CVE-2026-81723 NLTK before 3.10.3 Quadratic CPU Exhaustion via XMLCorpusView
Attackers can cause denial of service in NLTK by providing malformed XML files that trigger quadratic CPU exhaustion during parsing.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-81723?
CVE-2026-81723 is a medium-severity denial of service vulnerability in nltk, published on 27 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
NLTK versions before 3.10.3 contain a quadratic CPU exhaustion vulnerability in XMLCorpusView._read_xml_fragment() that rescans accumulated XML fragments on every 1 KiB block read. Attackers can provide malformed XML corpus files to cause severe CPU consumption and denial of service through affected readers like BNCCorpusReader.
Weakness: CWE-400
Which versions are affected by CVE-2026-81723?
3 open source packages ship an affected version, including nltk.
| Package | Affected | Fixed in |
|---|---|---|
nltk PyPI | All versions | 3.10.3 |
nltk PyPI | All versions | 3.10.3 |
nltk PyPI | All versions | 3.10.3 |
How do I fix CVE-2026-81723?
Upgrade nltk to 3.10.3 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-81723 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-81723 exists.
Sources
Also known as DEBIAN-CVE-2026-81723, ECHO-3ec1-214e-209e, GHSA-vp2x-qp44-57v7, MINI-hvrv-3qj9-5jpj, PYSEC-2026-3871, UBUNTU-CVE-2026-81723.