Trojan-Downloader:W32/Mebroot.gen!B is a generic detection that identifies the downloader program responsible for fetching the installer for the Mebroot rootkit.
Security programs use generic detections that look for broad patterns of code or behavior to identify similar programs or files. If you suspect the file was incorrectly detected, go to: Removal: Suspect a file is incorrectly detected (a False Positive)?.
Based on the settings of your F-Secure security product, it will either move the file to the quarantine where it cannot spread or cause harm, or remove it.
A False Positive is when a file is incorrectly detected as harmful, usually because its code or behavior resembles known harmful programs. A False Positive will usually be fixed in a subsequent database update without any action needed on your part. If you wish, you may also:
Check for the latest database updates
First check if your F-Secure security program is using the latest updates, then try scanning the file again.
Submit a sample
After checking, if you still believe the file is incorrectly detected, you can submit a sample of it for re-analysis.
Note: If the file was moved to quarantine, you need to collect the file from quarantine before you can submit it.
Exclude a file from further scanning
If you are certain that the file is safe and want to continue using it, you can exclude it from further scanning by the F-Secure security product.
Note: You need administrative rights to change the settings.
This malware is discussed in further detail in the following Labs Weblog posts:
The downloader is known to be distributed to users via a malicious website (driveby download) or via an exploit.
When active, the downloader downloads an encrypted file on port 443 or 80 from:
where (encrypted file) is a defined string. This string is unique in every sample.
Once downloaded, the encrypted file is first saved in an allocated memory where it will be decrypted, then saved to a file in a temporary folder. The file will then be executed.
The encrypted file is encrypted with an RC2 encryption algorithm. The Cipher Hash that is used in the decryption is based on a defined string that is also unique in every sample.