CVE-2026-78681 NLTK before 3.10.3 Entity Expansion DoS via ElementTree

High 8.7 Published 25 Aug 2026 Updated 26 Aug 2026

Attackers can cause denial of service in NLTK by sending crafted XML payloads that trigger excessive entity expansion 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-2026-78681?

CVE-2026-78681 is a high-severity denial of service vulnerability in nltk, published on 25 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 use xml.etree.ElementTree to parse XML in multiple modules, which honors entity declarations in document DTDs. Attackers can craft XML payloads with nested entity declarations that expand from hundreds of bytes to megabytes in memory, causing denial of service.

Weakness: CWE-776

Which versions are affected by CVE-2026-78681?

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-2026-78681?

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-2026-78681 being exploited?

There is no known exploitation of CVE-2026-78681 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-78681, ECHO-2035-7342-4fb1, GHSA-97qj-x29f-37w7, MINI-r7fm-rx7v-v666, PYSEC-2026-3748.