Soup Sieve is a CSS selector library for Beautiful Soup. From 0.4 until 3.0, deeply nested CSS pseudo-class selectors can cause recursive parsing to exceed Python’s recursion limit and raise  RecursionError . Applications accepting untrusted selectors may experience failure of the affected request or task if the exception is not handled. The demonstrated behavior is recoverable and does not establish an interpreter crash or persistent service-wide denial of service. This vulnerability is fixed in version 3.0.