vulnerability / NVI-00014040 / Revision 1
NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars
GitHub lists this reviewed advisory for nltk. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
nltk
source listed<= 3.10.2
`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: ```python 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 (
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
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04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
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- GHSA-ff5c-cp5c-9wjf ↗
GitHub Advisory Database · vendor advisory
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Public revision history
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- Revision 1 ·
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