CVE-2026-126303 — NLTK before 3.10.4 Denial of Service via SteppingRecursiveDescentParser.parse()

Affected: nltk

NLTK before 3.10.4 contains an uncontrolled resource consumption vulnerability in SteppingRecursiveDescentParser.parse() that lacks any max_time deadline or step limit. Attackers influencing the grammar can supply an ambiguous self-embedding grammar and short token input to force unbounded top-down search, pinning CPU cores and exhausting worker pools.

GHSA-8fx7-8jr8-84rv — NLTK: Unbounded CPU consumption in SteppingRecursiveDescentParser.parse() with ambiguous or self-embedding grammars

Affected: PyPI: nltk

## Summary

`nltk.parse.recursivedescent.SteppingRecursiveDescentParser.parse()` performs an unbounded top-down search over ambiguous or self-embedding context-free grammars with no wall-clock or step limit. An actor who can influence the grammar and supply a short input to a caller that uses this parser can force a single `parse()` call to pin a CPU core indefinitely, producing a denial of service. The related base class `RecursiveDescentParser` gained a `max_time` wall-clock deadline, but the stepping subclass drives its own step loop and never arms or consults that deadline, and its constructor does not accept a `max_time` parameter, so the guard cannot be enabled on this path.

## Root Cause

`RecursiveDescentParser` bounds its recursive search inside `_parse`: it arms a wall-clock deadline on the first step and raises `TimeoutError` once `max_time` (default 5.0 seconds) is exceeded. `SteppingRecursiveDescentParser` was designed for interactive single-stepping and overrides both `parse()` and `_parse`. Its `parse()` runs `initialize()` followed by a `while self.step() is not None` loop that never enters the guarded base `_parse`. Its `_parse` override is a state-capturing stub that records history and returns a sentinel, and it never references the inherited `self._parse_deadline` or `self._max_time`. Its `__init__` signature omits `max_time` entirely, so a caller cannot enable or tighten a bound. The security invariant that a single parse must terminate within a bounded…

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CVE-2026-126303 — NLTK before 3.10.4 Denial of Service via SteppingRecursiveDescentParser.parse()

GHSA-8fx7-8jr8-84rv — NLTK: Unbounded CPU consumption in SteppingRecursiveDescentParser.parse() with ambiguous or self-embedding grammars