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Evil Colon Attacks: A Quick Guide

Evil-Colon attacks are cyberattacks that used user input based path operations to exploit the vulnerabilities on windows operating systems.


The high-tech era has made the emergence of new cyber attacks more common than social media trends. One such case of a rapidly evolving threat is the Evil-Colon attack, which shares similarities with Poison-NULL-byte attacks. Despite the fact that poison-NULL-Byte attacks are now non-functioning, it has been suggested that they could have led to new versions of hacking and malware on your systems in case of inappropriate handling. 

In one of his articles, Leon Juranic, a security researcher at Mend, detailed his encounter with the Evil-colon attack. He mentioned that during auditing a source code he discovered a case where an Evil-Colon could be used to evade the path sanitization process. By using novel strategies, the threat actors were able to exploit the vulnerabilities in applications running on Windows operating systems. The analysis concluded that as Evil-Colon is a specific issue in windows-based services, it is more likely to affect any Windows servers. 

When applications or servers use path-based operations, such as using user input when forming the file path, the information stored in that file can be modified by external code flows, which can cause severe security issues like arbitrary data injection, etc. Leon illustrated the working of Evil-Colon with the example of the Java application WriterFile.jsp source code. 

He stated that the working of Evil-Colon includes creating a file in the directory whereas, with sanitization, the new files will append .txt. After passing a colon character at the end of the user’s input, the file gets created as an Altered Data Stream with an arbitrary file extension. 

Later the file is again created in the directory, but as a colon character was added at the end of the filename and it stripped off the rest of the filename string into Alternate Data Stream, the file is recreated with the .jsp extension. 

He furthermore described how the possibility of altering the files that are created earlier in the applicating workflow can lead to serious security threats. When malicious actors can edit the existing files later in code, it will also allow them to modify the .jsp file content into anything they want. On further searching of the modified file in-depth, you will find a string named EVIL-CONTENT. 

Leon concluded his example by warning that, in real-world scenarios, JSP webshell scripts can allow threat actors to remotely execute codes on vulnerable servers or applications. 

To protect your files and data from the Evil-Colon attacks, it is important to remove colon characters from any possible path operations. The elimination of colon characters can be done by using filters, string check operations, etc.
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