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CVE-2026-49477 — Soup Sieve: Regular Expression Denial of Service (ReDoS) in soupsieve Selector Parser

Affected: facelessuser soupsieve

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.

GHSA-836r-79rf-4m37 — Soup Sieve: Regular Expression Denial of Service (ReDoS) via Selector Parser

Affected: PyPI: soupsieve

### Summary

The CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) contains a regular expression vulnerable to catastrophic backtracking. When processing an attribute selector with an unterminated quoted value, the `VALUE` regex pattern in `css_parser.py` enters exponential backtracking. A payload of only **300 bytes** causes the regex engine to hang for **over 3 seconds**, enabling a trivial Regular Expression Denial of Service (ReDoS) attack.

To be completely transparent, AI tools helped surface this issue. However, this was independently reproduced and carefully validated.

Any application that passes untrusted CSS selector strings to `soupsieve.compile()` or Beautiful Soup's `.select()` / `.select_one()` is affected.

### Details

**Affected code:** `soupsieve/css_parser.py`, line ~121 - `RE_VALUES` / `VALUE` regex pattern

The soupsieve CSS parser uses a compiled regular expression to tokenise attribute selector values. This pattern matches both quoted strings (`"value"` or `'value'`) and unquoted identifiers. The regex contains alternation branches for:

1. Double-quoted strings: `"[^"\\]*(?:\\.[^"\\]*)*"` 2. Single-quoted strings: `'[^'\\]*(?:\\.[^'\\]*)*'` 3. Unquoted identifiers

When an attribute selector contains an **unterminated quoted value** - e.g., `[a="xxxx...` (opening `"` but no closing `"`) -” the regex engine attempts to match the quoted-string branch. After that branch fails (no closing quote), the engine backtracks and at…