CVE-2026-126303: NLTK: Unbounded CPU consumption in SteppingRecursiveDescentParser.parse() with ambiguous or self-embedding grammars

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 maxtime 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 maxtime 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 maxtime (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.parsedeadline or self.maxtime. Its init signature omits maxtime entirely, so a caller cannot enable or tighten a bound. The security invariant that a single parse must terminate within a bounded time is therefore never enforced on this parser.

text Affected component: nltk.parse.recursivedescent.SteppingRecursiveDescentParser - parse() nltk/parse/recursivedescent.py:428-433 (drives its own step loop, no bound armed or checked) - parse() nltk/parse/recursivedescent.py:643-668 (stub override; never consults self.parsedeadline / self.maxtime) - init() nltk/parse/recursivedescent.py:410-418 (signature (self, grammar, trace=0); no maxtime parameter) Base guard for comparison: RecursiveDescentParser.parse nltk/parse/recursivedescent.py:141-150

Impact

An actor who controls or influences the grammar passed to SteppingRecursiveDescentParser and can supply a short token sequence to a caller that invokes parse() can cause that call to never return. The top-down search enumerates a derivation space that grows on the order of the Catalan numbers with no memoization of the search and no deadline, so one crafted grammar plus a short input deterministically consumes a CPU core. In a deployment that parses attacker-influenced input on a bounded worker pool, repeated requests exhaust the pool and deny service to legitimate users. No authentication, privilege, or user interaction is required in that worst-credible deployment. Exploitation requires that the target actually use this parser on untrusted input; because the stepping parser is primarily a debugging or interactive-stepping component that backs a GUI, real-world exposure is narrower than for the base parser, though parse() is a supported public method that behaves as a full parser.

Proof of Concept

Requires only the nltk package (3.10.3) and standard library; no data downloads or network access.

Reproduction steps: 1. Build an ambiguous, self-embedding grammar such as S -> 'a' S S | 'a'. 2. Confirm the base parser self-terminates: RecursiveDescentParser(g, maxtime=2.0).parse(['a']17) raises TimeoutError at about 2 seconds. 3. Run the stepping parser on the same grammar and input and observe it never returns.

python import time, inspect from nltk import CFG from nltk.parse.recursivedescent import RecursiveDescentParser, SteppingRecursiveDescentParser

g = CFG.fromstring("S -> 'a' S S | 'a'")

Base parser is bounded and self-terminates. try: list(RecursiveDescentParser(g, maxtime=2.0).parse(['a'] 17)) except TimeoutError: print("base: TimeoutError (guarded)")

Stepping subclass cannot be bounded: no maxtime parameter. print("stepping init sig:", inspect.signature(SteppingRecursiveDescentParser.init))

Stepping parser runs unbounded on the same input. t0 = time.perfcounter() list(SteppingRecursiveDescentParser(g).parse(['a'] 17)) # does not return print("stepping: returned after", time.perfcounter() - t0)

text base: TimeoutError (guarded) stepping init sig: (self, grammar, trace=0) stepping len3: 1 parse in 0.001s ; len7: 5 in 0.017s ; len9: 14 in 0.075s ; len11: 42 in 0.36s stepping len17: still running past a 10s watchdog, process alive, no RecursionError

Attack Chain

1. Exposure: A caller uses nltk.parse.SteppingRecursiveDescentParser, a public exported symbol (nltk/parse/init.py:97), invoking parse(tokens) on grammar and input influenced by an actor. 2. Control: The actor influences the grammar (an ambiguous, self-embedding CFG such as S -> 'a' S S | 'a') and supplies a short token list (for example 17 copies of 'a'). Both survive to the parser unchanged. 3. Path: parse() (recursivedescent.py:428) calls initialize() then loops while self.step() is not None; step() (:479) calls match()/expand() (:552/:512) which call the inherited match/expand (:172/:214), which resolve self.parse by MRO to the subclass stub (:643). 4. Guard: The maxtime wall-clock deadline exists only in the base parse (:141-150) and is never entered on this path; the stub parse records state and returns without consulting the deadline, and init exposes no maxtime to enable a bound. 5. Primitive: The unbounded top-down search enumerates a Catalan-number-sized derivation space with no memoization and no deadline. 6. Result: The parse() call never returns and pins a CPU core; repeated calls exhaust a bounded worker pool, denying service.

Bypass Evidence

The prior fix for this class of issue added a maxtime wall-clock deadline to the base RecursiveDescentParser.parse (recursivedescent.py:141-150, default 5.0 seconds). This report demonstrates that the fix does not cover the sibling SteppingRecursiveDescentParser entry point, and the maintainers separately hardened other parsers (chart, earley chart, viterbi, shift-reduce, non-projective dependency) with maxtime while leaving this subclass unbounded.

Challenges attempted and their outcomes: - Hypothesis that the inherited self.maxtime value (observed at 5.0) makes the path guarded: refuted; the value is inherited but never read on the stepping path, and the parser runs unbounded despite it being set. - Hypothesis that a caller can bound the parser via the constructor: refuted; inspect.signature(SteppingRecursiveDescentParser.init) is (self, grammar, trace=0), with no maxtime parameter. - Hypothesis that the failure is merely an early RecursionError rather than CPU exhaustion: refuted; the step loop is iterative over an explicit history stack, so no exception is raised within a 10 second watchdog, CPU time grows exponentially with input length, and a small negative control input returns in about 0.001 seconds. - Hypothesis that the triede/triedm records collapse the search: refuted; those records are keyed by the frozen full partial tree, so they deduplicate only identical productions at identical states and do not prevent the exponential enumeration.

Affected Versions

- Ecosystem: PyPI - Package: nltk - Confirmed affected range: <= 3.10.3 - Latest release checked: 3.10.3 (PyPI sdist nltk-3.10.3.tar.gz) - Fix status: 3.10.4

The stepping subclass has never carried a time or step bound in any released version; the prior fix added the guard only to the base class. The vulnerable methods were verified present in the exact published PyPI 3.10.3 source distribution (parse() at lines 428-433, stub parse at 643-668 with no deadline reference, and init without maxtime). Because the class predates the prior fix and no specific historical introduction tag was pinned, the lower bound is left open rather than invented; the upper bound is verified against the current published artifact (3.10.3). No released fix exists.

Suggested Fix

Enforce a bounded parse on the stepping parser: give SteppingRecursiveDescentParser its own wall-clock or step-count budget that is checked inside its step loop (for example in parse() and/or the parse/step path), and add a maxtime (or equivalent step-limit) parameter to init that is propagated and honored, mirroring the base RecursiveDescentParser and the other hardened sibling parsers. As an interim mitigation without a code change, callers should avoid invoking SteppingRecursiveDescentParser.parse() on grammars or input derived from untrusted sources, run any such parsing under an external timeout or a separate resource-limited process, and prefer a chart parser for untrusted input.

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Reported by zx (GitHub: @manus-pi).

Fix status (2026-09-28)

PR: #3895 "[WIP] Bound algorithmic-DoS and uncontrolled-recursion vectors (CWE-400/407/674/1333)", open, head b22ebe144.

Fix commit: e20c40e26 "Bound tokenizer/parser algorithmic DoS (ffr9/gpwc/8fx7)" (SteppingRecursiveDescentParser gains the maxtime parameter, default 5.0 s, and honours the parse deadline).

No probe. The advisory PoC re-run (grammar S -> 'a' S S | 'a', 17 tokens): on the PR head the parse raises TimeoutError after 5.0 s and the constructor signature is (self, grammar, trace=0, maxtime=5.0); on develop 2fa3f48fe the same parse ran past a 45 s watchdog, so develop is still VULNERABLE.

Release: none.