{
  "affected": [
    {
      "ecosystem_specific": {
        "fix": "bump",
        "range_state": "fixed",
        "resource": "nltk",
        "resource_purl": "pkg:pypi/nltk@3.10.3",
        "upstream_fixed_in": "3.10.3"
      },
      "package": {
        "ecosystem": "Homebrew",
        "name": "acronym",
        "purl": "pkg:brew/acronym"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.0.0"
            },
            {
              "fixed": "2.0.0_5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "database_specific": {
    "confidence": "high",
    "source": "matched",
    "strategy": "registry",
    "upstream_evidence": [
      {
        "ecosystem": "PyPI",
        "key": "pkg:pypi/nltk@3.10.3",
        "name": "nltk",
        "resource": "nltk",
        "strategy": "registry",
        "subject_version": "3.10.3"
      }
    ]
  },
  "details": "### 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\u0027s recursion limit and raises an **unhandled `RecursionError`** instead of the library\u0027s 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 \u2014 `nltk/jsontags.py` (`JSONTaggedDecoder.decode_obj`, guarded by `MAX_DECODE_DEPTH = 200`) and `nltk/sem/logic.py` (`LogicParser`, guarded by `MAX_PARSE_DEPTH = 200`) \u2014 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()` \u2192 `_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\u0027s 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()` \u2014 the same function from step 1.\n\nThis closes a recursive cycle (`_read_partial \u2192 _read_value \u2192 read_value \u2192 read_fstruct_value \u2192 read_partial \u2192 _read_partial \u2192 ...`) 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\u0027s own recursion-limit protection fires and raises `RecursionError`, which is not a subclass of `ValueError` (the exception type this parser\u0027s own `_error()` helper raises for normal, well-formed parse errors) and therefore propagates uncaught through this API.\n\nFor comparison, `nltk/sem/logic.py`\u0027s `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`\u0027s `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 \u2014 fits trivially in a single HTTP request body/query parameter.\n- Time to crash: \u003c2ms \u2014 no resource exhaustion is needed, only recursion depth.\n\nMinimal reproduction (no server required):\n```bash\npython3 -c \"\nfrom nltk.featstruct import FeatStruct\nFeatStruct(\u0027[a=\u0027 * 200 + \u00271\u0027 + \u0027]\u0027 * 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 \u2014 Python\u0027s 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()` \u2014 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\u0027s 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\u0027s normal `ValueError`-based parse error once the limit is exceeded instead of letting `RecursionError` propagate.",
  "id": "BREW-acronym-CVE-2026-81724",
  "modified": "2026-09-17T18:47:55Z",
  "published": "2026-09-02T08:43:54Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/nltk/nltk/security/advisories/GHSA-cw6x-m8jw-qmrh"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-81724"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nltk/nltk/commit/43c7b78cc8ea37e5cd3a129e27e32c415ea21cf1"
    },
    {
      "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-2026-3739.yaml"
    },
    {
      "type": "WEB",
      "url": "https://www.vulncheck.com/advisories/nltk-before-3.10.3-denial-of-service-via-uncontrolled-recursion"
    }
  ],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    },
    {
      "score": "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "NLTK: Uncontrolled recursion in nltk.featstruct.FeatStructReader causes unhandled RecursionError (DoS) via deeply nested feature-structure input",
  "upstream": [
    "PYSEC-2026-3739",
    "CVE-2026-81724",
    "GHSA-cw6x-m8jw-qmrh"
  ]
}

{
  "affected": [
    {
      "ecosystem_specific": {
        "fix": "bump",
        "range_state": "fixed",
        "resource": "nltk",
        "resource_purl": "pkg:pypi/nltk@3.10.3",
        "upstream_fixed_in": "3.10.3"
      },
      "package": {
        "ecosystem": "Homebrew",
        "name": "gptline",
        "purl": "pkg:brew/gptline"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.0.8"
            },
            {
              "fixed": "1.0.8_23"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "database_specific": {
    "confidence": "high",
    "source": "matched",
    "strategy": "registry",
    "upstream_evidence": [
      {
        "ecosystem": "PyPI",
        "key": "pkg:pypi/nltk@3.10.3",
        "name": "nltk",
        "resource": "nltk",
        "strategy": "registry",
        "subject_version": "3.10.3"
      }
    ]
  },
  "details": "### 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\u0027s recursion limit and raises an **unhandled `RecursionError`** instead of the library\u0027s 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 \u2014 `nltk/jsontags.py` (`JSONTaggedDecoder.decode_obj`, guarded by `MAX_DECODE_DEPTH = 200`) and `nltk/sem/logic.py` (`LogicParser`, guarded by `MAX_PARSE_DEPTH = 200`) \u2014 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()` \u2192 `_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\u0027s 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()` \u2014 the same function from step 1.\n\nThis closes a recursive cycle (`_read_partial \u2192 _read_value \u2192 read_value \u2192 read_fstruct_value \u2192 read_partial \u2192 _read_partial \u2192 ...`) 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\u0027s own recursion-limit protection fires and raises `RecursionError`, which is not a subclass of `ValueError` (the exception type this parser\u0027s own `_error()` helper raises for normal, well-formed parse errors) and therefore propagates uncaught through this API.\n\nFor comparison, `nltk/sem/logic.py`\u0027s `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`\u0027s `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 \u2014 fits trivially in a single HTTP request body/query parameter.\n- Time to crash: \u003c2ms \u2014 no resource exhaustion is needed, only recursion depth.\n\nMinimal reproduction (no server required):\n```bash\npython3 -c \"\nfrom nltk.featstruct import FeatStruct\nFeatStruct(\u0027[a=\u0027 * 200 + \u00271\u0027 + \u0027]\u0027 * 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 \u2014 Python\u0027s 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()` \u2014 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\u0027s 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\u0027s normal `ValueError`-based parse error once the limit is exceeded instead of letting `RecursionError` propagate.",
  "id": "BREW-gptline-CVE-2026-81724",
  "modified": "2026-09-17T19:32:01Z",
  "published": "2026-09-02T09:04:32Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/nltk/nltk/security/advisories/GHSA-cw6x-m8jw-qmrh"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-81724"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nltk/nltk/commit/43c7b78cc8ea37e5cd3a129e27e32c415ea21cf1"
    },
    {
      "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-2026-3739.yaml"
    },
    {
      "type": "WEB",
      "url": "https://www.vulncheck.com/advisories/nltk-before-3.10.3-denial-of-service-via-uncontrolled-recursion"
    }
  ],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    },
    {
      "score": "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "NLTK: Uncontrolled recursion in nltk.featstruct.FeatStructReader causes unhandled RecursionError (DoS) via deeply nested feature-structure input",
  "upstream": [
    "PYSEC-2026-3739",
    "CVE-2026-81724",
    "GHSA-cw6x-m8jw-qmrh"
  ]
}

{
  "affected": [
    {
      "ecosystem_specific": {
        "fix": "bump",
        "range_state": "fixed",
        "resource": "nltk",
        "resource_purl": "pkg:pypi/nltk@3.10.3",
        "upstream_fixed_in": "3.10.3"
      },
      "package": {
        "ecosystem": "Homebrew",
        "name": "safety",
        "purl": "pkg:brew/safety"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.3.1"
            },
            {
              "fixed": "3.8.1_2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "database_specific": {
    "confidence": "high",
    "source": "matched",
    "strategy": "registry",
    "upstream_evidence": [
      {
        "ecosystem": "PyPI",
        "key": "pkg:pypi/nltk@3.10.3",
        "name": "nltk",
        "resource": "nltk",
        "strategy": "registry",
        "subject_version": "3.10.3"
      }
    ]
  },
  "details": "### 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\u0027s recursion limit and raises an **unhandled `RecursionError`** instead of the library\u0027s 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 \u2014 `nltk/jsontags.py` (`JSONTaggedDecoder.decode_obj`, guarded by `MAX_DECODE_DEPTH = 200`) and `nltk/sem/logic.py` (`LogicParser`, guarded by `MAX_PARSE_DEPTH = 200`) \u2014 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()` \u2192 `_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\u0027s 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()` \u2014 the same function from step 1.\n\nThis closes a recursive cycle (`_read_partial \u2192 _read_value \u2192 read_value \u2192 read_fstruct_value \u2192 read_partial \u2192 _read_partial \u2192 ...`) 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\u0027s own recursion-limit protection fires and raises `RecursionError`, which is not a subclass of `ValueError` (the exception type this parser\u0027s own `_error()` helper raises for normal, well-formed parse errors) and therefore propagates uncaught through this API.\n\nFor comparison, `nltk/sem/logic.py`\u0027s `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`\u0027s `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 \u2014 fits trivially in a single HTTP request body/query parameter.\n- Time to crash: \u003c2ms \u2014 no resource exhaustion is needed, only recursion depth.\n\nMinimal reproduction (no server required):\n```bash\npython3 -c \"\nfrom nltk.featstruct import FeatStruct\nFeatStruct(\u0027[a=\u0027 * 200 + \u00271\u0027 + \u0027]\u0027 * 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 \u2014 Python\u0027s 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()` \u2014 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\u0027s 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\u0027s normal `ValueError`-based parse error once the limit is exceeded instead of letting `RecursionError` propagate.",
  "id": "BREW-safety-CVE-2026-81724",
  "modified": "2026-09-17T17:35:56Z",
  "published": "2026-09-02T09:53:14Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/nltk/nltk/security/advisories/GHSA-cw6x-m8jw-qmrh"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-81724"
    },
    {
      "type": "WEB",
      "url": "https://github.com/nltk/nltk/commit/43c7b78cc8ea37e5cd3a129e27e32c415ea21cf1"
    },
    {
      "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-2026-3739.yaml"
    },
    {
      "type": "WEB",
      "url": "https://www.vulncheck.com/advisories/nltk-before-3.10.3-denial-of-service-via-uncontrolled-recursion"
    }
  ],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    },
    {
      "score": "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "NLTK: Uncontrolled recursion in nltk.featstruct.FeatStructReader causes unhandled RecursionError (DoS) via deeply nested feature-structure input",
  "upstream": [
    "PYSEC-2026-3739",
    "CVE-2026-81724",
    "GHSA-cw6x-m8jw-qmrh"
  ]
}

{
  "affected": [
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python-nltk",
            "binary_version": "2.0~b9-0ubuntu4.1~esm6"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:14.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@2.0~b9-0ubuntu4.1~esm6?arch=source\u0026distro=esm-infra-legacy/trusty"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "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"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python-nltk",
            "binary_version": "3.1-1ubuntu0.1+esm4"
          },
          {
            "binary_name": "python3-nltk",
            "binary_version": "3.1-1ubuntu0.1+esm4"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:16.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.1-1ubuntu0.1+esm4?arch=source\u0026distro=esm-apps-legacy/xenial"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "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"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python-nltk",
            "binary_version": "3.2.5-1ubuntu0.1+esm4"
          },
          {
            "binary_name": "python3-nltk",
            "binary_version": "3.2.5-1ubuntu0.1+esm4"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:18.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.2.5-1ubuntu0.1+esm4?arch=source\u0026distro=esm-apps/bionic"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "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"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
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            "binary_name": "python3-nltk",
            "binary_version": "3.4.5-2ubuntu0.1~esm4"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:20.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.4.5-2ubuntu0.1~esm4?arch=source\u0026distro=esm-apps/focal"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "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"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-nltk",
            "binary_version": "3.7-1ubuntu0.1~esm2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:22.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.7-1ubuntu0.1~esm2?arch=source\u0026distro=esm-apps/jammy"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "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"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-nltk",
            "binary_version": "3.8.1-1ubuntu0.1~esm2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:24.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.8.1-1ubuntu0.1~esm2?arch=source\u0026distro=esm-apps/noble"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "3.8.1-1",
        "3.8.1-1ubuntu0.1~esm1",
        "3.8.1-1ubuntu0.1~esm2"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-nltk",
            "binary_version": "3.9.2-1ubuntu0.1~esm2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:26.04:LTS",
        "name": "nltk",
        "purl": "pkg:deb/ubuntu/nltk@3.9.2-1ubuntu0.1~esm2?arch=source\u0026distro=esm-apps/resolute"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "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"
      ]
    }
  ],
  "aliases": [],
  "details": "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\u0027s recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.",
  "id": "UBUNTU-CVE-2026-81724",
  "modified": "2026-09-02T16:28:28Z",
  "published": "2026-08-27T17:21:00Z",
  "references": [
    {
      "type": "REPORT",
      "url": "https://ubuntu.com/security/CVE-2026-81724"
    },
    {
      "type": "REPORT",
      "url": "https://www.cve.org/CVERecord?id=CVE-2026-81724"
    },
    {
      "type": "REPORT",
      "url": "https://github.com/nltk/nltk/security/advisories/GHSA-cw6x-m8jw-qmrh"
    },
    {
      "type": "REPORT",
      "url": "https://www.vulncheck.com/advisories/nltk-before-3.10.3-denial-of-service-via-uncontrolled-recursion"
    }
  ],
  "related": [],
  "schema_version": "1.7.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    },
    {
      "score": "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",
      "type": "CVSS_V4"
    },
    {
      "score": "medium",
      "type": "Ubuntu"
    }
  ],
  "upstream": [
    "CVE-2026-81724"
  ]
}