Summary

When an input data has quotes in attribute values but process entities is not enabled, it breaks the attribute value into multiple attributes. This gives the room for an attacker to insert unwanted attributes to the XML/HTML.

Detail

Malicious Input

{
      a: {
        "@_attr": '" onClick="alert(1)'
      }
}

Output

<a attr="" onClick="alert(1)"></a>

Workarounds

If you're not ignoring attributes then keep processEntities flag true.