Threat description




CTB-Locker is ransomware that encrypts files on the affected machine and demands payment in return for the decryption key needed to restore access to the files.


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.

Restore from backup

Like most ransomware, though the malware itself can be removed, the encryption used to take the files hostage is sufficient to make it very difficult to decrypt the files without the necessary decryption key.

In such circumstances, the recommended course of action is to report the crime to the relevant authorities and restore the affected data from a recent, clean backup.

Technical Details

CTB-Locker is downloaded and installed on a system by a separate trojan-downloader program. Once installed on the system, it encrypts files on the system and displays a demand for payment in return for a decryption key to restore access to the affected files.

For more information, see:

Spread in spam e-mails

The CTB-Locker is spread in spam e-mail messages containing a file attachment (typically, of type ZIP or CAB). The attachment may be double-zipped - i.e., a ZIP file contains a second ZIP file. The attachment contains an executable program that uses the .SCR extension, which makes it appear to be a screensaver program. This executable is identified as Trojan-Downloader:W32/Dalexis.

If the user opens the attachment and runs the executable program, Dalexis will contact a predetermined list of compromised websites and download an encrypted copy of CTB-Locker on the user's machine. It will then decrypt and run the ransomware.

Encryption & ranson

Once run, CTB-Locker will encrypt files on the machine and append the original filenames with a randomly generated 7-character long extension.

It will then display a ransom notice and instructions for making the payment, as well as a countdown timer showing how long the user has to pay the ransom demanded. The desktop background is changed to display the same notice and a copy of the instructions is also saved to the My Documents folder.


Details of CTB-Locker were originally included in the Cryptolocker description. They have since been moved to this threat description to minimize confusion between these two ransomware families.

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