PYSEC-2026-3072

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 attempts to match the remaining input against subsequent alternation branches and parent patterns. The structure of the pattern causes catastrophic backtracking where the number of backtracking steps grows exponentially with the length of the content between the opening quote and the end of the string.

Root cause: The regex pattern does not anchor or guard against the case where a quoted string is never terminated. The overlapping character classes across alternation branches create...