CVE-2026-80205 NLTK before 3.10.0 ReDoS via Text.findall() unvalidated regex

High 8.7 Published 26 Aug 2026 Updated 1 Sep 2026

Attackers can cause denial of service in NLTK applications by supplying malicious regex patterns to findall methods, leading to CPU exhaustion.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-80205?

CVE-2026-80205 is a high-severity denial of service vulnerability in nltk, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

NLTK versions before 3.10.0 contain a regular expression denial of service vulnerability in Text.findall() and TokenSearcher.findall() methods that accept user-supplied regular expressions without validation or timeout. Attackers can supply crafted regex patterns that cause catastrophic backtracking, resulting in indefinite CPU saturation and denial of service to all users of the Python process.

Weakness: CWE-1333

Which versions are affected by CVE-2026-80205?

3 open source packages ship an affected version, including nltk. It affects 1 product.

Package	Affected	Fixed in
`nltk` PyPI	All versions	`3.10.0`
`nltk` PyPI	All versions	`3.10.0`
`nltk` PyPI	All versions	`3.10.0`

Products include Nltk Nltk.

How do I fix CVE-2026-80205?

Upgrade nltk to 3.10.0 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-2026-80205 being exploited?

There is no known exploitation of CVE-2026-80205 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-80205, ECHO-fcb3-e4dd-f503, GHSA-rrv8-h7p8-rx55, MINI-m8f9-f95j-84f2, PYSEC-2026-3750, UBUNTU-CVE-2026-80205.