Expat through 2.8.3 contains an out-of-bounds read vulnerability that allows attackers to trigger memory corruption by processing XML with external entity parsers created via XMLExternalEntityParserCreate. A struct size mismatch between ELEMENTTYPE members causes storeAtts to read the attIndex member past allocated memory boundaries, resulting in failure to normalize whitespace in non-CDATA attributes or a wild pointer dereference causing a segfault. This vulnerability was introduced by the fix for CVE-2026-66046.

When an Expat parser with a registered ElementDeclHandler parses an inline document type definition containing a deeply nested content model a C stack overflow occurs.

The Expat XML parser mishandles certain kinds of malformed input documents, resulting in buffer overflows during processing and error reporting. The overflows can manifest as a segmentation fault or as memory corruption during a parse operation. The bugs allow for a denial of service attack in many applications by an unauthenticated attacker, and could conceivably result in remote code execution.