fsspec: Server-Side Template Injection in ReferenceFileSystem leads to Remote Code Execution

Why it is Amber

• CVSS 8.8 high
• Public exploit or proof of concept referenced
• Remotely exploitable without logging in
• On your watchlist: PyPI

Affected packages

References

• https://github.com/advisories/GHSA-27vj-qcqg-25rc
• https://github.com/fsspec/filesystem_spec/security/advisories/GHSA-27vj-qcqg-25rc
• https://nvd.nist.gov/vuln/detail/CVE-2026-104851
• https://github.com/fsspec/filesystem_spec/pull/2029
• https://github.com/fsspec/filesystem_spec/pull/2039
• https://github.com/fsspec/filesystem_spec/commit/86438783f93b1398ef245b92f0e6063b445b611c
• https://github.com/fsspec/filesystem_spec/commit/a1c16ab3f07f354aa371c38f7b1b07ea7fd4c5c8
• https://github.com/fsspec/filesystem_spec/releases/tag/2026.6.0
• https://github.com/advisories/GHSA-27vj-qcqg-25rc

Original 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.