01 Mar, 2023

XML Bomb (Billion Laughs Attack)

Vulnerability Assessment as a Service (VAaaS)

Tests systems and applications for vulnerabilities to address weaknesses.

XML Bomb (Billion Laughs Attack) is a type of denial-of-service (DoS) attack that targets XML parsers by overwhelming them with a massive amount of nested entities and repeated data. The attack is designed to exploit the way XML parsers expand entities, which can result in the parser using an excessive amount of memory and CPU resources, ultimately leading to a denial of service for the targeted system. The “Billion Laughs” part of the attack refers to the repeated use of a small set of characters to create a large number of nested entities, resulting in an exponentially growing amount of data.

Example of vulnerable code on different programming languages:


in Python:

				
					import xml.etree.ElementTree as ET
xml_string = "<root>" + ("&x;" * 1000000) + "</root>"
parser = ET.XMLParser()
parser.feed(xml_string)
root = parser.close()

				
			


This Python code creates an XML string with a root element containing one million occurrences of the “&x;” entity. The code then creates an instance of the ElementTree XMLParser and feeds the XML string into it using the parser’s feed method. Finally, the close method is called on the parser to retrieve the parsed root element. The use of a large number of nested entities in the XML string can cause the parser to use an excessive amount of memory and CPU resources, potentially leading to a denial of service.

• in Java:

				
					import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;

public class XmlBombExample {
  public static void main(String[] args) throws Exception {
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    DocumentBuilder builder = factory.newDocumentBuilder();
    String xmlString = "<root>" + ("&x;" * 1000000) + "</root>";
    Document doc = builder.parse(new ByteArrayInputStream(xmlString.getBytes()));
  }
}

				
			


This Java code creates an XML string with a root element containing one million occurrences of the “&x;” entity. The code then creates an instance of the DocumentBuilderFactory class and uses it to create a new DocumentBuilder instance. The XML string is then parsed using the parse method of the DocumentBuilder class, which returns a Document object representing the parsed XML. The use of a large number of nested entities in the XML string can cause the parser to use an excessive amount of memory and CPU resources, potentially leading to a denial of service.

• in PHP:

				
					$loadEntities = libxml_disable_entity_loader(false);
$xmlString = "<root>" . str_repeat("&x;", 1000000) . "</root>";
$xml = simplexml_load_string($xmlString);
libxml_disable_entity_loader($loadEntities);

				
			


This PHP code creates an XML string with a root element containing one million occurrences of the “&x;” entity using the str_repeat function. The libxml_disable_entity_loader function is used to enable the loading of external entities in order to allow the “&x;” entity to be expanded during parsing. The simplexml_load_string function is then used to parse the XML string and return a SimpleXMLElement object representing the parsed XML. Finally, the libxml_disable_entity_loader function is called again with the original $loadEntities value to disable entity loading again. The use of a large number of nested entities in the XML string can cause the parser to use an excessive amount of memory and CPU resources, potentially leading to a denial of service.

Examples of exploitation XML Bomb (Billion Laughs Attack)

HTTP POST Request

An attacker could send an HTTP POST request to a vulnerable web application containing a large XML payload with nested entities. The payload could be designed to trigger an XML Bomb (Billion Laughs Attack) and cause the web application’s XML parser to use an excessive amount of memory and CPU resources, ultimately leading to a denial of service.

SOAP Message

An attacker could send a SOAP message to a web service that includes a large XML payload with nested entities. The payload could be designed to trigger an XML Bomb (Billion Laughs Attack) and cause the web service’s XML parser to use an excessive amount of memory and CPU resources, ultimately leading to a denial of service.

XML File

An attacker could send an XML file containing an XML Bomb payload to a vulnerable application that parses XML files. The payload could be designed to trigger an XML Bomb (Billion Laughs Attack) and cause the application’s XML parser to use an excessive amount of memory and CPU resources, ultimately leading to a denial of service.

Privilege escalation techniques for XML Bomb (Billion Laughs Attack)

Use the system crash to launch additional attacks:

If the target system crashes or becomes unresponsive due to an XML Bomb attack, an attacker could use this as an opportunity to launch additional attacks, such as attempting to exploit other vulnerabilities or steal sensitive data.

Gain access to restricted resources:

If the target system is configured to restrict access to certain resources or files, an attacker could potentially use the disruption caused by an XML Bomb attack to gain access to these resources while the system is in a vulnerable state.

Use the disruption to evade detection:

If the target system has intrusion detection or logging mechanisms in place, an XML Bomb attack could potentially be used to disrupt these systems, allowing an attacker to carry out additional attacks without being detected.

Exploit vulnerabilities in recovery processes:

After an XML Bomb attack, the target system may attempt to recover from the disruption by restarting services or processes. An attacker could potentially exploit vulnerabilities in these recovery processes to gain elevated privileges or access to sensitive resources.

General methodology and checklist for XML Bomb (Billion Laughs Attack)

Methodology:

  1. Identify the XML parser being used by the target system: Different XML parsers may have different vulnerabilities or limitations when it comes to processing large XML payloads. It is important to identify the specific parser being used by the target system before testing for an XML Bomb attack.

  2. Craft a test XML payload: Using a tool like XXEinjector or manually creating an XML file, craft a test XML payload that contains nested entities and is designed to trigger an XML Bomb attack.

  3. Send the test XML payload to the target system: Send the test XML payload to the target system via an HTTP POST request, SOAP message, or other method supported by the system.

  4. Monitor the system’s response: Monitor the target system’s response to the test XML payload to determine whether it is vulnerable to an XML Bomb attack. Signs of vulnerability may include excessive memory or CPU usage, unresponsiveness, or a system crash.

  5. Mitigate the vulnerability: If the target system is vulnerable to an XML Bomb attack, work with the system administrators or developers to implement mitigations, such as limiting the size of XML payloads that can be processed or implementing input validation to prevent the processing of malicious XML payloads.

  6. Retest the system: Once mitigations have been implemented, retest the system to ensure that it is no longer vulnerable to an XML Bomb attack.

Checklist:

  1. Determine which XML parser the target system uses to parse incoming XML payloads. Common parsers include the built-in parsers in programming languages like Java and Python, as well as standalone parsers like libxml2.

  2. Use a tool like XXEinjector or manually create an XML file that contains nested entities and is designed to trigger an XML Bomb attack. The payload should be carefully designed to maximize the amount of memory and CPU resources used by the parser.

  3. Send the test XML payload to the target system using a method supported by the system, such as an HTTP POST request or a SOAP message.

  4. Monitor the target system’s response to the test XML payload to determine whether it is vulnerable to an XML Bomb attack. Look for signs of excessive memory or CPU usage, unresponsiveness, or a system crash.

  5. If the target system is vulnerable to an XML Bomb attack, evaluate the severity of the vulnerability based on the impact it could have on the system and the likelihood of an attacker successfully carrying out the attack.

  6. If the vulnerability is deemed to be severe, work with the system administrators or developers to implement mitigations, such as limiting the size of XML payloads that can be processed or implementing input validation to prevent the processing of malicious XML payloads.

  7. Once mitigations have been implemented, retest the system to ensure that it is no longer vulnerable to an XML Bomb attack.

  8. Document the findings of the test, including any vulnerabilities found and mitigations implemented. This documentation can be used to improve the security posture of the target system and to inform future testing efforts.

Tools set for exploiting XML Bomb (Billion Laughs Attack)

Manual tools:

  • XXEinjector: A tool for exploiting XXE vulnerabilities in XML parsers, including XML Bomb attacks. XXEinjector can be used to craft custom payloads and test for vulnerabilities in a variety of XML parsers.

  • XML Bomb Toolkit: A set of tools for testing and exploiting XML Bomb vulnerabilities in various XML parsers. The toolkit includes a custom XML Bomb payload generator and a tool for measuring CPU usage during an attack.

  • Burp Suite: A popular web application testing tool that includes an intercepting proxy for capturing and modifying HTTP traffic. Burp Suite can be used to test for XML Bomb vulnerabilities by intercepting and modifying XML payloads.

  • SoapUI: A testing tool for SOAP-based web services that includes a built-in XML editor and XML validation tools. SoapUI can be used to test for XML Bomb vulnerabilities by sending specially crafted SOAP messages.

  • Wireshark: A network protocol analyzer that can be used to capture and analyze XML traffic. Wireshark can be used to test for XML Bomb vulnerabilities by capturing and analyzing XML payloads sent to a target system.

Automated tools:

  • Acunetix: A web application security scanner that includes checks for XML Bomb vulnerabilities. Acunetix can be used to automatically scan a target system for vulnerabilities and provide detailed reports on any vulnerabilities found.

  • AppScan: A web application security scanner that includes checks for XML Bomb vulnerabilities. AppScan can be used to automatically scan a target system for vulnerabilities and provide detailed reports on any vulnerabilities found.

  • Netsparker: A web application security scanner that includes checks for XML Bomb vulnerabilities. Netsparker can be used to automatically scan a target system for vulnerabilities and provide detailed reports on any vulnerabilities found.

  • OpenVAS: An open source vulnerability scanner that includes checks for XML Bomb vulnerabilities. OpenVAS can be used to automatically scan a target system for vulnerabilities and provide detailed reports on any vulnerabilities found.

  • Nessus: A vulnerability scanner that includes checks for XML Bomb vulnerabilities. Nessus can be used to automatically scan a target system for vulnerabilities and provide detailed reports on any vulnerabilities found.

  • Nmap: A network mapping and vulnerability scanning tool that includes checks for XML Bomb vulnerabilities. Nmap can be used to scan a target network for vulnerable systems and provide detailed reports on any vulnerabilities found.

  • Metasploit: A popular penetration testing framework that includes exploits for XML Bomb vulnerabilities in various XML parsers. Metasploit can be used to automate the process of testing for vulnerabilities and carrying out attacks.

  • Nikto: A web server scanner that includes checks for XML Bomb vulnerabilities. Nikto can be used to automatically scan a target system for vulnerabilities and provide detailed reports on any vulnerabilities found.

  • Vega: A web application scanner and testing platform that includes checks for XML Bomb vulnerabilities. Vega can be used to automatically scan a target system for vulnerabilities and provide detailed reports on any vulnerabilities found.

  • ZAP: A web application scanner and security testing tool that includes checks for XML Bomb vulnerabilities. ZAP can be used to automatically scan a target system for vulnerabilities and provide detailed reports on any vulnerabilities found.

Browser plugins:

  • FoxyProxy: A Firefox and Chrome plugin that allows users to easily switch between different proxy settings. FoxyProxy can be used to intercept and modify XML payloads sent to a target system.

  • Tamper Data: A Firefox plugin that allows users to view and modify HTTP/HTTPS requests and responses. Tamper Data can be used to intercept and modify XML payloads sent to a target system.

  • Hackbar: A Firefox plugin that provides a simple interface for testing and modifying HTTP requests. Hackbar can be used to craft custom

Average CVSS score of stack XML Bomb (Billion Laughs Attack)

The CVSS score for an XML Bomb (Billion Laughs Attack) vulnerability can vary depending on the severity of the vulnerability and the impact it has on the target system. However, in general, the CVSS score for an XML Bomb vulnerability is likely to be high, since the attack can result in denial of service and potentially allow an attacker to execute arbitrary code on the target system. The exact score will depend on the specifics of the vulnerability and the environment in which it is found, so it’s difficult to provide an average CVSS score for the stack as a whole. However, it’s safe to assume that any XML parser that is vulnerable to an XML Bomb attack should be considered a serious security risk and should be patched or replaced as soon as possible.

The Common Weakness Enumeration (CWE)

• CWE-400: Uncontrolled Resource Consumption (‘Resource Exhaustion’) This CWE refers to vulnerabilities that allow an attacker to consume excessive resources, leading to a denial of service. The XML Bomb attack is a specific type of uncontrolled resource consumption, where a maliciously crafted XML file causes an XML parser to consume excessive resources, resulting in a denial of service.

• CWE-404: Improper Resource Shutdown or Release This CWE refers to vulnerabilities where a resource is not properly released or closed after use, which can lead to resource exhaustion or other types of attacks. In the case of an XML Bomb attack, improper release of resources could allow an attacker to continue consuming resources long after the attack should have been stopped.

• CWE-611: Improper Restriction of XML External Entity Reference This CWE refers to vulnerabilities where an XML parser can be tricked into referencing an external entity that is controlled by an attacker. This can lead to a variety of attacks, including information disclosure, denial of service, and arbitrary code execution.

• CWE-712: OWASP Top Ten 2013 Category A8 – Cross-Site Request Forgery (CSRF) This CWE refers to vulnerabilities where an attacker can execute unauthorized actions on behalf of a victim user, typically by tricking the user into performing a malicious action without their knowledge or consent. Although the XML Bomb attack is not typically associated with CSRF attacks, it could potentially be used as part of a larger attack that includes CSRF.

• CWE-770: Allocation of Resources Without Limits or Throttling This CWE refers to vulnerabilities where resources can be allocated without any limits or throttling, leading to a denial of service or other types of attacks. In the case of an XML Bomb attack, the parser may allocate resources without any limits, allowing an attacker to consume all available resources on the system.

• CWE-771: Allocation of System Resources Without Limits or Throttling This CWE is similar to CWE-770, but specifically refers to vulnerabilities where system-level resources can be allocated without any limits or throttling. In the case of an XML Bomb attack, an attacker could potentially consume all available system resources, leading to a complete denial of service.

• CWE-789: Uncontrolled Memory Allocation This CWE refers to vulnerabilities where memory can be allocated without any controls or limits, leading to a denial of service or other types of attacks. In the case of an XML Bomb attack, an attacker could potentially cause the XML parser to allocate excessive amounts of memory, leading to a denial of service.

• CWE-937: OWASP Top Ten 2017 Category A8 – Insecure Deserialization This CWE refers to vulnerabilities where serialized data is not properly validated, leading to potential security issues such as code execution, denial of service, or data tampering. In the case of an XML Bomb attack, the parser may be susceptible to insecure deserialization if it does not properly validate the XML file before parsing it.

• CWE-942: OWASP Top Ten 2017 Category A6 – Security Misconfiguration This CWE refers to vulnerabilities that arise from misconfigured security settings, such as incorrect file permissions, default passwords, or other configuration issues. In the case of an XML Bomb attack, a misconfigured XML parser may be more susceptible to the attack due to weak security settings.

• CWE-943: Improper Neutralization of Special Elements used in an XML External Entity Reference This CWE refers to vulnerabilities where an XML parser does not properly neutralize special characters or elements in external entity references, leading to potential security issues such as information disclosure or code execution. In the case of an XML Bomb attack

CVES related to XML Bomb (Billion Laughs Attack)

• CVE-2014-2730 – The XML parser in Microsoft Office 2007 SP3, 2010 SP1 and SP2, and 2013, and Office for Mac 2011, does not properly detect recursion during entity expansion, which allows remote attackers to cause a denial of service (memory consumption and persistent application hang) via a crafted XML document containing a large number of nested entity references, as demonstrated by a crafted text/plain e-mail message to Outlook, a similar issue to CVE-2003-1564.

XML Bomb (Billion Laughs Attack) exploits

  • Classic XML Bomb: This involves creating an XML document with nested entities that reference themselves recursively, creating an exponentially expanding document.

  • External Entity Expansion: This technique involves using external entities to reference other external entities, leading to a significant increase in the size of the document.

  • Base64 Encoding: In this technique, the malicious payload is base64-encoded to make it difficult to detect by traditional signature-based detection systems.

  • DTD-Based Attacks: Attackers can use Document Type Definitions (DTDs) to reference external entities that point to a large amount of data, leading to a denial of service.

  • XXE Injection: This technique involves injecting malicious XML code into a vulnerable application to exploit an XML External Entity (XXE) vulnerability.

  • Blind XXE Injection: Similar to XXE Injection, this technique does not provide the attacker with direct feedback, but instead relies on timing-based side channels to exfiltrate data.

  • Parameter Entity Expansion: This technique involves using parameter entities to reference other parameter entities, leading to an exponentially expanding document.

Practicing in test for XML Bomb (Billion Laughs Attack)

Set up a test environment: Set up a virtual machine or a container that mimics the target environment. This will allow you to test various defenses without risking damage to the actual environment.

Create an XML Bomb payload: Use one of the techniques mentioned earlier to create an XML Bomb payload. Make sure the payload is large enough to cause a Denial-of-Service (DoS) condition.

Test for vulnerability: Test the target application or system for XML External Entity (XXE) vulnerabilities. There are various automated tools available that can help with this, such as OWASP ZAP, Burp Suite, and Acunetix.

Test for defenses: Test various defenses against XML Bomb attacks, such as input validation, firewall rules, and WAF (Web Application Firewall) rules. Try to bypass these defenses by modifying the payload or using a different technique.

Mitigate the vulnerability: If the target application or system is vulnerable, take steps to mitigate the vulnerability. This may involve patching the system, applying security configurations, or updating the application to a more secure version.

Retest: Once you have mitigated the vulnerability, retest the system to ensure that the vulnerability has been successfully mitigated and that the defenses are effective against XML Bomb attacks.

Repeat the process: Keep testing and retesting the system regularly to ensure that it remains secure against XML Bomb attacks and other vulnerabilities.

For study XML Bomb (Billion Laughs Attack)

OWASP XML External Entity (XXE) Prevention Cheat Sheet: This resource provides detailed guidance on how to prevent XXE vulnerabilities, which are often exploited in XML Bomb attacks. It includes information on secure coding practices, input validation, and other defenses against XXE attacks.

XML Bomb: Billion Laughs Attack: This is a detailed article on XML Bomb attacks, including how they work, how to create an XML Bomb payload, and how to defend against these attacks.

XML Security Cheat Sheet: This resource provides an overview of XML security issues and how to address them. It includes guidance on input validation, proper use of XML parsers, and other defenses against XML-based attacks.

Web Application Security Testing Cheat Sheet: This is a comprehensive resource on web application security testing, including how to test for XXE vulnerabilities and other common security issues. It includes information on various testing tools and techniques, as well as best practices for securing web applications.

OWASP Top Ten Project: The OWASP Top Ten Project is a widely recognized resource on web application security, which includes a list of the top 10 security risks facing web applications. It includes information on XML-related vulnerabilities, such as XXE and XML injection.

Online courses and tutorials: There are various online courses and tutorials available on web application security testing and XML security. These include courses on platforms such as Udemy, Coursera, and edX, as well as tutorials and articles on websites such as OWASP and SANS Institute.

Books with review of XML Bomb (Billion Laughs Attack)

“Web Application Security: A Beginner’s Guide” by Bryan Sullivan and Vincent Liu: This book provides an introduction to web application security and covers topics such as input validation, session management, and access control. It also includes information on XML-based attacks, including XXE and XPath injection.

“The Web Application Hacker’s Handbook: Finding and Exploiting Security Flaws” by Dafydd Stuttard and Marcus Pinto: This book is a comprehensive guide to web application security testing and includes information on XML-based attacks, including XXE vulnerabilities. It covers various testing techniques and tools, and provides practical examples of how to find and exploit security flaws in web applications.

“OWASP Testing Guide v4” by OWASP: This is a comprehensive guide to web application security testing, developed by the Open Web Application Security Project (OWASP). It includes information on testing for XML-based vulnerabilities, including XXE and XML injection.

“XML Security” by Blake Dournaee: This book provides an overview of XML security issues and covers topics such as XML encryption, digital signatures, and access control. It includes information on how to defend against XML-based attacks, including XML Bomb attacks.

“Securing XML: Protecting Your Data with XML Security” by Blake Dournaee: This book provides a practical guide to XML security, covering topics such as XML encryption, digital signatures, and access control. It includes information on how to prevent XML-based attacks, including XML Bomb attacks.

“Web Security, Privacy & Commerce” by Simson Garfinkel and Gene Spafford: This book provides an overview of web security and covers topics such as secure coding practices, authentication, and access control. It includes information on XML-based vulnerabilities, including XXE attacks.

“Secure Coding in C and C++” by Robert Seacord: While this book is focused on secure coding practices in C and C++, it includes information on how to prevent XML-based attacks, including XXE vulnerabilities. It covers topics such as input validation, data integrity, and memory management.

“Hacking Exposed Web Applications, Third Edition” by Joel Scambray, Mike Shema, and Caleb Sima: This book is a comprehensive guide to web application security and covers topics such as input validation, session management, and access control. It includes information on XML-based attacks, including XXE vulnerabilities.

“Mastering Web Application Security” by Dafydd Stuttard and Marcus Pinto: This book provides an in-depth guide to web application security testing and includes information on XML-based attacks, including XXE vulnerabilities. It covers various testing techniques and tools, and provides practical examples of how to find and exploit security flaws in web applications.

“The Tangled Web: A Guide to Securing Modern Web Applications” by Michal Zalewski: This book covers web application security from a technical perspective and includes information on XML-based attacks, including XXE vulnerabilities. It covers various attack vectors and provides practical advice on how to secure web applications against these types of attacks.

How to be protected from XML Bomb (Billion Laughs Attack)

  1. Disable external entity processing: One of the primary ways to prevent XML Bomb is to disable external entity processing in your XML parser. Many XML parsers have the option to turn off external entity processing, and doing so will prevent an XML document from loading external entities.

  2. Use a secure parser: It’s important to use a secure XML parser that is not vulnerable to XML Bomb attacks. Some secure XML parsers include SAX, StAX, and DOM.

  3. Set limits on resource usage: You can set limits on resource usage for your XML parser, such as limiting the amount of memory or CPU usage. This can prevent the parser from being overwhelmed by a large XML document.

  4. Use input validation: You can validate input from untrusted sources, such as user input or data received from a network, to ensure that it conforms to expected patterns and does not contain malicious payloads.

  5. Keep your software up-to-date: Make sure to keep your software up-to-date with the latest security patches and updates, as vulnerabilities that allow XML Bomb attacks can be fixed with updates.

  6. Use a web application firewall: A web application firewall can help detect and block XML Bomb attacks before they reach your web application.

  7. Use a content delivery network: A content delivery network can help mitigate XML Bomb attacks by providing caching and load balancing services that can help distribute the load of a large XML document across multiple servers.

Mitigations for XML Bomb (Billion Laughs Attack)

  1. Disable External Entity Processing: One of the primary ways to prevent XML Bomb is to disable external entity processing in your XML parser. Many XML parsers have the option to turn off external entity processing, and doing so will prevent an XML document from loading external entities.

  2. Use a Secure XML Parser: It’s important to use a secure XML parser that is not vulnerable to XML Bomb attacks. Some secure XML parsers include SAX, StAX, and DOM.

  3. Set Limits on Resource Usage: You can set limits on resource usage for your XML parser, such as limiting the amount of memory or CPU usage. This can prevent the parser from being overwhelmed by a large XML document.

  4. Use Input Validation: You can validate input from untrusted sources, such as user input or data received from a network, to ensure that it conforms to expected patterns and does not contain malicious payloads.

  5. Use Web Application Firewall (WAF): A WAF can help detect and block XML Bomb attacks before they reach your web application.

  6. Use Content Delivery Network (CDN): A CDN can help mitigate XML Bomb attacks by providing caching and load balancing services that can help distribute the load of a large XML document across multiple servers.

  7. Regularly Update Software: Make sure to keep your software up-to-date with the latest security patches and updates, as vulnerabilities that allow XML Bomb attacks can be fixed with updates.

  8. Use a Vulnerability Scanner: Use a vulnerability scanner tool to scan your application and check for vulnerabilities that can be exploited by XML Bomb or other similar attacks.

  9. Implement Rate Limiting: Implement rate limiting to restrict the number of requests or XML documents that can be processed within a specific time frame to avoid being overwhelmed by large payloads.

  10. Educate Developers: Educate your developers on how to write secure code, including how to prevent XML Bomb attacks, and conduct regular security training sessions to ensure that everyone is up-to-date with the latest security practices.

Conclusion

XML Bomb (Billion Laughs Attack) is a type of denial-of-service attack that targets XML parsers by exploiting the use of entities in XML documents. The attack involves creating an XML document with nested entities that expand exponentially, consuming system resources and causing a denial of service.

XML Bomb attacks can have serious consequences, including crashing servers, disrupting services, and causing downtime. It is important for developers and system administrators to be aware of the risks associated with XML Bomb attacks and to implement mitigation techniques to prevent or reduce the impact of such attacks.

Mitigation techniques include limiting the size and depth of XML documents, disabling external entity processing, and using secure XML parsers. It is also important to keep software and systems up-to-date with the latest security patches and to conduct regular security audits and vulnerability assessments.

While XML Bomb attacks can be difficult to detect and prevent, implementing a combination of mitigation techniques and best practices can help reduce the risk of these attacks and improve the overall security of XML-based applications and systems.

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