CVE-2026-126298

## 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 who can influence the tree that an application normalizes to Chomsky Normal Form causes sustained single-threaded CPU consumption (availability loss). The path is network-reachable (AV:N) and needs no privileges or user interaction (PR:N, UI:N). Because the cost grows quadratically with the attacker-chosen child count, roughly 1.5 MB of treebank input costs about 14 seconds on one core and scales as N^2 above that. Confidentiality and integrity are not affected. The NLTK regular-expression wall