COSW Ransomware (.cosw File Extension)

What is Cosw?

Discovering files with the .cosw extension on your computer is a clear indicator that you've been targeted by the Cosw ransomware. This malware aims directly at your finances—it encrypts your files and demands a ransom for their decryption. Before encrypting your files, it performs various modifications to your system settings to deactivate security defenses and block the execution of antivirus programs.

The Cosw ransomware utilizes the Salsa20 encryption algorithm. Although this cipher is not the most robust form of encryption, decrypting it through conventional means remains impractical.

The security modifications made by the Cosw virus are extensive. It deactivates Microsoft Defender and tweaks system configurations to prevent the installation of most antivirus software. Furthermore, it modifies the HOSTS file, obstructing access to websites that offer ransomware removal and file decryption advice.

Cosw Encryption process

The Cosw 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 - ".cosw". 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 Cosw 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.

Cosw 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:

External Links:

Indicators of Compromise(IOC)