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Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.9, selector_iter in src/soupsieve/css_parser.py trims the raw selector with RE_WS_END, an end-anchored WSC whitespace-and-comment expression used with search(), so the regular expression engine retries a greedy scan at every starting offset. An attacker-controlled valid selector containing a long internal whitespace run, or a selector containing a long CSS comment run followed by another token, causes quadratic CPU work before tokenization. User-controlled selectors can reach the path through soupsieve.compile() and BeautifulSoup.select(), while applications using only hard-coded selectors are unaffected. This root cause is separate from the IDENTIFIER and VALUE backtracking vulnerability because the cost occurs in RE_WS_END.search during trimming rather than token matching. The resulting CPU consumption can hold the Python GIL, exhaust workers, and stall a service without causing 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