RestrictedPython: Sandbox escape via string.Formatter field resolution

published 2026-09-16 54 d ago modified 2026-10-01 38 d ago also GHSA-hp3v-5vw7-fx9w , PYSEC-2026-4161 , EUVD-2026-80780

cvss: 8.4; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.6%; EPSS 0.6% chance of exploitation in 30 days, percentile 50%
public exploits: 0; none known

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.

Patch / fix

fixed in 1 distros 1 links 2

pypi restrictedpython 8.4

restrictedpython Debian 14 8.4-1

debian advisories DEBIAN-CVE-2026-76825

CISA SSVC

Exploitation: none
Automatable: no
Technical impact: total
Assessed: 53 d ago (2026-09-16)

Exploit prediction

0.62% chance of exploitation in the next 30 days, higher than 50% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	8.4	CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:H/A:H
OSV	3.1	8.4	CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:H/A:H
GitHub	3.1	8.4	CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:H/A:H

Affected packages

ecosystem	package	affected	fixed in
pypi	restrictedpython	<8.4	8.4

Affected products

zopefoundation RestrictedPython

Classification

Weakness

CWE-200 Exposure of Sensitive Information to an Unauthorized Actor
CWE-470 Use of Externally-Controlled Input to Select Classes or Code
CWE-680 Integer Overflow to Buffer Overflow
CWE-693 Protection Mechanism Failure

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary