CVE-2021-27291 is a high-severity (CVSS 7.5) CWE-1333 vulnerability in pygments. 1 public exploit reference exists, so weaponization risk is real. A fix is available for pygments — see the affected versions and patch details below.
Pygments vulnerable to Regular Expression Denial of Service (ReDoS)
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-27291 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
In pygments 1.1+, fixed in 2.7.4, the lexers used to parse programming languages rely heavily on regular expressions. Some of the regular expressions have exponential or cubic worst-case complexity and are vulnerable to ReDoS. By crafting malicious input, an attacker can cause a denial of service.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | pygments | ≥ 1.1&&< 2.7.4 | 2.7.4pip install --upgrade 'pygments==2.7.4' |
Affected Products
debian linuxdebianfedorafedoraprojectpygmentspygmentsResearch 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 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-2021-27291 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.
| 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-2021-27291 in your dependencies?
Find it across PyPI, including transitive dependencies.