CVE-2021-3828 is a high-severity (CVSS 7.5) CWE-1333 vulnerability in nltk. 1 public exploit reference exists, so weaponization risk is real. A fix is available for nltk — see the affected versions and patch details below.
NLTK Vulnerable to REDoS
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-3828 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
nltkReal-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
The nltk package is vulnerable to ReDoS (regular expression denial of service). An attacker that is able to provide as an input to the [_read_comparison_block()(https://github.com/nltk/nltk/blob/23f4b1c4b4006b0cb3ec278e801029557cec4e82/nltk/corpus/reader/comparative_sents.py#L259) function in the file nltk/corpus/reader/comparative_sents.py may cause an application to consume an excessive amount of CPU.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | nltk | all versions | 3.6.4pip install --upgrade 'nltk==3.6.4' |
Affected Products
nltknltkResearch 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 nltk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update nltk to 3.6.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3828 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.
Frequently Asked Questions
Is CVE-2021-3828 in your dependencies?
Find it across PyPI, including transitive dependencies.