Nowadays, when antivirus software encounters a giant chunk of encrypted code it triggers an alarm within the antivirus system causing it to quarantine the entire chunk of code, just to be safe. But, the AV program might recognize the presence of the encryption key. This type of encryption would not trigger a virus signature scanner because the scanner couldn’t identify encrypted module as anything. The second way is a bit of a blunt instrument – where the virus encrypts the entire body of the virus leaving only the encrypted virus and a cryptographic key to decrypt it. In this way the virus remains hidden but can be unpacked and used fairly easily. A virus maker might XOR each byte in a virus with some sort of constant value so that it only has to be repeated to decrypt the virus. It’s a simple and fast way of encoding that doesn’t require its own, separate algorithm to decipher it. The first is an older and very small footprint type of encryption that uses the XOR cipher.Ī XOR cipher is a simple form of encoding that encrypts the input by a using a simple key that is XORed against the input to create an output. Viruses encrypt themselves to avoid signature detection in 3 common ways. It doesn’t get any positive matches and believes no virus is present. They change their signature so that it is unique on every infected machine. Some classes of viruses hide themselves by tracking the code snippets anti-virus programs use to identify them and then altering that code snippet every time the virus is injected into a new machine. Here’s where self-modification enters the picture. Instead, antivirus companies use snippets of viruses – more like search strings. To do it with 100% accuracy the anti-virus software would have to compare the entire virus code base against the entire code base of the computer it’s trying to protect. That’s how it works in theory but in the world of anti-virus software this is not failsafe. It’s not unlike taking a section of one’s DNA and comparing it to the same section of the same DNA. Basically, they continually scan files on your computer, take samples of code from them and compare them to a database of known virus snippets. Understanding self-modification requires understanding how anti-virus programs scan for virus signatures. One method of doing this is called ‘Self-Modification.’ Viruses, like Woody Allen in ‘Zelig’ have the ability to change their identity and look and feel. This is a great example of how a system protected by a current, anti-virus anti-malware program can still be insecure. Then those patches have to work their way to the consumer. It’s a hit or miss process that extends the working life of a virus until the injected code or altered file is identified and then patched. what does its code profile look like? Or they may compare the file to a working database of Windows OS files known to be clean. They might examine what’s known as the virus signature by comparing a sample to a known sample of one or more viruses –e.g. Security software companies use several techniques. The interception can be made possible by injecting code into the actual OS files that handle the read request.įinding and preventing this is very hard. In other words, it intercepts the AV program’s request and either denies the request or it gives the anti-virus program a fake, clean version of the requested file. What does a virus trying to hide itself in this situation do? It represents itself as the Windows OS to the anti-virus software. Windows OS: “OK, I recognize you as one of the good guys. It goes a little like this:ĪV program: ‘Hey Windows! I want to examine file name 22450d384281.dll to see if there’s a virus hiding in there. As such, an anti-virus program made by another company has to query the Windows OS by sending a read request to the Windows OS for the files it wants to examine. Read request intercepts take advantage of the fact that large parts of the Windows operating system are proprietary and inaccessible by non-Windows software. Ever wonder how viruses hide themselves from anti-virus programs? Here are five ways.
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