CVE-2025-66034 is a medium-severity (CVSS 6.3) CWE-91 vulnerability in fonttools. 1 public exploit reference exists, so weaponization risk is real. A fix is available for fonttools — see the affected versions and patch details below.
fontTools is Vulnerable to Arbitrary File Write and XML injection in fontTools.varLib
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-66034.
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-2025-66034 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 377,166 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
fonttoolsReal-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
fontTools is a library for manipulating fonts, written in Python. In versions from 4.33.0 to before 4.60.2, the fonttools varLib (or python3 -m fontTools.varLib) script has an arbitrary file write vulnerability that leads to remote code execution when a malicious .designspace file is processed. The vulnerability affects the main() code path of fontTools.varLib, used by the fonttools varLib CLI and any code that invokes fontTools.varLib.main(). This issue has been patched in version 4.60.2.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | fonttools | ≥ 4.33.0&&< 4.60.2 | 4.60.2pip install --upgrade 'fonttools==4.60.2' |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fonttools, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fonttools to 4.60.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-66034 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-2025-66034 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-66034. 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.
This issue is rated Moderate rather than Important because the exploitability hinges on several limiting technical factors despite the high integrity impact. The arbitrary file-write and XML-injection pathways are only reachable when a user or automated workflow locally processes a malicious .designspace file, giving…
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat OpenShift AI 2.25 | rhoai/odh-kserve-agent-rhel9:1770055852 | RHSA-2026:2106 |
| Red Hat OpenShift AI 2.25 | rhoai/odh-openvino-model-server-rhel9:1770621450 | RHSA-2026:2695 |
| Red Hat OpenShift AI 3.3 | rhoai/odh-kserve-agent-rhel9:1770754605 | RHSA-2026:3713 |
Frequently Asked Questions
Is CVE-2025-66034 in your dependencies?
O3 Security finds CVE-2025-66034 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.