Introduction

As for several types of attacks, you can break XML External Object Attacks (XXE attacks) into two types, i.e. in and out of the band. XXE in-band attacks are more prevalent and allow the attacker to receive an instant response to the XXE payload. There is no instant response from the web application in the case of out-of-band XXE attacks (also called blind XXE). In this article, we will discuss  XXE payload, XML injection payloads, XXE attack payload, blind XXE payload and, what is XML external entity injection.

In this article let us look at:

  1. What is XML external entity injection?
  2. Types of XXE Attacks
  3. XML External Entity or XXE Injection Payloads

1. What is XML external entity injection?

XXE or XML External Entity injection is a major weakness that makes it possible for an attacker to read local server data, access internal networks, check for internal ports, or execute remote server commands. It targets XML parsing applications. This attack happens when an XML input that includes references to an external object is being interpreted by a weakly designed XML parser. By embedding malicious inline DOCTYPE definitions into the XML data, the attacker takes advantage of it. When the malicious XML input is processed by the webserver, the entities are expanded, which potentially results in access to the file system of a web server, remote file system access, or connection to arbitrary hosts over HTTP or HTTPS.

Attacks can require the leakage of local files that may include personal information, Passwords, or private user info, for example, using the schema or relative path system identifier file. Since the attack occurs in relation to the XML document processing application, An attacker will pivot to other internal structures using this trusted application, disclose other internal content to any unprotected internal services via HTTP or HTTPS requests, or launch a CSRF attack.

In some situations, by dereferencing a malicious URI, a Vulnerable XML Processor Library to problems with client-side memory leakage may be used, enabling the execution of arbitrary code under the framework account. Other attacks can provide access to local resources. If too many threads or protocols are not released, and the data will not be returned., possibly impacting the availability of applications.

2. Types of XXE Attacks

  • Exploit XXE to recover files, where the external object that holds a file’s contents is specified and returned in the application’s response.
  • Exploit XXE to carry out SSRF attacks that specify an external entity based on the URL of the back-end system.
  • To exfiltrate out of band data, Blind XXE is used when sensitive data is transferred from the application server to an attacker-managed computer.
  • Use blind XXE to extract data from the error messages.

3. XML External Entity or XXE Injection Payloads

An XML External Entity attack is a type of attack against an XML input parsing application. This attack occurs when an XML parser that is weakly designed processes XML information containing an external object relation. XXE Attack Type Description Exploiting XXE to Retrieve Files Where an external entity containing the contents of a file is defined and returned in the response of the application. Exploiting XXE to conduct SSRF attacks Where an external object is defined as a URL-based backend framework. 

Conclusion

XML External Entity (XXE) in-band as well as out-of-band vulnerabilities are very severe and affect virtually every web application that parses XML documents. They’re the # 4 hazard in the OWASP Top 10 2017 chart. XXE can be used to induce a denial of service, in addition to stealing system data and source code from local servers. XXE can also be used by attackers to initiate Server Side Request Forgery (SSRF) attacks against other servers on the internal network.

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