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

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.