NUSM Ransomware (.nusm File Extension)

What is Nusm?

The Nusm virus, a type of ransomware within the STOP/Djvu family, specifically targets your finances. This malware encrypts your files using a sophisticated cipher and then demands a ransom (approximately $1000) for decryption. To thwart any alternative decryption methods, the Nusm virus implements various system modifications.

These alterations primarily affect security settings. Initially, the malware disables Microsoft Defender and blocks any third-party security software present. Subsequently, it modifies Group Policies to prevent the execution of antivirus program installation files. The ultimate maneuver involves restricting access to websites offering insights on file decryption and malware eradication. Such changes can impede future PC operations, underscoring the importance of their prompt reversal.

Regarding the distribution of Nusm ransomware, its emergence isn't coincidental; it's often bundled with seemingly innocuous software. "Free" versions of inherently paid applications, crafted by anonymous developers, serve as a common vector. During the modification process, these creators can embed any malicious payload into the program's executable, ranging from spyware and coin miners to ransomware or data stealers. Sidestepping such hazards is advisable, particularly given the legal challenges in holding the culprits accountable.

Nusm Encryption process

The Nusm 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 - ".nusm". 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 Nusm 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.

Nusm 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)