NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars — GHSA-FF5C-CP5C-9WJF

Summary

`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;…

Timeline

Published by source

2026-09-02

Risk context

• Reported severity or threat level: medium.
• Reported CVSS score: 6.9 out of 10.
• Review exposure for the affected product or family: pip: nltk.

Product

pip: nltk

What to do

General, cautious steps (verify details in the official source):

• Review exposure and plan remediation based on risk and environment.
• Identify affected product versions in your inventory and verify whether you are impacted.
• Apply vendor patches/updates or recommended mitigations as soon as available.
• Read the official advisory for exact affected versions and remediation steps.

Official advisory

• Read the official source
• CVE Record
• NVD

Evidence and provenance

Canonical advisory ID

GHSA-FF5C-CP5C-9WJF

Source

GitHub

Vendor

GitHub

Published by source

2026-09-02

Last source activity

2026-09-02

Additional identifiers

CVE-2026-12876

Curated by

hlukh.ch

Related advisories and connections

These advisories are connected by shared CVE, product, vendor, source, or time signals.