Bredolab Hook Overwrite Sequence

ThreatFire continues to prevent high levels of activity from the Bredolab downloaders this week. The ongoing spam activity described several weeks ago is not abating. Our research then began to pry into the several kernel level hook overwrite attempts that Bredolab implements with the end goal of evading behavioral based security products. ThreatFire effectively prevents this malware, while other behavioral based products do not seem to perform quite so well, their kernel mode hooks duly overwritten and bypassed.

Two of the kernel hook overwrite attempts abuse straightforward Windows vulnerabilities, and they both have been patched. The other Bredolab hook overwrite attempt targets a mechanism that isn’t officially a vulnerability. When users are not logged in as admin, Bredolab is not effective. Here is the short list of the targeted vulnerabilities, in the order called by the Bredolab code:

1st Bredolab targeted vulnerability – MS07-017 – GDI Local Elevation of Privilege Vulnerability

2nd Bredolab targeted vulnerability- MS08-025 – Windows Kernel Usermode Callback Local Privilege Escalation Vulnerability

3rd Bredolab targeted vulnerability- Flaw allows local users with the SeDebugPrivilege privilege to execute arbitrary code as kernel

Just before exploiting the vulnerabilities to gain access to the kernel, Bredolab copies ntkrnlpa.exe from the drive to a location in virtual memory, examining the code for the addresses of nine kernel APIs that are frequently hooked by security solutions. It finds them and stores the virtual addresses for these api’s in its text section for use in the overwrites:

The first exploit attempt to overwrite security solutions’ hooks involves abusing Windows graphics functionality. After calling MapViewofFile and searching for the api’s listed above in the mapped copy of ntkrnlpa.exe, Bredolab maliciously initializes a Palette object:

Hook overwriting shellcode is delivered via a carefully crafted GetNearestPaletteIndex call:

cr0 manipulation in the shellcode to obtain write permissions on kernel memory here:

The first method will fail for Bredolab if the system is MS07-017 patched (patch your systems!). To account for that issue, Bredolab will check for the patch, and if present, deliver its next exploit.

First, it calls GetDesktopWindow to retrieve a handle to the desktop. Next, it sets up the first of two interrupt trampolines to NtUserMessageCall

After the two are setup, it then tricks ZwSetIntervalProfile to call user mode code from the kernel, passing a pointer to its hook overwrite function

Sometimes these first two exploits do not work on a system for the malware. But Bredolab arrives with a solution for that situation. When the first two are patched, Bredolab checks that its calling user has SeDebugPrivilege privilege

If SeDebugPrivilege is present, Bredolab calls ZwSystemDebugControl with two interesting parameters: Debug_Control_Code=9 and SysDbgCopyMemoryChunks_1. Providing that debug code to the call, Bredolab copies arbitrary code from user space to kernel space:

Using a bug in the read I/O sub-function of NtSystemDebugControl, not shown here, Bredolab writes to kernel memory. It modifies an IDT entry with a pointer to its malicious code, and provides control to the code by again calling ZwSetIntervalProfile.

While the bulk of the attacks appear in the U.S., outbreaks of this stuff occured the past year throughout Italy, England, Germany and Russia as well. Unfortunately, there remains large enough numbers of unpatched systems in these countries to gain these attackers’ attention.

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