Numerous components utilizing PHP's DOMDocument, SimpleXML, and xml_parse functionality are vulnerable to two types of attacks:

• XML eXternal Entity (XXE) Injection attacks. The above mentioned extensions are insecure by default, allowing external entities to be specified by adding a specific DOCTYPE element to XML documents and strings. By exploiting this vulnerability an application may be coerced to open arbitrary files and/or TCP connections.
• XML Entity Expansion (XEE) vectors, leading to Denial of Service vectors. XEE attacks occur when the XML DOCTYPE declaration includes XML entity definitions that contain either recursive or circular references; this leads to CPU and memory consumption, making Denial of Service exploits trivial to implement.

References

• https://framework.zend.com/security/advisory/ZF2014-01
• https://github.com/FriendsOfPHP/security-advisories/blob/master/zendframework/zendframework1/ZF2014-01.yaml