CVE-2026-104851: fsspec: Server-Side Template Injection in ReferenceFileSystem leads to Remote Code Execution

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.