CVE-2023-41039: Sandbox escape via various forms of "format" in RestrictedPython

Severity: high (CVSS 7.7 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2023-08-30 · updated 2026-06-17
Weakness: CWE-74

Description

RestrictedPython is a restricted execution environment for Python to run untrusted code. Python's "format" functionality allows someone controlling the format string to "read" all objects accessible through recursive attribute lookup and subscription from objects he can access. This can lead to critical information disclosure. With `RestrictedPython`, the format functionality is available via the `format` and `format_map` methods of `str` (and `unicode`) (accessed either via the class or its instances) and via `string.Formatter`. All known versions of `RestrictedPython` are vulnerable. This issue has been addressed in commit `4134aedcff1` which has been included in the 5.4 and 6.2 releases. Users are advised to upgrade. There are no known workarounds for this vulnerability.

Affected products

zopefoundation RestrictedPython: before 5.4; from 6.0 before 6.2 — fixed in 5.4, 6.2

How to fix

Upgrade to a fixed version: zopefoundation RestrictedPython 5.4, 6.2. Check the vendor advisory for mitigations.

References

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