Uncontrolled recursion DoS in JustHTML() via deeply nested HTML

Overview

Summary

justhtml through 1.9.1 allows denial of service via deeply nested HTML. During parsing, JustHTML.__init__() always reaches TreeBuilder.finish(), which unconditionally calls _populate_selectedcontent(). That function recursively traverses the DOM via _find_elements() / _find_element() without a depth bound, allowing attacker-controlled deeply nested input to trigger an unhandled RecursionError on CPython. Depending on the host application's exception handling, this can abort parsing, fail requests, or terminate a worker/process.

Details

TreeBuilder.finish() (treebuilder.py#L476) unconditionally calls _populate_selectedcontent(self.document) at line 494. _populate_selectedcontent() (treebuilder.py#L1243) calls _find_elements() (treebuilder.py#L1280) to recursively search the DOM tree for <select> elements:

def _find_elements(self, node: Any, name: str, result: list[Any]) -> None:
    """Recursively find all elements with given name."""
    if node.name == name:
        result.append(node)
    if node.has_child_nodes():
        for child in node.children:
            self._find_elements(child, name, result)  # recursive call

When the DOM tree depth exceeds CPython's default recursion limit (1000), this raises an unhandled RecursionError. The full call path is:

JustHTML(html) → tokenizer.run() → tree_builder.finish() → _populate_selectedcontent(document) → _find_elements(root, "select", selects) (recursive)

Deeply nested DOM trees can be produced by nesting <div> tags ~1000 levels deep. On CPython with the default recursion limit, approximately 11 KB of <div> nesting is sufficient to trigger the error. The exact depth threshold is environment-dependent (CPython version, recursion limit setting, call stack depth at invocation).

Additional recursive functions are affected on already-parsed deep trees:

• Node.clone_node(deep=True) (node.py#L523) — called during sanitization
• _node_to_html() (serialize.py#L580) — used by to_html(pretty=True)
• _to_markdown_walk() (node.py#L817) — used by to_markdown()

Note: the library already uses iterative traversal in several comparable functions (e.g., _node_to_html_compact at serialize.py#L197, _to_text_collect at node.py#L161, _is_blocky_element at serialize.py#L405, apply_to_children at transforms.py#L1642), demonstrating the correct pattern.

PoC

from justhtml import JustHTML

html = "<div>" * 1000 + "x" + "</div>" * 1000
doc = JustHTML(html)  # raises RecursionError

Test environment: CPython 3.14.3, macOS ARM64 (Apple Silicon), justhtml 1.9.1, default recursion limit (1000)

The error occurs with both sanitize=True (default) and sanitize=False.

Impact

An attacker who can supply HTML for parsing can trigger an unhandled RecursionError during JustHTML() construction. The error is triggered during construction and is not avoided by justhtml configuration alone; mitigating it requires host-application exception handling or input constraints. Depending on the host application's exception handling, this can abort parsing, fail requests, or terminate a worker/process.

Suggested Fix

Convert the recursive tree traversal functions to iterative implementations using an explicit stack. Example for _find_elements:

def _find_elements(self, node: Any, name: str, result: list[Any]) -> None:
    stack = [node]
    while stack:
        current = stack.pop()
        if current.name == name:
            result.append(current)
        if current.has_child_nodes():
            stack.extend(reversed(current.children))

The same conversion should be applied to _find_element, clone_node(deep=True), _node_to_html(), and _to_markdown_walk().

Affected packages

• justhtml < 1.10.0

Remediation

Upgrade to a patched release:

• justhtml 1.10.0