Search
Find a vulnerability
Search criteria
Related vulnerabilities
BREW-ACRONYM-CVE-2026-12876 (GHSA-FF5C-CP5C-9WJF)
Vulnerability from osv_homebrew – Published: 2026-09-03 08:45 – Updated: 2026-09-17 18:47 – Source website
VLAI
Summary
NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars
Details
nltk.parse.RecursiveDescentParser (and SteppingRecursiveDescentParser) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely — a denial of service.
Proof of concept
Both of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop:
from nltk import CFG
from nltk.parse import RecursiveDescentParser
# (a) left recursion -> unbounded recursion
g = CFG.fromstring("S -> S S | 'a'")
list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs
# (b) ambiguous grammar -> exponential number of parses
g = CFG.fromstring("S -> 'a' S | 'a' S S | 'a'")
list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs
Impact
An application that runs RecursiveDescentParser on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only.
Sibling
The RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.
Severity
{
"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": "`nltk.parse.RecursiveDescentParser` (and `SteppingRecursiveDescentParser`) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely \u2014 a denial of service.\n\n## Proof of concept\n\nBoth of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop:\n\n```python\nfrom nltk import CFG\nfrom nltk.parse import RecursiveDescentParser\n\n# (a) left recursion -\u003e unbounded recursion\ng = CFG.fromstring(\"S -\u003e S S | \u0027a\u0027\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24)) # hangs\n\n# (b) ambiguous grammar -\u003e exponential number of parses\ng = CFG.fromstring(\"S -\u003e \u0027a\u0027 S | \u0027a\u0027 S S | \u0027a\u0027\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24)) # hangs\n```\n\n## Impact\n\nAn application that runs `RecursiveDescentParser` on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only.\n\n## Sibling\n\nThe RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.",
"id": "BREW-acronym-CVE-2026-12876",
"modified": "2026-09-17T18:47:55Z",
"published": "2026-09-03T08:45:00Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-ff5c-cp5c-9wjf"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/pull/3649"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/commit/43aaca1b9024138421c97f970bf13ee19ac8129d"
},
{
"type": "PACKAGE",
"url": "https://github.com/nltk/nltk"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/releases/tag/v3.10.3"
}
],
"schema_version": "1.7.3",
"severity": [
{
"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 resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars",
"upstream": [
"GHSA-ff5c-cp5c-9wjf",
"CVE-2026-12876",
"PYSEC-2026-3869"
]
}
BREW-GPTLINE-CVE-2026-12876 (GHSA-FF5C-CP5C-9WJF)
Vulnerability from osv_homebrew – Published: 2026-09-03 09:08 – Updated: 2026-09-17 19:32 – Source website
VLAI
Summary
NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars
Details
nltk.parse.RecursiveDescentParser (and SteppingRecursiveDescentParser) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely — a denial of service.
Proof of concept
Both of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop:
from nltk import CFG
from nltk.parse import RecursiveDescentParser
# (a) left recursion -> unbounded recursion
g = CFG.fromstring("S -> S S | 'a'")
list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs
# (b) ambiguous grammar -> exponential number of parses
g = CFG.fromstring("S -> 'a' S | 'a' S S | 'a'")
list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs
Impact
An application that runs RecursiveDescentParser on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only.
Sibling
The RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.
Severity
{
"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": "`nltk.parse.RecursiveDescentParser` (and `SteppingRecursiveDescentParser`) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely \u2014 a denial of service.\n\n## Proof of concept\n\nBoth of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop:\n\n```python\nfrom nltk import CFG\nfrom nltk.parse import RecursiveDescentParser\n\n# (a) left recursion -\u003e unbounded recursion\ng = CFG.fromstring(\"S -\u003e S S | \u0027a\u0027\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24)) # hangs\n\n# (b) ambiguous grammar -\u003e exponential number of parses\ng = CFG.fromstring(\"S -\u003e \u0027a\u0027 S | \u0027a\u0027 S S | \u0027a\u0027\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24)) # hangs\n```\n\n## Impact\n\nAn application that runs `RecursiveDescentParser` on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only.\n\n## Sibling\n\nThe RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.",
"id": "BREW-gptline-CVE-2026-12876",
"modified": "2026-09-17T19:32:01Z",
"published": "2026-09-03T09:08:38Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-ff5c-cp5c-9wjf"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/pull/3649"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/commit/43aaca1b9024138421c97f970bf13ee19ac8129d"
},
{
"type": "PACKAGE",
"url": "https://github.com/nltk/nltk"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/releases/tag/v3.10.3"
}
],
"schema_version": "1.7.3",
"severity": [
{
"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 resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars",
"upstream": [
"GHSA-ff5c-cp5c-9wjf",
"CVE-2026-12876",
"PYSEC-2026-3869"
]
}
BREW-SAFETY-CVE-2026-12876 (GHSA-FF5C-CP5C-9WJF)
Vulnerability from osv_homebrew – Published: 2026-09-03 10:05 – Updated: 2026-09-17 17:35 – Source website
VLAI
Summary
NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars
Details
nltk.parse.RecursiveDescentParser (and SteppingRecursiveDescentParser) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely — a denial of service.
Proof of concept
Both of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop:
from nltk import CFG
from nltk.parse import RecursiveDescentParser
# (a) left recursion -> unbounded recursion
g = CFG.fromstring("S -> S S | 'a'")
list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs
# (b) ambiguous grammar -> exponential number of parses
g = CFG.fromstring("S -> 'a' S | 'a' S S | 'a'")
list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs
Impact
An application that runs RecursiveDescentParser on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only.
Sibling
The RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.
Severity
{
"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": "`nltk.parse.RecursiveDescentParser` (and `SteppingRecursiveDescentParser`) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely \u2014 a denial of service.\n\n## Proof of concept\n\nBoth of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop:\n\n```python\nfrom nltk import CFG\nfrom nltk.parse import RecursiveDescentParser\n\n# (a) left recursion -\u003e unbounded recursion\ng = CFG.fromstring(\"S -\u003e S S | \u0027a\u0027\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24)) # hangs\n\n# (b) ambiguous grammar -\u003e exponential number of parses\ng = CFG.fromstring(\"S -\u003e \u0027a\u0027 S | \u0027a\u0027 S S | \u0027a\u0027\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24)) # hangs\n```\n\n## Impact\n\nAn application that runs `RecursiveDescentParser` on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only.\n\n## Sibling\n\nThe RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.",
"id": "BREW-safety-CVE-2026-12876",
"modified": "2026-09-17T17:35:56Z",
"published": "2026-09-03T10:05:22Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-ff5c-cp5c-9wjf"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/pull/3649"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/commit/43aaca1b9024138421c97f970bf13ee19ac8129d"
},
{
"type": "PACKAGE",
"url": "https://github.com/nltk/nltk"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/releases/tag/v3.10.3"
}
],
"schema_version": "1.7.3",
"severity": [
{
"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 resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars",
"upstream": [
"GHSA-ff5c-cp5c-9wjf",
"CVE-2026-12876",
"PYSEC-2026-3869"
]
}
GHSA-FF5C-CP5C-9WJF
Vulnerability from github – Published: 2026-09-02 14:33 – Updated: 2026-09-02 14:33
VLAI
Summary
NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars
Details
nltk.parse.RecursiveDescentParser (and SteppingRecursiveDescentParser) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely — a denial of service.
Proof of concept
Both of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop:
from nltk import CFG
from nltk.parse import RecursiveDescentParser
# (a) left recursion -> unbounded recursion
g = CFG.fromstring("S -> S S | 'a'")
list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs
# (b) ambiguous grammar -> exponential number of parses
g = CFG.fromstring("S -> 'a' S | 'a' S S | 'a'")
list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs
Impact
An application that runs RecursiveDescentParser on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only.
Sibling
The RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.
Severity
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 3.10.2"
},
"package": {
"ecosystem": "PyPI",
"name": "nltk"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.10.3"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-12876"
],
"database_specific": {
"cwe_ids": [
"CWE-407",
"CWE-674"
],
"github_reviewed": true,
"github_reviewed_at": "2026-09-02T14:33:38Z",
"nvd_published_at": null,
"severity": "MODERATE"
},
"details": "`nltk.parse.RecursiveDescentParser` (and `SteppingRecursiveDescentParser`) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely \u2014 a denial of service.\n\n## Proof of concept\n\nBoth of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop:\n\n```python\nfrom nltk import CFG\nfrom nltk.parse import RecursiveDescentParser\n\n# (a) left recursion -\u003e unbounded recursion\ng = CFG.fromstring(\"S -\u003e S S | \u0027a\u0027\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24)) # hangs\n\n# (b) ambiguous grammar -\u003e exponential number of parses\ng = CFG.fromstring(\"S -\u003e \u0027a\u0027 S | \u0027a\u0027 S S | \u0027a\u0027\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24)) # hangs\n```\n\n## Impact\n\nAn application that runs `RecursiveDescentParser` on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only.\n\n## Sibling\n\nThe RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.",
"id": "GHSA-ff5c-cp5c-9wjf",
"modified": "2026-09-02T14:33:39Z",
"published": "2026-09-02T14:33:38Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-ff5c-cp5c-9wjf"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/pull/3649"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/commit/43aaca1b9024138421c97f970bf13ee19ac8129d"
},
{
"type": "PACKAGE",
"url": "https://github.com/nltk/nltk"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/releases/tag/v3.10.3"
}
],
"schema_version": "1.4.0",
"severity": [
{
"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 resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars"
}
PYSEC-2026-3869
Vulnerability from pysec - Published: 2026-09-10 09:44 - Updated: 2026-09-10 11:02
VLAI
Details
nltk.parse.RecursiveDescentParser (and SteppingRecursiveDescentParser) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely — a denial of service.
Proof of concept
Both of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop:
from nltk import CFG
from nltk.parse import RecursiveDescentParser
# (a) left recursion -> unbounded recursion
g = CFG.fromstring("S -> S S | 'a'")
list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs
# (b) ambiguous grammar -> exponential number of parses
g = CFG.fromstring("S -> 'a' S | 'a' S S | 'a'")
list(RecursiveDescentParser(g).parse(["a"] * 24)) # hangs
Impact
An application that runs RecursiveDescentParser on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only.
Sibling
The RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.
Severity
Impacted products
| Name | purl | nltk | pkg:pypi/nltk |
|---|
Aliases
{
"affected": [
{
"package": {
"ecosystem": "PyPI",
"name": "nltk",
"purl": "pkg:pypi/nltk"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.10.3"
}
],
"type": "ECOSYSTEM"
}
],
"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"
]
}
],
"aliases": [
"CVE-2026-12876",
"GHSA-ff5c-cp5c-9wjf"
],
"details": "`nltk.parse.RecursiveDescentParser` (and `SteppingRecursiveDescentParser`) enumerate parses top-down with no bound on the number of recursive steps. A small, crafted context-free grammar makes a short input consume unbounded CPU (and/or exhaust the Python recursion stack), pinning a process indefinitely \u2014 a denial of service.\n\n## Proof of concept\n\nBoth of the following hang on a 24-token input (killed after 8s; growth is super-linear in input length), on NLTK develop:\n\n```python\nfrom nltk import CFG\nfrom nltk.parse import RecursiveDescentParser\n\n# (a) left recursion -\u003e unbounded recursion\ng = CFG.fromstring(\"S -\u003e S S | \u0027a\u0027\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24)) # hangs\n\n# (b) ambiguous grammar -\u003e exponential number of parses\ng = CFG.fromstring(\"S -\u003e \u0027a\u0027 S | \u0027a\u0027 S S | \u0027a\u0027\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24)) # hangs\n```\n\n## Impact\n\nAn application that runs `RecursiveDescentParser` on a grammar (or an input) drawn from an untrusted source can be driven into an unbounded CPU / stack-exhaustion loop by a tiny payload. No confidentiality or integrity impact; single-process availability only.\n\n## Sibling\n\nThe RegexpTokenizer ReDoS reported alongside this (CVE-2026-12875) is a different class (caller-supplied regex) and is addressed under GHSA-w3v8-gmh9-3wv7.",
"id": "PYSEC-2026-3869",
"modified": "2026-09-10T11:02:15.848558Z",
"published": "2026-09-10T09:44:59.291918Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-ff5c-cp5c-9wjf"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/pull/3649"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/commit/43aaca1b9024138421c97f970bf13ee19ac8129d"
},
{
"type": "PACKAGE",
"url": "https://github.com/nltk/nltk"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/releases/tag/v3.10.3"
},
{
"type": "PACKAGE",
"url": "https://pypi.org/project/nltk"
},
{
"type": "ADVISORY",
"url": "https://github.com/advisories/GHSA-ff5c-cp5c-9wjf"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-12876"
}
],
"severity": [
{
"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 resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars"
}
UBUNTU-CVE-2026-12876 (CVE-2026-12876)
Vulnerability from osv_ubuntu – Published: 2026-08-14 00:00 – Updated: 2026-08-14 00:00 – Source website
VLAI
Details
[Unknown description]
Severity
N/A (UNKNOWN)
References
{
"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": [
{
"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": "[Unknown description]",
"id": "UBUNTU-CVE-2026-12876",
"modified": "2026-08-14T00:00:00Z",
"published": "2026-08-14T00:00:00Z",
"references": [
{
"type": "REPORT",
"url": "https://ubuntu.com/security/CVE-2026-12876"
},
{
"type": "REPORT",
"url": "https://www.cve.org/CVERecord?id=CVE-2026-12876"
},
{
"type": "REPORT",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-ff5c-cp5c-9wjf"
}
],
"related": [],
"schema_version": "1.7.0",
"severity": [
{
"score": "medium",
"type": "Ubuntu"
}
],
"upstream": [
"CVE-2026-12876"
]
}