document supplied inline or fetched from an attacker-controlled URL can provide template expressions that execute Python code when the reference filesystem is opened, including through consumers such as xarray, before referenced data is read. The _process_gen path is reached whenever a document includes a gen array, while the other paths depend on template-related options and values. This issue is fixed in version 2026.6.0.
References7
- github.com/fsspec/filesystem_spec/commit/86438783f93b1398ef245b…
- github.com/fsspec/filesystem_spec/commit/a1c16ab3f07f354aa371c3…
- github.com/fsspec/filesystem_spec/pull/2029
- github.com/fsspec/filesystem_spec/pull/2039
- github.com/fsspec/filesystem_spec/releases/tag/2026.6.0
- github.com/fsspec/filesystem_spec/security/advisories/GHSA-27vj…
- github.com/fsspec/filesystem_spec/security/advisories/GHSA-27vj…
Metrics
- CVSS 3.1
-
8.8
HIGH
Score source: CNA
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H - EPSS — exploit probability
- 0% percentile: 41 · as of 10 November 2026
- CWE
- CWE-94CWE-1336
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.