Don’t write it, read it instead! [by Chun Feng]

Article

The bootkit malware Trojan:Win32/Popureb.E has made some changes in its code compared to previous samples (specifically, Trojan:Win32/Popureb.B), and now it introduces a driver component to prevent the malicious MBR and other malicious data stored as disk sectors from being changed. The driver component protects the data in an unusual way – by hooking the DriverStartIo routine in a hard disk port driver (for example, atapi.sys). The following steps describe the trick:

1. It calls IoGetDeviceAttachmentBaseRef( ) to retrieve the bottom device object in the disk device stack, that is, the real physical disk device object.
2. Then it hooks the DriverStartIo routine in the found device's DRIVER_OBJECT structure (see the picture below). 
3. The hooked DriverStartIo routine monitors the disk write operations: If it finds the write operation is trying to overwrite the MBR or the disk sectors containing malicious code, it simply replaces the write operation with a read operation. The operation will still succeed, however, the data will never actually be written onto the disk. 

If your system is infected with Trojan:Win32/Popureb.E, we advise fixing the MBR using the Windows Recovery Console to return the MBR to a clean state.
To fix the MBR:
1. Open a Windows Recovery Console
• For Windows XP: Installing and using the Recovery Console in Windows XP
• For Windows Vista: System Recovery Options in Windows Vista
• For Windows 7: System Recovery Options in Windows 7
2. Use the tool BOOTREC.exe to fix the MBR as in:

bootrec.exe /fixmbr

More information about using the tool BOOTREC.exe available here.
3. Restart the computer and you can then scan the system to remove any remaining malware.

If you opt to use Windows Restore to remove the malware instead, you must still fix the MBR first, and then restore the system.
-- Chun Feng
Update 6/28/2011: A clarification was made to the blog content concerning remediation efforts for the malware mentioned.

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MBR, bootrec, recovery console, Win32/Popureb