CVE-2026-104851 in filesystem_spec
Summary
by MITRE • 10/02/2026
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.
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Analysis
by VulDB Data Team • 10/02/2026
The fsspec library serves as a foundational specification and Python implementation framework for filesystem interfaces, enabling unified access to various storage backends such as local disks, cloud object stores, and remote HTTP resources. Within this ecosystem, the ReferenceFileSystem class plays a critical role in handling Kerchunk reference JSON documents, which are commonly used to describe virtual datasets by mapping logical paths to physical locations without requiring data duplication. However, versions of fsspec ranging from 0.9.0 up to version 2026.6.0 contain a severe server-side request forgery and remote code execution vulnerability rooted in the unsafe evaluation of Jinja2 template expressions within these reference documents. This flaw allows an attacker who can supply or influence the content of a Kerchunk JSON file to execute arbitrary Python code on the host system when the filesystem is initialized or accessed by legitimate applications such as xarray for data analysis.
The technical root cause lies in the internal processing methods _process_references1._render_jinja, _process_templates, and _process_gen located within fsspec/implementations/reference.py. When a ReferenceFileSystem instance processes a reference document, it invokes Jinja2's Template.render() method without adequate sanitization or sandboxing of the template context. Specifically, if the JSON document contains fields that are interpreted as Jinja2 templates, these expressions are evaluated directly against Python objects available in the rendering scope. This includes access to built-in functions and modules, which can be exploited to instantiate arbitrary classes, import standard library modules like os or subprocess, and execute system commands. The vulnerability is triggered through multiple code paths depending on the structure of the input document; the _process_gen path activates when a gen array is present in the JSON, while other paths depend on specific template-related options and values being set within the configuration.
The operational impact of this vulnerability is severe because it facilitates remote code execution with the privileges of the user running the Python application consuming fsspec. Attackers can craft malicious Kerchunk reference files that appear benign but contain hidden Jinja2 payloads designed to exfiltrate data, install backdoors, or pivot further into internal networks upon being opened by vulnerable software stacks. This risk is particularly acute in scientific computing and big data environments where datasets are frequently shared via URLs or embedded directly in scripts using libraries like xarray. Since the vulnerability occurs during the initialization phase of reading metadata rather than at runtime data processing, it affects a wide range of downstream applications that rely on fsspec for transparent file access without realizing they are parsing untrusted template logic.
This issue is classified under CWE-94 as Improper Control of Generation of Code (Code Injection) and aligns with MITRE ATT&CK technique T1059, specifically Command and Scripting Interpreter subcategories such as Python or Jinja2 Template injection. The vulnerability highlights the dangers of using templating engines for data configuration without strict isolation mechanisms. To mitigate this risk, organizations must immediately upgrade to fsspec version 2026.6.0 or later where the unsafe rendering logic has been remediated by restricting template context access and validating input fields before evaluation. In environments where upgrading is not immediately feasible, administrators should enforce strict allowlists for reference file sources, avoid loading Kerchunk documents from untrusted external URLs, and consider implementing sandboxing solutions like PyPySandbox or restricted execution contexts to limit the impact of any potential code injection attempts until a permanent patch can be deployed across all dependent systems.