Summary
NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars
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
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.
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.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Frequently Asked Questions
- What is CVE-2026-12876? CVE-2026-12876 is a medium-severity security vulnerability in nltk (pip), affecting versions <= 3.10.2. It is fixed in 3.10.3.
- Which versions of nltk are affected by CVE-2026-12876? nltk (pip) versions <= 3.10.2 is affected.
- Is there a fix for CVE-2026-12876? Yes. CVE-2026-12876 is fixed in 3.10.3. Upgrade to this version or later.
- Is CVE-2026-12876 exploitable, and should I be worried? Whether CVE-2026-12876 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-12876 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-12876? Upgrade
nltkto 3.10.3 or later.