This blog aims to equip cybersecurity professionals, especially those on blue teams, with the necessary knowledge to detect and mitigate XXE (XML External Entity) Attacks. Our goal is to deepen your understanding of these threats and provide actionable strategies for enhancing your security posture.
XXE Attacks exploit vulnerabilities in XML parsers by allowing the injection of external entities. This can lead to significant security breaches, including unauthorized data access, server-side request forgery (SSRF), and denial of service (DoS). Understanding these attacks is crucial for defending our digital environments against potential exploits.
In a nutshell, attackers manipulate XML input to include references to external entities. These entities can then be resolved and processed by the XML parser, potentially accessing sensitive data or executing malicious requests.
The potential damage includes unauthorized data access, denial of service, and even server-side request forgery—enabling attackers to interact with internal systems.
XXE, or XML External Entity attacks, involve exploiting the XML parsing process to inject external entities. This can lead to exposure of sensitive data, denial of service, or SSRF.
Vulnerable systems typically include web applications and services that process XML input without proper configuration to disable external entity processing.
In this example, a Java application processes XML input from users without disabling external entities:
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
dbf.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
DocumentBuilder db = dbf.newDocumentBuilder();
Document doc = db.parse(new InputSource(new StringReader(xml)));
This script is vulnerable because it does not explicitly disable external entities. An attacker could inject malicious entities that the XML parser would resolve, leading to potential data breaches or other malicious activities.
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "file:///etc/passwd" >]>
<foo>&xxe;</foo>
2024-06-04 12:34:56,789 INFO [main] org.apache.catalina.core.Standar WrapperValve.invoke Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.io.FileNotFoundException: /etc/passwd (No such file or directory)] with root cause
java.io.FileNotFoundException: /etc/passwd (No such file or directory)
Detecting XXE attacks requires a keen eye on both automated systems and manual review processes. Here's how security teams can enhance their detection capabilities:
Stay alert for signs that might indicate an attempt or a successful XXE attack:
Review your application and server logs regularly for entries like the following, which could indicate an XXE attempt:
java.io.FileNotFoundException: /etc/passwd (No such file or directory)
Leverage both automated tools and manual techniques to enhance your detection efforts:
Implement regular expressions to catch suspicious patterns in log files. Here's a regex pattern that helps detect potential XXE payloads:
<!DOCTYPE[^>]*\\[[^\\]]*\\]>
This regex matches DOCTYPE declarations with embedded entity definitions.
When analyzing logs, you can use specific queries to filter out potential XXE attempts. For instance, in a logging tool like Splunk, you might use:
source="/var/log/application.log" "DOCTYPE" AND "<!ENTITY"
This query checks for logs indicating XML DOCTYPE declarations with entity definitions.
While automated tools can catch many attacks, manual review remains crucial:
Consider a scenario where your IDS flags a suspicious XML payload. Follow up by:
To prevent XXE attacks, follow these best practices:
Disable External Entities: Always configure XML parsers to disable external entity processing. How?
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
dbf.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
dbf.setFeature("<http://apache.org/xml/features/disallow-doctype-decl>", true);
dbf.setFeature("<http://xml.org/sax/features/external-general-entities>", false);
dbf.setFeature("<http://xml.org/sax/features/external-parameter-entities>", false);
Let's examine a documented case of an XXE attack that targeted a major web application. For confidentiality, specific names are omitted.
If you're looking for a hands-on course to detecting web attacks, you can check this out: Detecting Web Attacks - 2
The target was a widely used web application that had an unpatched vulnerability in its XML parsing configuration. The vulnerability allowed attackers to use external entity declarations to access sensitive files.
The attack was first noticed when unusual outbound requests were detected in the server logs. Here’s a more detailed look into the logs and the investigation process.
2024-06-04 03:21:12,345 INFO [main] org.apache.http.impl.execchain.MainClientExec.execute Sending request to <http://malicious-server.com> with parameters [file=/etc/passwd]
2024-06-04 03:21:13,567 INFO [main] org.apache.http.impl.execchain.MainClientExec.execute Received response from <http://malicious-server.com>
Initial Detection
Detailed Log Analysis
Requests like the following were frequently seen in the logs:
<?xml version="1.0"?>
<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/passwd">]>
<foo>&xxe;</foo>
System Impact Assessment
Root Cause Analysis
Temporary Measures
Permanent Fixes
The application was updated to explicitly disable external entity processing in the XML parser configuration.
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
dbf.setFeature("<http://xml.org/sax/features/external-general-entities>", false);
dbf.setFeature("<http://xml.org/sax/features/external-parameter-entities>", false);
dbf.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
Security Patches
User Notification
XXE (XML External Entity) attacks are a serious threat to web applications, leading to unauthorized data access, server-side request forgery, and denial of service. Detecting and preventing these attacks require a combination of automated tools like Web Application Firewalls (WAFs) and Intrusion Detection Systems (IDS), as well as manual techniques such as code reviews and security audits.
Key strategies include disabling external entity processing in XML parsers, validating input rigorously, and maintaining vigilant monitoring of logs and network traffic. A proactive approach, continuous education, and adherence to best practices are essential to protect your systems from XXE attacks.
By understanding the mechanisms of XXE attacks and implementing robust detection and prevention measures, you can significantly enhance your security posture and safeguard your digital environment. For further learning, consult resources like OWASP, PortSwigger, and SANS. Stay informed and proactive in defending against evolving threats.
