An XXE attack is referred to as an attack that takes place against an application parsing XML input. This attack abuses a rarely used but broadly available feature of XML parsers. Attackers use XXE or XML External Entity to cause DoS or Denial of Service. It also results in gaining access to remote and local content and services. In simple words, an XEE attack is a web security vulnerability using which an attacker interferes with XML Data application processing.
OWASP defines XML External Entity as an attack against an XML input parsing application. It is also referred to as XML External Entity Injection. This attack takes place due to web security based vulnerability when a reference to an external entity containing XML input gets possessed by an XML parser that is weakly configured. As a result of this attack, denial of service, confidential data disclosure, port scanning from the machine perspective where the parser is located, server-side request forgery, and other system impact results.
XML-based downstream integrations or web services and applications are prone to attack in conditions-
This vulnerability can be easily understood with the help of pertinent XEE examples. Here are a few that shall help clarify things.
With time, several public XXE problems have come to light. It also includes an attack on embedded devices. It is prudent to note that XXE attacks can occur in several unexpected places like deeply nested dependencies. If accepted, the easiest way to get a malicious XML file uploaded;
Example 1- In this case, the attacker tries to extract data from the server.
<?xml version=”1.0″ encoding=”ISO-8859-1″?> <!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM “file:///etc/passwd” >]> <foo>&xxe;</foo>
Example 2- The attacker probe’s the private network of the server by changing the above entity line to-
<!ENTITY xxe SYSTEM “https://192.168.1.1/private” >]>
Example 3 – The attacker includes a potentially endless file to use a denial of service attack.
<!ENTITY xxe SYSTEM “file:///dev/random” >]>
Example 4- In this example, the simple web application output accepts XML input. It then parses it and gives the result.
XML can be used to declare attributes, elements, and text. These XML documents are of a particular type, and the document type can be declared by specifying the type definition. XML Parser validates XML document if it adheres to this definition before document processing. There are two definition types. One is DTD or a Document Type Definition and XSD or XML Schema Definition. Document Type Definitions witness XXE vulnerabilities though it is considered legacy. These are sourced from the ancestor of XML or SGML.
Example 5- In this example of XXE payload, foo with an element called bar Document Type Definition is an alias for World. Thus, any bar & time is used, and the XML parser replaces the entity with the word World.
An attacker uses XML entities that may seem harmless, causing a denial of service by embedding entities with entities. This attack is popularly known as the Billion Laughs attack. It also overloads XML parser memory. In some cases, the XML parsers limit the memory amount that can be used automatically.
There is a file /etc/fstab that contains XML lookalike characters even when they are not XML. It causes the XML parser to try and pars these elements but notice that it is not a valid or genuine XML document.
Thus, XXE or XML External Entity gets limited in two ways-
These were a few of XXE attack examples that give an idea of the issue.
It is important to look for ways and means to curb these XXE XML external entity attacks. For this, it is important to go for developer training that helps identify issues and mitigate XXE attacks. Following is required for prevention XXE-
In case such controls are not working or not possible, check virtual patching, Web Application Firewalls, Interactive Application Security Testing, or API Security Gateways for the detection, monitoring, and blocking XXE attacks.
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