What is XXE (XML External Entity) Vulnerability?
XML (eXtensible Markup Language) is a markup language used for describing and structuring data in a machine-readable format. One of the key features of XML is its ability to be easily processed by software. Because XML is a plain text format, it can be read and written by a wide variety of programming languages and platforms. However, this also allows attackers to exploit it.
The XML External Entity (XXE) vulnerability is a type of security vulnerability that affects applications that process XML input. It arises when an attacker is able to inject specially crafted XML data into an application and cause it to disclose sensitive information or perform unauthorized actions.
The vulnerability is caused by the use of XML entities, which are essentially placeholders that refer to a piece of data that is stored externally. When an application parses an XML document containing entity references, it may fetch the contents of the referenced entity from an external source such as a file, URL or database. This can be exploited by an attacker to read arbitrary files, execute remote code, or perform other malicious actions.
For instance, an attacker can craft an XML file containing a malicious external entity that references a sensitive file on the server, such as /etc/passwd. If the application processes this file, it may disclose the contents of the sensitive file to the attacker.
Types of XXE Attacks
The three main types of XXE attacks are:
- In-band XXE: This is the most common type of XXE attack. The attacker can receive the response containing the sensitive data or action performed by the XXE attack in-band with the original request. For instance, an attacker may send an XML file with a malicious external entity that references a sensitive file on the server. If the server processes the file and responds with the contents of the file, the attacker can receive this information in the response.
- Out-of-band XXE: The attacker receives the response containing the sensitive data or action performed by the XXE attack out-of-band with the original request. This can be achieved by exploiting features in the server or application that allow outbound communication, such as DNS or HTTP requests. For example, an attacker may send a malicious XML file that references a domain they control. When the server processes the file, it may perform an HTTP request to the malicious domain, allowing them to receive the sensitive information in the request.
- Blind XXE: The attacker does not receive a response from the server or application. This can occur when the application does not provide any feedback to the attacker, or the feedback is not accessible to the attacker. For example, an attacker may send a malicious XML file that references a sensitive file on the server, but the server does not provide any response. This type of attack can be more difficult to detect and exploit, as the attacker must rely on other techniques to confirm the success of the attack.
What are the Potential Consequences of an XXE Attack for Businesses?
The damage caused by an XXE attack can have significant financial and reputational impacts on businesses. XXE attacks can be used for various types of cybercrime on businesses, including:
- Denial of service (DoS): This involves overloading the server with malicious XML requests, which can result in the server becoming unresponsive or even crashing.
- Local file inclusion (LFI): An attacker can access sensitive files on the server by referencing them in the malicious XML file. This can result in the attacker obtaining sensitive information such as passwords, configuration files, or other confidential data.
- Server-side request forgery (SSRF): XXE attacks can be used to force the server to make requests to external systems on behalf of the attacker, which can be used for various purposes, including port scanning, network reconnaissance, and data exfiltration.
- Port scanning: An attacker can use XXE vulnerabilities to scan for open ports on the server and can identify potential vulnerabilities and attack vectors.
- Remote code execution (RCE): Attackers can execute arbitrary code on the server, resulting in a complete compromise of the server and potentially the entire network. This can allow them to take control of the server, steal sensitive data, or launch further attacks.
How to Detect and Prevent XML External Entities Attacks
There are several ways to detect XML external entity vulnerabilities and attacks:
- Manual code review: One of the most effective ways to detect XXE vulnerabilities is by manually reviewing the code. This involves checking all the code that processes XML input for the use of entity references and ensuring that appropriate input validation is performed on all XML data, including entity references. This can be time-consuming but is highly effective in identifying potential vulnerabilities.
- Code scans: It is important to scan code for vulnerabilities in the early stages of development to identify and fix issues before they make it to production. This can be done through continuous integration/continuous delivery (CI/CD) pipelines or automated code review tools that scan code as it is committed to version control.
- Application security testing: automated tools like software composition analysis (SCA), static application security testing (SAST) and dynamic application security testing (DAST) can be used to automatically detect potential XXE vulnerabilities in the code. These tools can scan the code and identify potential security issues such as the use of external entities without proper validation. This approach can be faster and more efficient than manual code review, but it may generate false positives or false negatives.
To prevent XXE attacks, it is important to follow best practices such as disabling external entity support, using whitelists of trusted external entities, and validating XML input against a schema. It is also essential to keep software and applications up-to-date with the latest security patches and best practices. Additionally, it is critical to educate developers and users on the risks associated with XXE vulnerabilities and how to prevent them.
Conclusion
In conclusion, XXE vulnerabilities are a serious threat to businesses that process XML data. An attacker can use an XXE attack to read sensitive information, execute arbitrary code, or perform unauthorized actions on a server or application. The consequences of an XXE attack can be severe, including loss of revenue, legal consequences, and damage to brand reputation.
Businesses can take several steps to detect and prevent XXE vulnerabilities, such as performing manual code reviews, using static code analysis tools, and scanning code in the early stages of development. By taking these measures, you can minimize the risk of XXE attacks and protect your sensitive data from compromise.
Author Bio: Gilad David Maayan
Gilad David Maayan is a technology writer who has worked with over 150 technology companies including SAP, Imperva, Samsung NEXT, NetApp and Check Point, producing technical and thought leadership content that elucidates technical solutions for developers and IT leadership. Today he heads Agile SEO, the leading marketing agency in the technology industry.
