Detecting, Fixing, and Defending Against XXE Attacks in Python and Java

6 min readJun 16, 2023

Introduction:

XML External Entity (XXE) attacks pose a significant threat to web applications that process XML inputs. These attacks exploit vulnerabilities in XML parsers, enabling attackers to read local files, perform server-side requests, and execute remote code. In this comprehensive guide, we will explore the detection and mitigation of XXE attacks during the early coding cycle. We provide real-world code examples in both Python and Java, along with detailed explanations of the fixes and additional preventive measures. Furthermore, we will discuss the impact of XXE attacks on compliance audits and various industry standards like OWASP and SANS Top 25.

Understanding XXE Attacks:

XXE attacks occur when malicious XML content with external entity references is injected, which can lead to information disclosure, server-side request forgery, and denial of service. These attacks exploit insecure XML parsing and entity resolution, allowing attackers to access sensitive data or manipulate server behavior.

Impact of XXE Attacks:

The impact of XXE attacks can be severe:

Information disclosure: Attackers can access sensitive files such as configuration files, passwords, or private keys, leading to potential data breaches.
Server-side request forgery: XXE can enable attackers to make requests to internal resources or external systems, potentially leading to unauthorized access or further exploitation.
Denial of service: XXE attacks can exhaust server resources by overwhelming the system with excessive XML entity expansions, resulting in a denial of service.

Scenarios and Industry Standards:

XXE vulnerabilities are a common concern across industries and are listed in industry standards such as OWASP and SANS Top 25 Most Dangerous Software Errors. Compliance audits, such as the Payment Card Industry Data Security Standard (PCI DSS) and General Data Protection Regulation (GDPR), emphasize the need for protecting against XXE and other web application vulnerabilities.

Detecting XXE Vulnerabilities:

Identify XML parsing: Look for code segments that parse XML input, such as xml.etree.ElementTree in Python or DocumentBuilder in Java.
Analyze entity references: Check if the XML parser resolves external entity references. Look for patterns like

that indicate potential XXE vulnerability.

3. Review input validation: Examine how input validation is performed on XML inputs. Lack of proper validation or reliance on user-controlled XML can increase the risk of XXE vulnerabilities.

Mitigating XXE Vulnerabilities:

Disable external entity resolution: Configure the XML parser to disable the resolution of external entities. In Python, use etree.parse(source, forbid_entities=True) to prevent entity expansion.
Use whitelist-based validation: Implement a whitelist of allowed elements, attributes, and namespaces. Reject or sanitize any user-defined or untrusted inputs that do not conform to the whitelist.
Utilize secure XML parsers: Replace insecure parsers with secure alternatives that automatically prevent XXE attacks, such as defusedxml in Python or XMLInputFactory with secure settings in Java.

Example Vulnerable Source Code (Python):

Consider the following Python code snippet vulnerable to XXE attacks:

Mitigating XXE Vulnerabilities:

Disable external entity resolution: To mitigate XXE attacks, use the forbid_entities parameter while parsing the XML

2. Use whitelist-based validation: Implement a whitelist of allowed elements, attributes, and namespaces. Reject or sanitize any user-defined or untrusted inputs that do not conform to the whitelist.

3. Utilize secure XML parsers: Instead of the default XML parser, use the defusedxml library, which provides a secure alternative:

Let’s look at another approach to fix this vulnerable Python source code.

In the fixed code:

We import the xml.etree.ElementTree module, which is the default XML parsing module in Python.
We define the XML content with the malicious entity reference (&xxe;).
We create an instance of ET.XMLParser() to customize the XML parser settings.
We set the forbid_external_entities property to True to disable the resolution of external entities.
We set the forbid_entity_expansion property to True to prevent entity expansion.
The parser the argument is passed to the ET.fromstring() function, ensuring that the secure parser instance is used for parsing the XML content.
The parsed XML tree can be accessed and processed as needed.

By configuring the XML parser with the appropriate settings, we can effectively mitigate XXE vulnerabilities in Python.

Example Vulnerable Source Code (Java):

Consider the following Java code snippet vulnerable to XXE attacks:

Mitigating XXE Vulnerabilities:

Disable external entity resolution: Configure the XML parser to disable the resolution of external entities by setting the feature FEATURE_SECURE_PROCESSING to true on the DocumentBuilderFactory object.
Use whitelist-based validation: Implement a whitelist of allowed elements, attributes, and namespaces. Reject or sanitize any user-defined or untrusted inputs that do not conform to the whitelist.
Utilize secure XML parsers: Instead of using the vulnerable DocumentBuilder, switch to SAXParser or XMLInputFactory with secure settings:

Network-Level Defenses:

In addition to code-level fixes, implementing network-level defenses adds an extra layer of protection against XXE attacks:

Web Application Firewall (WAF): Deploy a WAF that includes XXE attack detection and prevention rules. A WAF can inspect incoming XML requests, detect malicious patterns, and block or sanitize them before reaching the application.
Content Security Policy (CSP): Configure CSP headers to restrict the loading of external resources and enforce the same-origin policy. This prevents the loading of external entities or malicious DTDs.
Network Segmentation: Implement proper network segmentation to isolate critical systems from the web application servers. Restricting access to internal resources and using strict firewall rules can limit the impact of successful XXE attacks.

Conclusion:

Detecting, fixing, and defending against XXE attacks are crucial steps in ensuring the security of web applications. By understanding the impact of XXE attacks, implementing code-level fixes, and employing network-level defenses, developers can significantly reduce the risk of XXE vulnerabilities. Compliance with industry standards such as OWASP, SANS Top 25, PCI DSS, and GDPR further reinforces the importance of protecting against XXE attacks. Stay vigilant, adopt secure coding practices, and regularly update defenses to safeguard sensitive data and maintain the integrity of web applications.