CVE-2021-23926: Improper Restriction of Recursive Entity References in Apache XMLBeans

The vulnerability stemmed from uncontrolled recursive entity processing in the SAX parser implementation. The patch adds three key mitigations: 1) Byte counting in characters() to limit entity size, 2) Entity nesting depth tracking via startEntity/endEntity, and 3) A configurable byte limit through XmlOptions. The affected methods - characters(), startEntity(), and endEntity() - handled entity content processing without these restrictions in vulnerable versions, making them the entry points for entity expansion attacks. Their unpatched behavior would appear in stack traces during exploitation as they directly process entity content without validation.