26 Feb, 2024

Insufficient protection against XML entity attacks

Insufficient Protection Against XML Entity Attacks

“Insufficient Protection Against XML Entity Attacks” refers to a security vulnerability in applications that parse XML input without implementing proper defenses against malicious entities. XML entities are placeholders for data that can be referenced within XML documents, and attackers may exploit this feature to launch various types of attacks. Here’s a description of this vulnerability:

Description:

XML Entity Expansion:

Attack Vector: Malicious actors can leverage XML entity expansion to cause excessive resource consumption, leading to denial-of-service (DoS) attacks.

Example: An attacker includes malicious XML entities that, when expanded, result in a large number of entities and consume system resources.

External Entity Injection (XXE):

Attack Vector: Attackers may inject external entities, leading to disclosure of sensitive information or remote code execution.

Example: An attacker injects an external entity that reads sensitive files from the server, exfiltrating data.

Parameter Entities Abuse:

Attack Vector: Exploiting parameter entities can lead to the injection of crafted content into the XML document.

Example: An attacker manipulates parameter entities to inject unauthorized content into the XML document structure.

DTD (Document Type Definition) Attacks:

Attack Vector: Malicious DTDs can be used to impact the application’s behavior or exfiltrate sensitive information.

Example: An attacker crafts a DTD that forces the application to make unintended requests, revealing sensitive data.

Consequences:

Information Disclosure: Exploiting XML entity attacks may result in the disclosure of sensitive information such as configuration files or database contents.

Denial of Service: Excessive resource consumption from entity expansion attacks can lead to system unresponsiveness.

Remote Code Execution: In some cases, successful exploitation may lead to remote code execution, allowing attackers to take control of the affected system.

Mitigation Strategies:

Disable External Entities:

Implementation: Configure XML parsers to disable external entity expansion, mitigating XXE attacks.

Example: In Java, set the FEATURE_SECURE_PROCESSING feature to disable external entity expansion.

Use a Secure Parser:

Implementation: Choose and use a secure XML parser that has built-in protections against entity expansion attacks.

Example: Use parsers that have specific features like “Entity Expansion Limit” or “XXE Prevention.”

Input Validation and Whitelisting:

Implementation: Validate and sanitize XML input to ensure it adheres to expected structures, and use whitelists for allowed entities.

Example: Reject any input that includes external entities or disallowed DTD structures.

Content Security Policies:

Implementation: Implement content security policies to restrict the types of entities and DTD structures allowed in XML input.

Example: Define and enforce strict policies on XML content structures and entity usage.

Regular Security Audits:

Implementation: Conduct regular security audits and penetration testing to identify and address vulnerabilities related to XML entity attacks.

Example: Use automated scanning tools to detect and report potential XML entity vulnerabilities.

				
					import xml.etree.ElementTree as ET

def parse_xml(xml_data):
    try:
        # Parse the XML input
        root = ET.fromstring(xml_data)

        # Process the XML content
        for element in root.findall('.//user'):
            username = element.find('username').text
            password = element.find('password').text
            print(f"User: {username}, Password: {password}")

    except ET.ParseError as e:
        print(f"Error parsing XML: {e}")

# Example XML input with an external entity declaration
xml_input = """
<!DOCTYPE data [
    <!ENTITY xxe SYSTEM "file:///etc/passwd">
]>
<data>
    <user>
        <username>john</username>
        <password>secret</password>
    </user>
    <user>
        <username>&xxe;</username>
        <password>malicious</password>
    </user>
</data>
"""

# Process the XML input
parse_xml(xml_input)
				
			

In this example:

The XML input contains a Document Type Definition (DTD) with an external entity declaration (<!ENTITY xxe SYSTEM “file:///etc/passwd”>).

The parse_xml function parses the XML input using Python’s xml.etree.ElementTree.

The code attempts to print user information from the XML, including the username and password.

Scanners that detect vulnerability

OWASP ZAP (Zed Attack Proxy):

Description: ZAP is an open-source security testing tool designed for finding vulnerabilities in web applications.

Example: Use ZAP to intercept and modify XML requests, injecting malicious entities and checking for proper handling and protection mechanisms.

Burp Suite:

Description: Burp Suite is a web application security testing tool that includes features for scanning and crawling web applications.

Example: Employ Burp Suite to perform active scanning on XML input, identifying and reporting XML entity vulnerabilities.

XXEinjector:

Description: XXEinjector is a tool specifically designed for testing and exploiting XXE vulnerabilities.

Example: Use XXEinjector to inject various XML entities and observe how the application responds to different entity expansion scenarios.

OXML_xxe:

Description: OXML_xxe is a tool for testing XXE vulnerabilities in OpenXML file formats.

Example: Test XML processing in applications that handle OpenXML file formats, injecting malicious entities to assess vulnerability.

Yasuo:

Description: Yasuo is a Ruby script that performs a static analysis of XML files to identify potential XXE vulnerabilities.

Example: Run Yasuo on XML files to detect external entity declarations and other XXE-related issues.

Average CVSS score

Assigning an average Common Vulnerability Scoring System (CVSS) score specifically for “Insufficient Protection Against XML Entity Attacks” is challenging because CVSS scores are typically assigned to individual vulnerabilities rather than broader categories. The CVSS score for a vulnerability is influenced by various factors, including the impact, exploitability, and complexity of the specific vulnerability.

CWE information

CWE-611: Improper Restriction of XML External Entity Reference (‘XXE’):

Description: This weakness involves situations where an XML parser processes external entity references from untrusted sources, leading to XML External Entity (XXE) attacks.

Potential Consequences: Information disclosure, denial of service, and remote code execution.

CWE-827: Improper Control of Document Type Definition:

Description: This weakness involves improper control of Document Type Definition (DTD) declarations, leading to security vulnerabilities such as XXE attacks.

Potential Consequences: Information disclosure, denial of service, and remote code execution.

CWE-917: Improper Neutralization of Special Elements used in an Expression Language Statement (‘Expression Language Injection’):

Description: This weakness involves improper handling of expressions within XML documents, leading to injection vulnerabilities.

Potential Consequences: Injection attacks, including XXE, and potential compromise of system integrity.

CWE-942: Permissive XML External Entity Policy:

Description: This weakness involves allowing XML External Entities without proper validation, leading to vulnerabilities like XXE attacks.

Potential Consequences: Information disclosure, denial of service, and remote code execution.

Conclusion and Mitigation

Insufficient protection against XML entity attacks poses a significant risk to applications that process XML input, allowing attackers to exploit vulnerabilities for unauthorized access, information disclosure, or even denial-of-service attacks. Mitigating this type of vulnerability is crucial to ensure the secure handling of XML data and prevent exploitation by malicious actors.

Key Points:

XML Entity Vulnerabilities:

XML entity attacks, such as XML External Entity (XXE) attacks, can lead to information disclosure, unauthorized access, and other security risks.

Impact on Applications:

Applications that parse XML input without adequate protection mechanisms may be susceptible to various types of XML entity attacks.

Mitigation Strategies:

Disable External Entities:

Implementation: Configure XML parsers to disable external entity expansion to prevent XXE attacks.

Example: In Java, set the FEATURE_SECURE_PROCESSING feature to disable external entity expansion.

Use a Secure XML Parser:

Implementation: Choose and use a secure XML parser that explicitly prevents entity expansion from untrusted sources.

Example: Select parsers that have features like “Entity Expansion Limit” or “XXE Prevention.”

Input Validation and Whitelisting:

Implementation: Validate and sanitize XML input to ensure it adheres to expected structures, and use whitelists for allowed entities.

Example: Reject any input that includes external entities or disallowed DTD structures.

Content Security Policies:

Implementation: Implement content security policies to restrict the types of entities and DTD structures allowed in XML input.

Example: Define and enforce strict policies on XML content structures and entity usage.

Regular Security Audits:

Implementation: Conduct regular security audits and penetration testing to identify and address vulnerabilities related to XML entity attacks.

Example: Use automated scanning tools to detect and report potential XML entity vulnerabilities.

Educate Developers:

Implementation: Educate developers on secure coding practices for handling XML input and the risks associated with XML entity attacks.

Example: Provide training sessions on secure XML processing and the importance of validating user input.

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