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27 de marzo de 2017

Apache Struts Vulnerability



This month is being fearful with the amount of attacks I'm watching against web applications because attackers have found an easy way to get into systems taking advantage of the last Apache Struts2 vulnerability with CVE-2017-5638. Thanks to the Ethical Hacker course I received two years ago and my knowledge about XML and Web Services, I'm going to highlight some steps to know how to exploit this vulnerability, of course, to know if our web services are vulnerable, and we are also going to see what we can do to protect our organization from this attack.

This is a critical vulnerability due to an error handling issue when we send crafted HTTP request to a web server containing a malicious “Content-Type” or “Content-Disposition” field. The remote attacker can exploit this to execute arbitrary code within the context of the application, via a crafted request. Therefore, systems can be compromised and remote attackers can gain control of vulnerable systems. Actually, the affected products are from Struts 2.3.5 to 2.3.31 and from Struts 2.5 to 2.5.10.

How can we know if our web servers are vulnerable? First, asking to our developers to know the Struts version they are using for developing web applications. If we are not lucky, we can check it by our own. Struts 2 is built with Maven, as a result we can find the pom.xml file from the source tree or in jars under META-INF folder:

Struts version

Another way to know if our web applications are vulnerable is with the Nmap tool which has an script NSE called http-vuln-cve2017-5638 that help us to detect whether the specified URL is vulnerable to the CVE-2017-5638. As we know, this is a useful tool for scanning and enumeration before attacking.

Nmap NSE script http-vuln-cve2017-5638

Nmap request and Server response

As you can see, my lab is vulnerable, this is because I have installed a Windows Server 2012 with Apache Tomcat 6 server + MySQL database 5.5 + Java JRE environment + Struts 2.3.20 Apps. This is an easy laboratory to deploy because after installing Apache, MySQL and Java, we just have to copy the struts2-blank.war file into the Apache Software Foundation → Tomcat 6.0 → webapps folder and restart the Tomcat service.

Struts Hello World Webpage
 
In addition, I have tested this vulnerability with the script published by Hack Players where I have been able to see the “Content-Type” header into HTTP requests while I was executing arbitrary code like “dir”, “whoami” or “ipconfig”.

Python script CVE-2017-5638.py
  
Python request and Server response

Once we know if our web servers are vulnerable, and while developers upgrade or fix applications, we have to take measures. The first and the traditional way is with an IPS engine where we should check if our firewall has the signature to block this kind of attack.

IPS Sensor
 
The second step would be to install or upgrade an IDS system to check if all attacks are being blocked by the firewall. If we have alarms inside our network, maybe the IPS engine is not working properly or it is not well configured.

IDS Rule Detection

Finally, the most professional way is to install and upgrade a Web Application Firewall (WAF) where we'll configure the strings allowed into HTTP headers, even for “Content-Type” and “Content-Disposition”. This is the best way to block Web Application Vulnerabilities, Preventing Web Scraping and implementing Virtual Patching as well.

Regards my friend and remember, open you eyes and protect your web servers.

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