CVE-2025-2099 is a medium-severity (CVSS 5.3) CWE-1333 vulnerability in transformers. A fix is available for transformers — see the affected versions and patch details below.
Regular Expression Denial of Service (ReDoS) in huggingface/transformers
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.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-2099.
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-2025-2099 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
transformersReal-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
A Regular Expression Denial of Service (ReDoS) exists in the preprocess_string() function of the transformers.testing_utils module. In versions before 4.50.0, the regex used to process code blocks in docstrings contains nested quantifiers that can trigger catastrophic backtracking when given inputs with many newline characters. An attacker who can supply such input to preprocess_string() (or code paths that call it) can force excessive CPU usage and degrade availability.
Fix: released in 4.50.0, which rewrites the regex to avoid the inefficient pattern. ([GitHub][1])
- Affected:
< 4.50.0 - Patched:
4.50.0
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | transformers | all versions | 4.50.0pip install --upgrade 'transformers==4.50.0' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for transformers, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update transformers to 4.50.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-2099 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-2025-2099 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-2099. 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.
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Ansible Automation Platform 2.5 for RHEL 8 | ansible-automation-platform-25/lightspeed-chatbot-rhel8:2.5.250730-2 | RHSA-2025:12791 |
Frequently Asked Questions
Is CVE-2025-2099 in your dependencies?
O3 Security finds CVE-2025-2099 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.