Soup Sieve: Uncontrolled recursion in CSS parser lets nested pseudo-class selectors exhaust the Python call stack

Summary The Soup Sieve CSS selector parser recurses once per level of nested pseudo-class selector (:is(), :not(), :where(), :has()) without any limit on nesting depth. An unauthenticated attacker who can supply a CSS selector string to soupsieve.compile(), soupsieve.select(), or Beautiful Soup's soup.select() / soup.selectone() can force the parser to exceed Python's recursion limit with a small crafted input (about 1.6 KB, roughly 330 nesting levels), raising an uncontrolled RecursionError. Because this exception is not part of the parser's documented error contract, applications that catch only ValueError or SelectorSyntaxError see an unexpected failure of the request or task that parsed the selector. Root Cause The parser dispatches each opening pseudo-class token to parsepseudoopen, which calls back into parseselectors to parse the pseudo-class body. Every nesting level therefore adds Python stack frames, and no counter, depth guard, or RecursionError handler bounds this descent. The token-count guard SELECTORLIMIT = 8192 (added for the earlier comma-separated-list memory issue) counts parsed tokens, not recursion depth; at the failing nesting depth the token count is only…

curl https://cve.zerocrew.in/api/cve/CVE-2026-126193