CVE-2026-81725 NLTK before 3.10.3 Regular Expression Denial of Service via Pl196xCorpusReader
Attackers can cause denial of service in NLTK by supplying malformed TEI blocks that trigger quadratic CPU consumption via regex patterns.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-81725?
CVE-2026-81725 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 before 3.10.3 contains a regular expression denial of service vulnerability in Pl196xCorpusReader that allows attackers to cause quadratic CPU consumption by supplying malformed TEI blocks with many unmatched opening tags. Attackers can exploit lazy regex patterns in the read_block method through public APIs like words() and tagged_words() to force repeated rescans and achieve near-quadratic runtime growth.
Weakness: CWE-400
Which versions are affected by CVE-2026-81725?
3 open source packages ship an affected version, including nltk. It affects 1 product.
| 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 |
Products include Nltk Nltk.
How do I fix CVE-2026-81725?
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-2026-81725 being exploited?
There is no known exploitation of CVE-2026-81725 in the wild and no public exploit code.
Other vulnerabilities in nltk
- CVE-2026-0848 Arbitrary Code Execution in NLTK StanfordSegmenter via Untrusted JAR Loading
- CVE-2025-14009 Zip Slip Vulnerability in nltk/nltk Leading to Remote Code Execution
- CVE-2026-78683 NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization
- CVE-2026-79657 NLTK before 3.10.3 Remote Code Execution via Unsafe Pickle Deserialization
- CVE-2026-79675 NLTK before 3.10.3 JVM Argument Injection via Per-Call Options
Sources
Also known as DEBIAN-CVE-2026-81725, ECHO-c4c0-fd69-5811, GHSA-8mpw-7fpc-4gqj, MINI-v5qj-7q8q-jhfc, PYSEC-2026-3752, UBUNTU-CVE-2026-81725.