CVE-2023-41039 is a high-severity (CVSS 8.3) CWE-74 vulnerability in restrictedpython. A fix is available for restrictedpython — see the affected versions and patch details below.
Sandbox escape via various forms of "format" in RestrictedPython
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2023-41039.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2023-41039 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
restrictedpython🐍restrictedpythonReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Impact
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.
Patches
The issue will be fixed in 5.4 and 6.2.
Workarounds
There are no workarounds to fix the issue without upgrading.
References
- https://docs.python.org/3/library/stdtypes.html#str.format_map
- http://lucumr.pocoo.org/2016/12/29/careful-with-str-format/
- https://www.exploit-db.com/exploits/51580
For more information
If you have any questions or comments about this advisory:
- Open an issue in the RestrictedPython issue tracker
- Email us at [email protected]
Credits
Thanks for analysing and reporting the go to:
- Abhishek Govindarasu
- Ankush Menat
- Ward Theunisse
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | restrictedpython | all versions | 5.4pip install --upgrade 'restrictedpython==5.4' |
| 🐍PyPI | restrictedpython | ≥ 6.0&&< 6.2 | 6.2pip install --upgrade 'restrictedpython==6.2' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for restrictedpython, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update restrictedpython to 5.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-41039 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2023-41039 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2023-41039. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Frequently Asked Questions
Is CVE-2023-41039 in your dependencies?
O3 Security finds CVE-2023-41039 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.