NOSU Ransomware (.nosu File Extension)

What is Nosu?

Nosu ransomware is part of the STOP/Djvu ransomware family, which typically targets individuals. This focus is evident not only through statistics but also by examining the specific distribution methods and actions this malware undertakes post-infection. It employs the Salsa20 cipher for file encryption, a method that is effectively impervious to cracking. All decryption keys are stored on a remote server under the control of cybercriminals.

This malware enacts several modifications to system configurations, notably disabling Microsoft Defender and hindering the execution of anti-malware software installation files. These adjustments are facilitated through Group Policies, a Windows interface that provides access to advanced system settings.

Nosu ransomware often proliferates via questionable websites that offer paid programs for free. These sites attract visitors with the allure of bypassing payment, providing hacked versions of software that circumvent licensing requirements. During the hacking of the source code, perpetrators can seamlessly integrate any form of malware. The specific type of malware incorporated may vary based on the payment made for its distribution.

Alternative vectors for the spread of Nosu ransomware include so-called hack tools. Applications such as keygens, Windows activation utilities, cheat engines, and similar tools serve as ideal conduits for malware. Utilizing these tools not only exposes users to the risk of Nosu infection but also poses the threat of legal repercussions for the use of unlicensed software.

Nosu Encryption process

The Nosu uses the Salsa20 encryption algorithm. That is not the strongest method, but it still provides an overwhelming amount of possible decryption keys. To brute force the 78-digit number of keys, you need 3.5 unvigintillion years (1*10^65), even if you use the most powerful regular PC. Quantum computers can show a bit better results, but it is still too slow to get your files back while you are alive.

The exact algorithm of encryption is next: malware scans each folder for the files it is able to encrypt. Then, when it finds the target, it makes a copy of your file, removes the original one, encrypts the copy and leaves it instead of the removed original. Such a procedure is done to prevent the situation when you have already opened the file, so ransomware is not able to read it because of the Windows restrictions. To each encrypted copy, the virus adds the specific extension - ".nosu". Then, ransomware creates a _readme.txt file in the folder where the encrypted file is located, and goes to the next folder.

Such an encryption method can be exploited for file recovery. Since the original file is deleted, you may try to recover it using the file recovery tools. The less time is passed - the bigger the chance to get your files back, so hurry up!

Another specific moment that can help you to use the files even after the encryption is the fact that Nosu ransomware encrypts only the first 150KB of each file. Hence, you can try to run a big file, such as video or music, without the encryption. Similar feature also works with other ransomware families - Dharma, Conti and Makop encrypt the same 150KB.

Nosu note:

Ransom note is the same for the whole STOP/DJVU family. In fact, it is one of the main signs of to which family the certain ransomware belongs. Here is the typical note for STOP/Djvu family:

Indicators of Compromise(IOC)