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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.
CVSS Justification ==================== AV:N -> Since service accept a selector over HTTP/API, attack is network AC:H -> In RH offerings, exploit needs an uncommon path that forwards untrusted CSS selectors into soupsieve; default hard-coded use is unaffected PR:N -> No privileges assumed on that path; if only an authenticated admin UI ever accepted selectors, this would be PR:L instead UI:N -> No second user interaction required S:U -> DoS stays in the consuming process/component; no scope change C:N -> No confidentiality impact; CPU exhaustion only I:N -> No integrity impact; no data modification or code execution A:H -> Successful hit can hold the Global Interpreter Lock, exhaust workers and stall the service - full/serious availability loss while the attack continues, not mild degradation