When an attacker sends the above request, the XML parser will first attempt to process the %dtd parameter entity by making a request to. In the following example, a parameter entity is used to define a general entity, which is then called from the XML document. This character instructs the XML parser that a parameter entity (not a general entity) is being defined. Parameter entities are only used in Document Type Definitions (DTDs).Ī parameter entity starts with the % character. In addition to general entities, XML also supports parameter entities. This will not actually work because the XML specification does not allow you to include external entities in combination with internal entities. # /etc/fstab: static file system informa. Therefore, in theory, an attacker could send a request similar to the following. Special XML characters in CDATA (Character Data) tags are ignored by the XML parser. There are legitimate cases when you may need to store XML special characters in XML files. XML already has a workaround for this problem.
#XEE 2.2 HOW TO#
The primary problem for an attacker using XXE is how to access text files with XML-like content (files that contain XML special characters such as &, ). XXE cannot be used to obtain binary files.XXE can only be used to obtain files or responses that contain “valid” XML.Therefore, this limits XML External Entity (XXE) in the following two important ways. This will cause the XML parser to try and parse these elements, only to notice that it’s not a valid XML document. etc/fstab is a file which contains some characters that look like XML (even though they’re not XML). 圎rror: Specification mandate value for attribute system, line 3, column 15. If the XML parser is configured to process external entities (by default, many popular XML parsers are configured to do so), the web server will return the contents of a file on the system, potentially containing sensitive data. This is where XXE becomes a type of a Server Side Request Forgery (SSRF) attack.Īn attacker can create make the following request using a URI (known in XML as the system identifier). In fact, XML entities can come from just about anywhere – including external sources (hence the name XML External Entity). This is because you do not have to define XML entities in the XML document. Hello World World World World World World World World World World World World World World World World World World World World World World World World World World World World World World World World World World World World World World World WorldĪttackers can use XML entities for much more than reducing application availability.