{"id":"PYSEC-2026-3869","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.\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 | 'a'\")\nlist(RecursiveDescentParser(g).parse([\"a\"] * 24))   # hangs\n\n# (b) ambiguous grammar -\u003e exponential number of parses\ng = CFG.fromstring(\"S -\u003e 'a' S | 'a' S S | 'a'\")\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.","aliases":["CVE-2026-12876","GHSA-ff5c-cp5c-9wjf"],"modified":"2026-09-10T12:15:10.248081657Z","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"}],"affected":[{"package":{"name":"nltk","ecosystem":"PyPI","purl":"pkg:pypi/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"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/nltk/PYSEC-2026-3869.yaml"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","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"}]}