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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.8 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

EPSS

• CVE-2026-104851: 0.47% estimated exploitation activity in the next 30 days; percentile 41.09 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary fsspec.implementations.reference.ReferenceFileSystem parses a "references" JSON document (Kerchunk format) supplied either inline or via a URL. The parser renders fields from this JSON through un-sandboxed jinja2.Template(...).render(...) calls in three locations. An attacker who controls the JSON document — typically by hosting it at a URL that the victim opens with fsspec.filesystem("reference", fo=URL) or via xarray.opendataset("reference://...") — achieves arbitrary Python code execution on the victim machine, before any data is read. This mirrors the pattern of CVE-2024-34359 in llama-cpp-python, where externally-sourced template strings were rendered with the default unrestricted Jinja2 environment. Affected versions All versions of fsspec from 0.9.0 onward (vulnerable code introduced in commit 0fb8d56b684ee74ad9ad4587fd560bde1b116450, 2021-03-12). Confirmed on the latest released version 2025.10.0. Affected sinks All in fsspec/implementations/reference.py: | Sink |…

Published by the source

5 Oct 2026, 22:53 UTC

Last modified by the source

15 Oct 2026, 14:56 UTC

Source last checked

26 Oct 2026, 13:55 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-27vj-qcqg-25rc

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-27vj-qcqg-25rc

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-27vj-qcqg-25rc

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# Patch Next advisory brief: CVE-2026-104851

Advisory: fsspec: Server-Side Template Injection in ReferenceFileSystem leads to Remote Code Execution
IDs: CVE-2026-104851, GHSA-27vj-qcqg-25rc
Record: https://patchnext.com/advisories/cve-2026-104851 (revision 3, source checked 26 Oct 2026, 13:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fsspec 2026.6.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI fsspec: affected >= 0.9.0, < 2026.6.0; fixed in 2026.6.0

## Suggested inspection (read-only)
- `python -m pip show fsspec`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fsspec.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-27vj-qcqg-25rc (GitHub-reviewed advisory GHSA-27vj-qcqg-25rc lists affected ranges: PyPI fsspec >=0.9.0 <2026.6.0)
- Verified: https://osv.dev/vulnerability/GHSA-27vj-qcqg-25rc (OSV record GHSA-27vj-qcqg-25rc lists affected ranges: PyPI fsspec >=0.9.0 <2026.6.0 (fixed))

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