Basic Knowledge

XXE (XML External Entity)

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Ngoc Vo

Marketing Executive @CyStack|May 24, 2023

XXE (XML External Entity) Attacks and How to Prevent Them

Learn the inner workings of XML External Entity (XXE) vulnerabilities, their impact on IT systems, and effective strategies to prevent those attacks.

XXE attacks pose a significant threat to modern IT systems, exploiting vulnerabilities in XML parsers to access sensitive information or even execute arbitrary code.

This article will explore the mechanics of XXE and its potential consequences – valuable knowledge that can help patch this flaw and protect your system.

What is XML External Entity?

XML External Entity (XXE) is a type of security vulnerability that targets applications processing XML data. It occurs when an attacker exploits XML parsers to access sensitive data or execute remote code.

XML, or Extensible Markup Language, is widely used in web applications to store, transmit, and display data. It plays a significant role in content management systems, e-commerce sites, and various other web applications.

Parsers are responsible for reading and interpreting XML data, which is where the XXE attack comes into play. This attack is particularly dangerous due to its ability to bypass security measures and access restricted information.

XML and External Entities


XML is an important technology in the web development stack. Image: Freepik

Designed as a more flexible and extensible alternative to HTML, the syntax of XML is easy to understand, making it a popular choice for exchanging data between applications and systems.

One of its advantages is the ability to represent complex data structures in a human-readable and machine-readable format, facilitating data exchange between different systems.

An XML document consists of elements, attributes, and text, all enclosed within angled brackets.

Here is a simple example:

In this example, we see the elements “person,” “name,” “age,” and “email,” each with their respective opening and closing tags. The text between the tags is the actual data.

XML Parser

To process and interpret the data within an XML document, we need the help of an XML parser.

The parser reads the XML document and converts it into a format that can be easily understood and manipulated by applications or systems.

This program ensures that the XML document is well-formed and adheres to the XML syntax rules. It also identifies elements, attributes, and text content as it processes the document.

Entities and External Entities

Entities are a fundamental aspect of XML, allowing for the inclusion and reuse of data within an XML document. They can be categorized into two types: internal and external entities.

Internal entities are defined within the XML document, while external entities refer to data that is not part of the XML document but can be referenced and fetched by the XML parser.

For example:

In this example, the &ext; reference tells the XML parser to fetch the data from the external source and include it in the XML document.

How XXE Attacks Work

Hackers can take advantage of this flaw to break into your infrastructure. Image: Freepik

For an XXE attack to occur, two conditions must be met: the application must accept XML input, and the XML parser must have external entities enabled.

Attackers exploit external entities by crafting malicious XML documents that reference sensitive files or resources on the target system or execute remote code.

Consider the following malicious XML:

In this example, the attacker has created an XML document with an external entity reference to the /etc/passwd file, which contains sensitive user information on Unix-based systems like Linux.

When the vulnerable XML parser processes this document, it fetches the contents of the /etc/passwd file and includes it in the resulting XML, potentially exposing sensitive data to the attacker.

Variations of XXE Attacks

XXE attacks come in several variations, each with its own mechanism and typical goals.

In-band XXE Attacks

In-band XXE attacks are the most common form of XXE, where the attacker receives the extracted data directly within the application’s response.

This type of attack relies on the application processing the malicious XML data and returning the sensitive information in the response. The goal of in-band XXE attacks is usually to exfiltrate sensitive data, such as user credentials or system configuration files.

Out-of-band XXE Attacks

Out-of-band XXE attacks differ from in-band attacks as they involve the attacker receiving extracted data via a separate channel, rather than within the application’s response.

The attacker crafts a malicious XML document that instructs the XML parser to send the extracted data to an external system controlled by the attacker.

Out-of-band attacks are typically used when in-band attacks are not feasible or when the attacker wants to exfiltrate larger amounts of data.

Blind XXE Attacks

Blind XXE attacks occur when the attacker cannot directly observe the results of the exploited vulnerability. Instead, the attacker uses the XXE vulnerability to carry out actions such as launching a Denial of Service (DoS) attack or triggering server-side processes.

The objective of blind XXE attacks is to disrupt the target system or perform specific actions rather than exfiltrate data.

Impact of XXE Attacks

The consequences of XXE attacks can be severe, affecting both the target organization and its users.

Sensitive data exfiltration

Attackers can exploit external entities to access and exfiltrate critical files from the targeted system. For example, they might gain access to sensitive configuration files, user credentials, or proprietary business information.

This unauthorized access can have severe consequences for the organization, as it may lead to further security breaches or unauthorized activities within the system.

Server-Side Request Forgery (SSRF)

XXE attacks can also lead to SSRF incidents. In this scenario, the attacker manipulates the XML document to include external entities that trigger requests from the targeted server to another system.

These requests can bypass network security measures, such as firewalls, and potentially access restricted internal resources or launch attacks against other systems within the organization’s network.

This technique allows the attacker to pivot within the network and potentially exploit additional vulnerabilities, further compromising the organization’s security posture.

Distributed Denial of Service (DDoS)

By exploiting specific features of XML parsers, like entity expansion or external entity inclusion, attackers can craft malicious XML payloads that cause the targeted system to consume excessive resources or crash altogether.

Such attacks, known as “Billion Laughs” or “XML bomb,” can effectively render the targeted service unavailable, affecting its functionality and disrupting critical operations.

An XXE attack can lead to severe DDoS and overwhelm your system. Image: Freepik

Remote code execution

Although less common, this impact is particularly severe as it allows the attacker to execute arbitrary code on the targeted system.

By exploiting specific vulnerabilities in XML parsers or leveraging misconfigurations, an attacker can manipulate the XML document to include external entities that lead to the execution of malicious code.

This code execution can grant the attacker full control over the targeted system, enabling them to perform a wide range of actions, such as installing malware, exfiltrating data, or escalating their privileges within the network.

How to Prevent XXE Attacks

Disabling Document Type Definitions

One of the most effective ways to prevent XXE attacks is to disable Document Type Definitions (DTDs) entirely. By doing so, you’re blocking the very mechanism that allows external entities to be processed.

The steps to disable DTDs will vary depending on the language and parser you’re using, but the XML External Entity Prevention Cheat Sheet from OWASP is an excellent resource to consult for guidance on this matter.

Input Validation

Another useful strategy to thwart XXE attacks is input validation. By ensuring that the data received from users or external sources conforms to specific rules and expectations, you can reduce the likelihood of malicious payloads being processed.

This includes checking the XML structure, content, and size, as well as implementing secure coding practices to avoid potential vulnerabilities.

Considering Alternative Formats

It’s worth considering using alternative data formats that don’t rely on XML and are less susceptible to XXE attacks. JSON (JavaScript Object Notation), for instance, is a lightweight and widely used data interchange format that doesn’t have the same vulnerability to external entity attacks as XML.

By opting for JSON or other similar formats, you can minimize the risks associated with processing XML documents while still benefiting from a structured and efficient data exchange format.


XXE attacks pose a significant risk to IT systems, with consequences ranging from unauthorized access to critical system disruptions. It is crucial for you to stay informed and vigilant as well as employ appropriate prevention strategies and maintain a strong security posture to defend against these ever-evolving threats.

If you’re concerned about XXE or other potential attacks on your systems, consult CyStack for professional assistance. Our experienced team of security experts is ready to help you identify vulnerabilities, recommend effective solutions, and ensure your organization is well-protected against cyber threats.

By partnering with CyStack, you can confidently navigate the complex world of cybersecurity, knowing that your infrastructure is in good hands. Contact CyStack today and let us help you build a more secure future for your organization.

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