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Advisory details: gitlab / pypi/nltk/CVE-2026-78681
Advisory ID gitlab/pypi/nltk/CVE-2026-78681
Aliases CVE-2026-78681
GHSA-97qj-x29f-37w7
Summary NLTK: Entity-expansion DoS (billion laughs) via remaining raw ElementTree parses Several XML parsing sites in NLTK still used `xml.etree.ElementTree` directly, which honours `<!ENTITY>` declarations in a document's internal DTD subset. A crafted document a few hundred bytes long can expand to megabytes in memory (each nesting level multiplies by ten), a denial-of-service. Affected call sites (<= 3.10.2): - `nltk.chunk.named_entity.load_ace_file` — parses ACE annotation XML - `nltk.internals.ElementWrapper` — converts any given string to an Element - `nltk.downloader` — `Package.fromxml`, `Collection.fromxml`, `_find_collections`, `_find_packages` libexpat 2.6.0 added an input-amplification cap, but it only engages above an activation threshold (~8 MiB output) and depends on whichever libexpat the interpreter links; builds against older libexpat have no cap at all. External entities are not resolved by ElementTree, so this is a memory-amplification DoS (CWE-776), not XXE/file disclosure. This completes the earlier defusedxml adoption that these sites were missed by. Fix routes all of them through a new `nltk.xmlsec` module that refuses entity declarations, preferring `defusedxml` and falling back to a standard-library `xml.parsers.expat` pre-scan when defusedxml is absent. ---
Status Published
Exploitability 0.5
Weighted Severity 0.0
Risk
Affected and Fixed Packages Package Details
Introduced and Fixed Package Commit Patches Package Commit Patches Details
Weaknesses (3)
CWE-776 Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')
CWE-937 OWASP Top Ten 2013 Category A9 - Using Components with Known Vulnerabilities
CWE-1035 OWASP Top Ten 2017 Category A9 - Using Components with Known Vulnerabilities
System Score Found at
cvssv3.1 https://gitlab.com/gitlab-org/advisories-community/-/blob/main/pypi/nltk/CVE-2026-78681.yml
Reference id Reference type URL
https://github.com/nltk/nltk/commit/e91789c9a043296ad04912ce171c22776d45963b
https://github.com/nltk/nltk/releases/tag/v3.10.3
https://github.com/nltk/nltk/security/advisories/GHSA-97qj-x29f-37w7
https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3748.yaml
https://www.vulncheck.com/advisories/nltk-before-entity-expansion-dos-via-elementtree
CVE-2026-78681 https://nvd.nist.gov/vuln/detail/CVE-2026-78681
GHSA-97qj-x29f-37w7 https://github.com/advisories/GHSA-97qj-x29f-37w7
No exploits are available.

No EPSS data available for this advisory.

Patch URL
There are no known patches.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Found at https://gitlab.com/gitlab-org/advisories-community/-/blob/main/pypi/nltk/CVE-2026-78681.yml
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none

There are no SSVC decisions available.