Vulnerability · Published Aug 25, 2026 · Assigned by VulnCheck
CVE-2026-78681NLTK before 3.10.3 Entity Expansion DoS via ElementTree
High 7.5
In plain English
A high-severity security flaw in nltk from nltk. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 44% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Severity (CVSS 3.1)7.5High
Exploit odds (30 days)0.5%top 55.6% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | nltk | no fix listed | DEBIAN-CVE-2026-78681 |
| Debian:13 | nltk | no fix listed | DEBIAN-CVE-2026-78681 |
| Debian:14 | nltk | 3.10.3-1 | DEBIAN-CVE-2026-78681 |
| PyPI | nltk | 3.10.3 | GHSA-97qj-x29f-37w7 |
| PyPI | nltk | 3.10.3 | PYSEC-2026-3748 |
Affected products (as reported)
Weakness: CWE-776 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
References
- GitHub Security Advisory (GHSA-97qj-x29f-37w7) vendor-advisory
- VulnCheck Advisory: NLTK before 3.10.3 Entity Expansion DoS via ElementTree third-party-advisory