GHSA-r8cj-3554-33mr

## Summary

`justhtml` `1.18.0` fixes multiple low-severity denial-of-service hardening issues in CSS selector handling and linkification.

These issues are availability concerns. They do not allow script execution, data disclosure, or sanitizer bypass by themselves.

## Affected versions

- `justhtml` `< 1.18.0`

## Fixed version

- `justhtml` `1.18.0` released on May 4, 2026

## Impact

### CSS selector handling

Applications that evaluate attacker-controlled selector strings, or that run selector-based transform pipelines over attacker-controlled documents, could consume disproportionate CPU or memory.

The affected selector patterns included oversized selectors, large selector lists, oversized compound selectors, long combinator chains, deeply nested functional pseudo-classes such as `:not(...)`, repeated attribute/class token matching over large values, repeated sibling or ancestor scans, repeated positional pseudo-class work, and `:contains(...)` over large descendant text.

Programmatically constructed malformed DOM graphs could also trigger non-terminating or duplicate traversal in some selector paths, including cyclic/shared child graphs, cyclic parent chains, and cyclic text traversal for `:contains(...)`.

### Linkification

Attacker-controlled text containing punctuation-heavy input or URL candidates ending in long runs of unmatched closing brackets could cause repeated rescanning and consume disproportionate CPU when linkification was enabled.

## Default configuration

Ordinary sanitization of parsed HTML with the default `JustHTML(..., sanitize=True)` configuration is not expected to expose untrusted users to selector injection, because selectors are normally supplied by application code.

The main risk areas are:

- applications that accept selector strings from untrusted users and pass them to `query(...)`, `matches(...)`, or selector-based transforms - custom transform or sanitization pipelines that run selector matching over very large untrusted documents - applications that construct or mutate DOM trees programmatically from untrusted structure - applications that enable `Linkify(...)` over attacker-controlled text

## Fixes in 1.18.0

`1.18.0` adds generalized selector resource controls and removes several repeated-work hot paths:

- shared selector limits for parse and match operations - structural caps for selector length, selector lists, compound selectors, complex selectors, and parse depth - match-operation and string-byte budgets - per-query matcher state for caches and cycle guards - precomputed or cached ancestor, sibling, positional, attribute-token, text-content, `:not(...)`, `:empty`, and `:nth-child(...)` work - consistent enforcement across public parsing, `query(...)`, tag-only query fast paths, transform selector compilation, and sanitization transform matching - linkification hardening for punctuation-heavy inputs and trailing bracket trimming

## CWE mapping

- CWE-400: Uncontrolled Resource Consumption - CWE-407: Inefficient Algorithmic Complexity - CWE-835: Loop with Unreachable Exit Condition

## Recommended action

Upgrade to `justhtml` `1.18.0`.

If users cannot upgrade immediately:

- do not pass untrusted selector strings to `query(...)`, `matches(...)`, or selector-based transforms - restrict the size of untrusted documents before selector matching or linkification - avoid constructing programmatic DOM graphs from untrusted structure - avoid enabling `Linkify(...)` on very large attacker-controlled text

## Credit

Discovered during an internal security review of `justhtml`.