- Severity: HIGH
- CVE: CVE-2026-126298
- Ecosystems: pip
- Packages: nltk
## Summary
`Tree.chomsky_normal_form()` binarizes a parse tree and defaults to right factoring. For a node with N children the right-factor branch repeatedly removes the first element of a working list with `list.pop(0)`, which shifts all remaining elements each time, so binarizing one wide node costs O(N^2). An application that converts an attacker-supplied treebank tree to Chomsky Normal Form can be forced into tens of seconds of single-threaded CPU by one wide flat tree. No authentication or user interaction is required.
## Root Cause
In the default right-factor branch the transform executes `curNode[0:] = [nodeCopy.pop(0), newNode]` inside a loop over the N children of a node. `list.pop(0)` is O(len) because it shifts every remaining element down by one; performed N-2 times over a shrinking length-N list this sums to O(N^2). The left-factor branch instead uses `nodeCopy.pop()` (removing the last element), which is O(1) and linear overall, so the two branches differ only by this front-versus-back removal. No cap is placed on the child count of a node, and the public `Tree.fromstring` parser accepts an arbitrarily wide bracketed tree.
“`text
Affected function:
– nltk.tree.transforms.chomsky_normal_form (nltk/tree/transforms.py; right-factor sink at line 168, O(1) left control at line 179)
Public entry points: nltk.tree.Tree.chomsky_normal_form (nltk/tree/tree.py:455, default factor=”right”); nltk.tree.Tree.fromstring (nltk/tree/tree.py:592).
“`
## Impact
An attacker…
Source: GitHub Security Advisories · 2026-11-14