CVE-2021-20270 is a high-severity (CVSS 7.5) CWE-835 vulnerability in pygments. A fix is available for pygments — see the affected versions and patch details below.
Infinite Loop in Pygments
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-20270 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
pygmentsReal-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
An infinite loop in SMLLexer in Pygments versions 1.5 to 2.7.3 may lead to denial of service when performing syntax highlighting of a Standard ML (SML) source file, as demonstrated by input that only contains the "exception" keyword.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | pygments | ≥ 1.5&&< 2.7.4 | 2.7.4pip install --upgrade 'pygments==2.7.4' |
Affected Products
debian linuxdebianfedorafedoraprojectpygmentspygmentsenterprise linuxredhatopenshift container platformredhatopenstack platformredhatDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for pygments, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update pygments to 2.7.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-20270 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
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.
In OpenShift Container Platform 3.11, the vulnerable version of python-pygments is embedded in the google-cloud-sdk package, which is shipped in the openshift-ansible container (aos3-installation-container). As the access to the openshift-ansible container is restricted only to cluster administrators, this component…
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Automation Hub 4.2 for RHEL 7 | automation-hub-0:4.2.2-1.el7pc | RHSA-2021:0781 |
| Red Hat Enterprise Linux 8 | python36:3.6-8050020210811103506.982725ab | RHSA-2021:4150 |
| Red Hat Enterprise Linux 8 | python27:2.7-8050020210811095446.3e7ace8b | RHSA-2021:4151 |
| Red Hat Enterprise Linux 8 | resource-agents-0:4.1.1-98.el8 | RHSA-2021:4139 |
| Red Hat Software Collections for Red Hat Enterprise Linux 7 | python27-babel-0:0.9.6-10.el7 | RHSA-2021:3252 |
Frequently Asked Questions
Is CVE-2021-20270 in your dependencies?
Find it across PyPI, including transitive dependencies.