{"id":"CVE-2026-66393","aliases":["DEBIAN-CVE-2026-66393","EUVD-2026-64310","GHSA-cv2g-m8rr-888c","GHSA-rf74-v2fm-23pw","PYSEC-2026-3724","UBUNTU-CVE-2026-66393"],"status":"published","title":"NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder","description":"NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.","severity":{"version":"4.0","score":8.7,"vector":"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","level":"high","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-11","score":0.00521,"percentile":0.4471,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-674"],"published":"2026-03-18T20:17:43Z","modified":"2026-10-28T02:31:05Z","fixes":[{"ecosystem":"PyPI","package":"nltk","fixed_in":["3.9.4"],"sources":["ghsa","osv"]},{"ecosystem":"Debian:14","package":"nltk","fixed_in":["3.10.0-1"],"sources":["osv"]}],"affected":[{"source":"ghsa","ecosystem":"PyPI","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"3.9.3"},{"fixed":"3.9.4"}]}]},{"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":"3.10.0-1"}]}],"versions":["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":"3.9.4"}]}],"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","3.2","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","3.9b1"]},{"source":"osv","ecosystem":"PyPI","package":"nltk","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.9.4"}]}],"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","3.2","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","3.9b1"]},{"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-rf74-v2fm-23pw","tags":["vendor-advisory","exploit","Exploit","Mitigation","Vendor Advisory","advisory","web","evidence","report"]},{"url":"https://www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoder","tags":["third-party-advisory","Third Party Advisory","advisory","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-66393","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-cv2g-m8rr-888c"},{"url":"https://github.com/nltk/nltk/commit/00cdcd392142e6c745e7120c8d50a24127df5fad","tags":["web"]},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3724.yaml","tags":["web"]},{"url":"https://github.com/advisories/GHSA-rf74-v2fm-23pw"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/66xxx/CVE-2026-66393.json","tags":["advisory"]},{"url":"https://security-tracker.debian.org/tracker/CVE-2026-66393","tags":["advisory"]},{"url":"https://github.com/nltk/nltk","tags":["package"]},{"url":"https://ubuntu.com/security/CVE-2026-66393","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-66393","tags":["report"]}],"sources":{"cve":[{"id":"CVE-2026-66393","title":"NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder","description":"NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.","status":"published","published":"2026-08-22T14:12:42.072Z","modified":"2026-08-26T17:55:08.015Z","cvss":[{"version":"4.0","score":8.7,"vector":"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","level":"high","assessor":"cna"},{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"cna"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T14:20:13.272106Z"}],"euvd":[{"id":"EUVD-2026-64310","description":"NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.","status":"published","published":"2026-08-22T14:12:42Z","modified":"2026-08-26T17:55:08Z","cvss":[{"version":"4.0","score":8.7,"vector":"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","level":"high"}],"updated_at":"2026-11-02T18:01:30.08255Z"}],"ghsa":[{"id":"GHSA-cv2g-m8rr-888c","title":"Duplicate Advisory: Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS","description":"## Duplicate Advisory\n\nThis advisory has been withdrawn because it is a duplicate of GHSA-rf74-v2fm-23pw. This link is maintained to preserve external references.\n\n## Original Description\nNLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.","status":"withdrawn","published":"2026-08-22T15:31:03Z","modified":"2026-09-02T15:33:18Z","cvss":[{"version":"4.0","score":8.7,"vector":"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/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":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T14:41:10.121131Z"},{"id":"GHSA-rf74-v2fm-23pw","title":"Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS","description":"### Summary\n`JSONTaggedDecoder.decode_obj()` in `nltk/jsontags.py` calls itself \nrecursively without any depth limit. A deeply nested JSON structure \nexceeding `sys.getrecursionlimit()` (default: 1000) will raise an \nunhandled `RecursionError`, crashing the Python process.\n\n### Affected code\nFile: `nltk/jsontags.py`, lines 47–52\n```python\n@classmethod\ndef decode_obj(cls, obj):\n    if isinstance(obj, dict):\n        obj = {key: cls.decode_obj(val) for (key, val) in obj.items()}\n    elif isinstance(obj, list):\n        obj = list(cls.decode_obj(val) for val in obj)\n```\n\n### Proof of Concept\n```python\nimport sys, json\nfrom nltk.jsontags import JSONTaggedDecoder\n\ndepth = sys.getrecursionlimit() + 50  # e.g. 1050\npayload = '{\"x\":' * depth + \"null\" + \"}\" * depth\n\n# Raises RecursionError, crashing the process\njson.loads(payload, cls=JSONTaggedDecoder)\n```\n\n### Impact\nAny code path that passes externally-supplied JSON to \n`JSONTaggedDecoder` is vulnerable to denial of service.\nThe severity depends on whether such a path exists in the \ncalling code (e.g. `nltk/data.py`).\n\n### Suggested Fix\nAdd a depth parameter with a hard limit:\n```python\n@classmethod\ndef decode_obj(cls, obj, _depth=0):\n    if _depth \u003e 100:\n        raise ValueError(\"JSON nesting too deep\")\n    if isinstance(obj, dict):\n        obj = {key: cls.decode_obj(val, _depth + 1) \n               for (key, val) in obj.items()}\n    elif isinstance(obj, list):\n        obj = list(cls.decode_obj(val, _depth + 1) for val in obj)\n```","status":"published","published":"2026-03-18T20:17:43Z","modified":"2026-09-02T15:33:52Z","cvss":[{"version":"4.0","score":5.1,"vector":"CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N","level":"medium"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T14:41:10.121131Z"}],"nvd":[{"id":"CVE-2026-66393","description":"NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.","status":"published","published":"2026-08-22T15:16:19.633Z","modified":"2026-10-14T13:10:00.187Z","cvss":[{"version":"4.0","score":8.7,"vector":"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/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":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"secondary"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T14:12:40.665005Z"}],"osv":[{"id":"CVE-2026-66393","title":"NLTK before 3.9.4 Denial of Service via JSONTaggedDecoder","description":"NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.","status":"published","published":"2026-08-22T14:12:42.072Z","modified":"2026-10-28T02:31:05.012152109Z","cvss":[{"version":"4.0","score":8.7,"vector":"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","level":"high"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T15:45:14.46177Z"},{"id":"DEBIAN-CVE-2026-66393","description":"NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.","status":"published","published":"2026-08-22T15:16:19.633Z","modified":"2026-10-15T09:47:40.340744328Z","cvss":[{"version":"4.0","score":8.7,"vector":"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/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":"2026-11-02T15:45:58.699079Z"},{"id":"GHSA-cv2g-m8rr-888c","title":"Duplicate Advisory: Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS","description":"## Duplicate Advisory\n\nThis advisory has been withdrawn because it is a duplicate of GHSA-rf74-v2fm-23pw. This link is maintained to preserve external references.\n\n## Original Description\nNLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.","status":"withdrawn","published":"2026-08-22T15:31:03Z","modified":"2026-09-10T03:51:13.899979725Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"},{"version":"4.0","score":8.7,"vector":"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/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-674"],"updated_at":"2026-11-02T15:46:45.639885Z"},{"id":"GHSA-rf74-v2fm-23pw","title":"Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS","description":"### Summary\n`JSONTaggedDecoder.decode_obj()` in `nltk/jsontags.py` calls itself \nrecursively without any depth limit. A deeply nested JSON structure \nexceeding `sys.getrecursionlimit()` (default: 1000) will raise an \nunhandled `RecursionError`, crashing the Python process.\n\n### Affected code\nFile: `nltk/jsontags.py`, lines 47–52\n```python\n@classmethod\ndef decode_obj(cls, obj):\n    if isinstance(obj, dict):\n        obj = {key: cls.decode_obj(val) for (key, val) in obj.items()}\n    elif isinstance(obj, list):\n        obj = list(cls.decode_obj(val) for val in obj)\n```\n\n### Proof of Concept\n```python\nimport sys, json\nfrom nltk.jsontags import JSONTaggedDecoder\n\ndepth = sys.getrecursionlimit() + 50  # e.g. 1050\npayload = '{\"x\":' * depth + \"null\" + \"}\" * depth\n\n# Raises RecursionError, crashing the process\njson.loads(payload, cls=JSONTaggedDecoder)\n```\n\n### Impact\nAny code path that passes externally-supplied JSON to \n`JSONTaggedDecoder` is vulnerable to denial of service.\nThe severity depends on whether such a path exists in the \ncalling code (e.g. `nltk/data.py`).\n\n### Suggested Fix\nAdd a depth parameter with a hard limit:\n```python\n@classmethod\ndef decode_obj(cls, obj, _depth=0):\n    if _depth \u003e 100:\n        raise ValueError(\"JSON nesting too deep\")\n    if isinstance(obj, dict):\n        obj = {key: cls.decode_obj(val, _depth + 1) \n               for (key, val) in obj.items()}\n    elif isinstance(obj, list):\n        obj = list(cls.decode_obj(val, _depth + 1) for val in obj)\n```","status":"published","published":"2026-03-18T20:17:43Z","modified":"2026-09-10T03:50:40.818763364Z","cvss":[{"version":"4.0","score":5.1,"vector":"CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N","level":"medium"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T15:46:55.877614Z"},{"id":"PYSEC-2026-3724","description":"NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.","status":"published","published":"2026-08-22T15:16:19.633Z","modified":"2026-08-28T02:30:02.708784842Z","cvss":[{"version":"4.0","score":8.7,"vector":"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/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-2026-66393","description":"NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.","status":"published","published":"2026-08-22T15:16:00Z","modified":"2026-09-02T19:36:36.299263283Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"},{"version":"4.0","score":8.7,"vector":"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","level":"high"}],"updated_at":"2026-11-02T16:56:53.710209Z"}]}}