{"id":"CVE-2026-81724","aliases":["DEBIAN-CVE-2026-81724","EUVD-2026-67074","GHSA-cw6x-m8jw-qmrh","GHSA-pf76-q698-37v8","PYSEC-2026-3739","UBUNTU-CVE-2026-81724"],"status":"published","title":"NLTK before 3.10.3 Denial of Service via Uncontrolled Recursion","description":"NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.","severity":{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-12","score":0.00456,"percentile":0.40055,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-674"],"published":"2026-08-27T14:51:17Z","modified":"2026-10-28T02:31:02Z","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-cw6x-m8jw-qmrh","tags":["vendor-advisory","exploit","Exploit","Vendor Advisory","advisory","web","evidence","report"]},{"url":"https://www.vulncheck.com/advisories/nltk-before-3.10.3-denial-of-service-via-uncontrolled-recursion","tags":["third-party-advisory","Third Party Advisory","advisory","web","report"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-81724","tags":["advisory"]},{"url":"https://github.com/nltk/nltk/commit/43c7b78cc8ea37e5cd3a129e27e32c415ea21cf1","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-2026-3739.yaml","tags":["web"]},{"url":"https://github.com/advisories/GHSA-cw6x-m8jw-qmrh"},{"url":"https://github.com/advisories/GHSA-pf76-q698-37v8"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/81xxx/CVE-2026-81724.json","tags":["advisory"]},{"url":"https://security-tracker.debian.org/tracker/CVE-2026-81724","tags":["advisory"]},{"url":"https://github.com/nltk/nltk","tags":["package"]},{"url":"https://ubuntu.com/security/CVE-2026-81724","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-81724","tags":["report"]}],"sources":{"cve":[{"id":"CVE-2026-81724","title":"NLTK before 3.10.3 Denial of Service via Uncontrolled Recursion","description":"NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.","status":"published","published":"2026-08-27T14:51:17.358Z","modified":"2026-08-27T18:29:04.021Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N","level":"medium","assessor":"cna"},{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium","assessor":"cna"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T13:36:18.075535Z"}],"euvd":[{"id":"EUVD-2026-67074","description":"NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.","status":"published","published":"2026-08-27T14:51:17Z","modified":"2026-08-27T18:29:04Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N","level":"medium"}],"updated_at":"2026-11-02T18:01:26.820985Z"}],"ghsa":[{"id":"GHSA-cw6x-m8jw-qmrh","title":"NLTK: Uncontrolled recursion in nltk.featstruct.FeatStructReader causes unhandled RecursionError (DoS) via deeply nested feature-structure input","description":"### Summary\n\n`nltk.featstruct.FeatStructReader` (used by `FeatStruct(str)` and by `FeatureGrammar.fromstring()`) parses feature-structure strings such as `[a=1]` with a recursive-descent parser that has no nesting-depth limit. A small, trivially-crafted input (~700 bytes) with deeply nested brackets drives the parser past Python's recursion limit and raises an **unhandled `RecursionError`** instead of the library's normal, catchable `ValueError`/`LogicalExpressionException`. Any application that parses user-supplied feature-structure or feature-grammar text (e.g. NLP teaching tools, grammar \"playgrounds\", unification-grammar-based NLU pipelines) can be crashed by an unauthenticated input with no special privileges. This is a Denial of Service issue (CWE-674, Uncontrolled Recursion), not a memory-safety or code-execution issue.\n\nThis appears to be the same bug class as two issues already fixed elsewhere in the codebase — `nltk/jsontags.py` (`JSONTaggedDecoder.decode_obj`, guarded by `MAX_DECODE_DEPTH = 200`) and `nltk/sem/logic.py` (`LogicParser`, guarded by `MAX_PARSE_DEPTH = 200`) — but `nltk/featstruct.py` does not have an equivalent guard.\n\n### Details\n\nThe recursive call chain (current `develop` branch, `nltk/featstruct.py`):\n\n1. `FeatStructReader.fromstring()` ([`featstruct.py:2184`](nltk/featstruct.py#L2184)) calls `read_partial()` → `_read_partial()` ([`featstruct.py:2250`](nltk/featstruct.py#L2250)).\n2. `_read_partial()` dispatches to `_read_partial_featdict()`, which calls `_read_value()` ([`featstruct.py:2436`](nltk/featstruct.py#L2436)) for each feature's value.\n3. `_read_value()` calls `read_value()` ([`featstruct.py:2442`](nltk/featstruct.py#L2442)), which matches the value against `VALUE_HANDLERS` ([`featstruct.py:2478`](nltk/featstruct.py#L2478)).\n4. If the value itself starts with `[` (a nested feature structure), the matched handler is `read_fstruct_value` ([`featstruct.py:2479`](nltk/featstruct.py#L2479), defined at [`featstruct.py:2495`](nltk/featstruct.py#L2495)):\n   ```python\n   def read_fstruct_value(self, s, position, reentrances, match):\n       return self.read_partial(s, position, reentrances)\n   ```\n   This calls `read_partial()` again, which re-enters `_read_partial()` — the same function from step 1.\n\nThis closes a recursive cycle (`_read_partial → _read_value → read_value → read_fstruct_value → read_partial → _read_partial → ...`) with **no depth counter, no `MAX_*_DEPTH` constant, and no `try/except RecursionError`** anywhere in the class. Each additional `[` in the input adds one more full cycle of Python stack frames. Once the input nests deeply enough, Python's own recursion-limit protection fires and raises `RecursionError`, which is not a subclass of `ValueError` (the exception type this parser's own `_error()` helper raises for normal, well-formed parse errors) and therefore propagates uncaught through this API.\n\nFor comparison, `nltk/sem/logic.py`'s `LogicParser` was hardened against exactly this class of issue:\n```python\n#: Maximum expression-nesting depth the recursive-descent parser will\n#: descend to. Deeply nested input would otherwise recurse until Python\n#: raises an uncaught RecursionError and crashes the caller\n#: (uncontrolled recursion, CWE-674); past this depth a normal\n#: LogicalExpressionException is raised instead. Configurable.\nMAX_PARSE_DEPTH = 200\n```\n(`nltk/sem/logic.py:102-107`), and `nltk/jsontags.py`'s `JSONTaggedDecoder` similarly has `MAX_DECODE_DEPTH = 200` with an explicit depth check. `nltk/featstruct.py` has no analogous protection.\n\n`FeatureGrammar.fromstring()` (`nltk/grammar.py`) parses feature structures embedded in FCFG grammar rules via the same `FeatStructReader`, so the same crash is reachable through grammar-string parsing as well as through `FeatStruct()` directly.\n\n### PoC\n\nVerified against the current `develop` branch in a clean virtualenv (Python 3.12, NLTK installed from this checkout via `pip install -e .`):\n\n```python\nfrom nltk.featstruct import FeatStruct\n\ndepth = 167\npayload = \"[a=\" * depth + \"1\" + \"]\" * depth   # 669 bytes\nFeatStruct(payload)\n```\n\nResult:\n```\nTraceback (most recent call last):\n  ...\n  File \".../nltk/featstruct.py\", line 2310, in _read_partial_featdict\n    value, position = self._read_value(name, s, position, reentrances)\n  File \".../nltk/featstruct.py\", line 2440, in _read_value\n    return self.read_value(s, position, reentrances)\n  File \".../nltk/featstruct.py\", line 2446, in read_value\n    return handler_func(s, position, reentrances, match)\n  [... repeats ~167 times ...]\nRecursionError: maximum recursion depth exceeded\n```\n\n- Crash threshold: nesting depth 167 (binary-searched between 50 and 200).\n- Payload size: 669 bytes — fits trivially in a single HTTP request body/query parameter.\n- Time to crash: \u003c2ms — no resource exhaustion is needed, only recursion depth.\n\nMinimal reproduction (no server required):\n```bash\npython3 -c \"\nfrom nltk.featstruct import FeatStruct\nFeatStruct('[a=' * 200 + '1' + ']' * 200)\n\"\n```\n\nIllustrative server-side context (not part of NLTK itself, but representative of how the bug becomes reachable):\n```python\nfrom flask import Flask, request\nfrom nltk.featstruct import FeatStruct\n\napp = Flask(__name__)\n\n@app.route(\"/parse\", methods=[\"POST\"])\ndef parse_grammar():\n    return {\"result\": str(FeatStruct(request.json[\"grammar\"]))}\n```\nA POST of `{\"grammar\": \"[a=\" * 200 + \"1\" + \"]\" * 200}` to this endpoint raises the uncaught `RecursionError` inside the request handler.\n\n### Impact\n\n**Vulnerability type:** Denial of Service via uncontrolled recursion (CWE-674). This is not a memory-corruption bug and does not lead to code execution or data disclosure — Python's own recursion-limit safety net converts what would be a C-level stack overflow into a catchable (but here, uncaught) `RecursionError`.\n\n**Who is affected:** Any application that passes externally-supplied text into `nltk.featstruct.FeatStruct()` or `nltk.grammar.FeatureGrammar.fromstring()` — for example, NLP/computational-linguistics teaching tools, unification-grammar demo services, or NLU pipelines that accept user-authored feature grammars. This is a narrower slice of NLTK's user base than, e.g., tokenization or POS tagging, since feature-structure/unification-grammar parsing is a more specialized part of the library.\n\n**Practical severity depends on deployment:**\n- In typical WSGI-style web frameworks (Flask/Django/FastAPI behind gunicorn/uwsgi), an uncaught exception inside a request handler is caught at the framework/server boundary: the single request fails (HTTP 500), the worker process itself survives, and unaffected requests are unimpacted.\n- In single-threaded or per-task-unprotected contexts (e.g. a queue-consuming worker without per-task exception isolation), the uncaught `RecursionError` can terminate the entire process; without a process supervisor that auto-restarts it, this is a persistent outage until manually restarted. An attacker who repeats the payload can keep such a worker in a crash loop for as long as the attack continues.\n\n**Suggested fix:** Add a depth counter and a `MAX_PARSE_DEPTH`-style constant to `FeatStructReader`, mirroring the existing fix in `nltk/sem/logic.py`, and raise the library's normal `ValueError`-based parse error once the limit is exceeded instead of letting `RecursionError` propagate.","status":"published","published":"2026-09-02T14:33:22Z","modified":"2026-09-02T14:33:23Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N","level":"medium"},{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T14:41:10.121131Z"},{"id":"GHSA-pf76-q698-37v8","title":"Duplicate Advisory: Uncontrolled recursion in nltk.featstruct.FeatStructReader causes unhandled RecursionError (DoS) via deeply nested feature-structure input","description":"## Duplicate Advisory\n\nThis advisory has been withdrawn because it is a duplicate of GHSA-cw6x-m8jw-qmrh. This link is maintained to preserve external references.\n\n## Original Description\nNLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.","status":"withdrawn","published":"2026-08-27T18:32:30Z","modified":"2026-09-02T14:33:13Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium"},{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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":"medium"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T14:41:10.121131Z"}],"nvd":[{"id":"CVE-2026-81724","description":"NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.","status":"published","published":"2026-08-27T17:21:03.1Z","modified":"2026-08-31T19:06:33.93Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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":"medium","assessor":"secondary"},{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium","assessor":"secondary"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T14:12:42.035353Z"}],"osv":[{"id":"CVE-2026-81724","title":"NLTK before 3.10.3 Denial of Service via Uncontrolled Recursion","description":"NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.","status":"published","published":"2026-08-27T14:51:17.358Z","modified":"2026-10-28T02:31:02.83485511Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/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":"2026-11-02T15:45:18.131258Z"},{"id":"DEBIAN-CVE-2026-81724","description":"NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.","status":"published","published":"2026-08-27T17:21:03.1Z","modified":"2026-10-01T03:03:43.327134687Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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":"medium"}],"updated_at":"2026-11-02T15:46:04.577397Z"},{"id":"GHSA-cw6x-m8jw-qmrh","title":"NLTK: Uncontrolled recursion in nltk.featstruct.FeatStructReader causes unhandled RecursionError (DoS) via deeply nested feature-structure input","description":"### Summary\n\n`nltk.featstruct.FeatStructReader` (used by `FeatStruct(str)` and by `FeatureGrammar.fromstring()`) parses feature-structure strings such as `[a=1]` with a recursive-descent parser that has no nesting-depth limit. A small, trivially-crafted input (~700 bytes) with deeply nested brackets drives the parser past Python's recursion limit and raises an **unhandled `RecursionError`** instead of the library's normal, catchable `ValueError`/`LogicalExpressionException`. Any application that parses user-supplied feature-structure or feature-grammar text (e.g. NLP teaching tools, grammar \"playgrounds\", unification-grammar-based NLU pipelines) can be crashed by an unauthenticated input with no special privileges. This is a Denial of Service issue (CWE-674, Uncontrolled Recursion), not a memory-safety or code-execution issue.\n\nThis appears to be the same bug class as two issues already fixed elsewhere in the codebase — `nltk/jsontags.py` (`JSONTaggedDecoder.decode_obj`, guarded by `MAX_DECODE_DEPTH = 200`) and `nltk/sem/logic.py` (`LogicParser`, guarded by `MAX_PARSE_DEPTH = 200`) — but `nltk/featstruct.py` does not have an equivalent guard.\n\n### Details\n\nThe recursive call chain (current `develop` branch, `nltk/featstruct.py`):\n\n1. `FeatStructReader.fromstring()` ([`featstruct.py:2184`](nltk/featstruct.py#L2184)) calls `read_partial()` → `_read_partial()` ([`featstruct.py:2250`](nltk/featstruct.py#L2250)).\n2. `_read_partial()` dispatches to `_read_partial_featdict()`, which calls `_read_value()` ([`featstruct.py:2436`](nltk/featstruct.py#L2436)) for each feature's value.\n3. `_read_value()` calls `read_value()` ([`featstruct.py:2442`](nltk/featstruct.py#L2442)), which matches the value against `VALUE_HANDLERS` ([`featstruct.py:2478`](nltk/featstruct.py#L2478)).\n4. If the value itself starts with `[` (a nested feature structure), the matched handler is `read_fstruct_value` ([`featstruct.py:2479`](nltk/featstruct.py#L2479), defined at [`featstruct.py:2495`](nltk/featstruct.py#L2495)):\n   ```python\n   def read_fstruct_value(self, s, position, reentrances, match):\n       return self.read_partial(s, position, reentrances)\n   ```\n   This calls `read_partial()` again, which re-enters `_read_partial()` — the same function from step 1.\n\nThis closes a recursive cycle (`_read_partial → _read_value → read_value → read_fstruct_value → read_partial → _read_partial → ...`) with **no depth counter, no `MAX_*_DEPTH` constant, and no `try/except RecursionError`** anywhere in the class. Each additional `[` in the input adds one more full cycle of Python stack frames. Once the input nests deeply enough, Python's own recursion-limit protection fires and raises `RecursionError`, which is not a subclass of `ValueError` (the exception type this parser's own `_error()` helper raises for normal, well-formed parse errors) and therefore propagates uncaught through this API.\n\nFor comparison, `nltk/sem/logic.py`'s `LogicParser` was hardened against exactly this class of issue:\n```python\n#: Maximum expression-nesting depth the recursive-descent parser will\n#: descend to. Deeply nested input would otherwise recurse until Python\n#: raises an uncaught RecursionError and crashes the caller\n#: (uncontrolled recursion, CWE-674); past this depth a normal\n#: LogicalExpressionException is raised instead. Configurable.\nMAX_PARSE_DEPTH = 200\n```\n(`nltk/sem/logic.py:102-107`), and `nltk/jsontags.py`'s `JSONTaggedDecoder` similarly has `MAX_DECODE_DEPTH = 200` with an explicit depth check. `nltk/featstruct.py` has no analogous protection.\n\n`FeatureGrammar.fromstring()` (`nltk/grammar.py`) parses feature structures embedded in FCFG grammar rules via the same `FeatStructReader`, so the same crash is reachable through grammar-string parsing as well as through `FeatStruct()` directly.\n\n### PoC\n\nVerified against the current `develop` branch in a clean virtualenv (Python 3.12, NLTK installed from this checkout via `pip install -e .`):\n\n```python\nfrom nltk.featstruct import FeatStruct\n\ndepth = 167\npayload = \"[a=\" * depth + \"1\" + \"]\" * depth   # 669 bytes\nFeatStruct(payload)\n```\n\nResult:\n```\nTraceback (most recent call last):\n  ...\n  File \".../nltk/featstruct.py\", line 2310, in _read_partial_featdict\n    value, position = self._read_value(name, s, position, reentrances)\n  File \".../nltk/featstruct.py\", line 2440, in _read_value\n    return self.read_value(s, position, reentrances)\n  File \".../nltk/featstruct.py\", line 2446, in read_value\n    return handler_func(s, position, reentrances, match)\n  [... repeats ~167 times ...]\nRecursionError: maximum recursion depth exceeded\n```\n\n- Crash threshold: nesting depth 167 (binary-searched between 50 and 200).\n- Payload size: 669 bytes — fits trivially in a single HTTP request body/query parameter.\n- Time to crash: \u003c2ms — no resource exhaustion is needed, only recursion depth.\n\nMinimal reproduction (no server required):\n```bash\npython3 -c \"\nfrom nltk.featstruct import FeatStruct\nFeatStruct('[a=' * 200 + '1' + ']' * 200)\n\"\n```\n\nIllustrative server-side context (not part of NLTK itself, but representative of how the bug becomes reachable):\n```python\nfrom flask import Flask, request\nfrom nltk.featstruct import FeatStruct\n\napp = Flask(__name__)\n\n@app.route(\"/parse\", methods=[\"POST\"])\ndef parse_grammar():\n    return {\"result\": str(FeatStruct(request.json[\"grammar\"]))}\n```\nA POST of `{\"grammar\": \"[a=\" * 200 + \"1\" + \"]\" * 200}` to this endpoint raises the uncaught `RecursionError` inside the request handler.\n\n### Impact\n\n**Vulnerability type:** Denial of Service via uncontrolled recursion (CWE-674). This is not a memory-corruption bug and does not lead to code execution or data disclosure — Python's own recursion-limit safety net converts what would be a C-level stack overflow into a catchable (but here, uncaught) `RecursionError`.\n\n**Who is affected:** Any application that passes externally-supplied text into `nltk.featstruct.FeatStruct()` or `nltk.grammar.FeatureGrammar.fromstring()` — for example, NLP/computational-linguistics teaching tools, unification-grammar demo services, or NLU pipelines that accept user-authored feature grammars. This is a narrower slice of NLTK's user base than, e.g., tokenization or POS tagging, since feature-structure/unification-grammar parsing is a more specialized part of the library.\n\n**Practical severity depends on deployment:**\n- In typical WSGI-style web frameworks (Flask/Django/FastAPI behind gunicorn/uwsgi), an uncaught exception inside a request handler is caught at the framework/server boundary: the single request fails (HTTP 500), the worker process itself survives, and unaffected requests are unimpacted.\n- In single-threaded or per-task-unprotected contexts (e.g. a queue-consuming worker without per-task exception isolation), the uncaught `RecursionError` can terminate the entire process; without a process supervisor that auto-restarts it, this is a persistent outage until manually restarted. An attacker who repeats the payload can keep such a worker in a crash loop for as long as the attack continues.\n\n**Suggested fix:** Add a depth counter and a `MAX_PARSE_DEPTH`-style constant to `FeatStructReader`, mirroring the existing fix in `nltk/sem/logic.py`, and raise the library's normal `ValueError`-based parse error once the limit is exceeded instead of letting `RecursionError` propagate.","status":"published","published":"2026-09-02T14:33:22Z","modified":"2026-09-10T03:51:15.361788366Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium"},{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/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":"2026-11-02T15:46:45.639885Z"},{"id":"GHSA-pf76-q698-37v8","title":"Duplicate Advisory: Uncontrolled recursion in nltk.featstruct.FeatStructReader causes unhandled RecursionError (DoS) via deeply nested feature-structure input","description":"## Duplicate Advisory\n\nThis advisory has been withdrawn because it is a duplicate of GHSA-cw6x-m8jw-qmrh. This link is maintained to preserve external references.\n\n## Original Description\nNLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.","status":"withdrawn","published":"2026-08-27T18:32:30Z","modified":"2026-09-10T03:50:53.771377088Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium"},{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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":"medium"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T15:46:52.891208Z"},{"id":"PYSEC-2026-3739","description":"NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.","status":"published","published":"2026-08-27T17:21:03.1Z","modified":"2026-09-01T09:15:03.464962431Z","cvss":[{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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":"medium"}],"updated_at":"2026-11-02T15:52:57.255153Z"},{"id":"UBUNTU-CVE-2026-81724","description":"NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.","status":"published","published":"2026-08-27T17:21:00Z","modified":"2026-09-02T16:28:28Z","cvss":[{"version":"3.1","score":5.3,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","level":"medium"},{"version":"4.0","score":6.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N","level":"medium"}],"updated_at":"2026-11-02T16:57:22.213888Z"}]}}