Affected: facelessuser soupsieve

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.9, the selector parser in src/soupsieve/css_parser.py defines IDENTIFIER with adjacent quantified groups over overlapping character classes, and VALUE embeds IDENTIFIER for attribute selectors. When an attacker-controlled selector contains a long identifier or unquoted attribute-value run followed by input that makes the overall match fail, the regular expression engine explores quadratically many splits between the overlapping groups. User-controlled selectors can reach this path through soupsieve.compile(), soupsieve.select(), or BeautifulSoup.select(), while applications using only hard-coded selectors are unaffected. The resulting CPU consumption can hold the Python GIL, exhaust application workers, and stall a service; successful plain identifier matches are linear, and the issue does not cause memory corruption or code execution. The issue is fixed in version 2.9.

Affected: PyPI: soupsieve

## Summary

soupsieve compiles CSS selector strings with a set of hand-written regular expressions. The shared `IDENTIFIER` sub-pattern (also embedded in `VALUE`, and therefore in attribute selectors) places two adjacent quantified groups over overlapping character classes: `(?:[classA]|ESC)+(?:[classB]|ESC)*`, where both classes match ordinary identifier characters such as `a`. When a selector contains a long identifier/value run that must ultimately fail to match (e.g. an attribute value with no closing `]`, or an identifier followed by an invalid character), the regex engine backtracks across all O(n) ways to split the run between the `+` group and the `*` group, giving O(n²) parse time. A single attacker-controlled selector of a few kilobytes stalls the interpreter for many seconds of CPU; tens of kilobytes reach minutes.

## Trust model (Q0)

The selector string is the input. It reaches this code via `soupsieve.compile()`, `soupsieve.select/iselect/match/filter`, and — most commonly — BeautifulSoup's `soup.select(selector)` / `soup.select_one(selector)`, which delegate to soupsieve. This is exploitable in any application that passes a user-controlled CSS selector to BeautifulSoup/soupsieve (scrapers that accept selectors, no-code extraction tools, admin/query UIs). Applications that only use hard-coded selectors are not affected.

## Root cause (exact anchors) — `src/soupsieve/css_parser.py`

```python # lines 122-126 IDENTIFIER = fr''' (?:(?:-?(?:[^\x00-\x2f\x30-\x40\x…