Posté par: igor51 le juillet 02, 2018, 20:00:23
Introduction
Through FireEye Dynamic Threat Intelligence (DTI), we observed RIG
Exploit Kit (EK) delivering a dropper that leverages the
injection technique to inject code that downloads and executes a
Monero miner (similar activity has been reported by
Micro). Apart from leveraging a relatively lesser known injection
technique, the attack chain has some other interesting properties that
we will touch on in this blog post.
Attack Chain
The attack chain starts when the user visits a compromised website
that loads the RIG EK landing page in an iframe. The RIG EK uses
various techniques to deliver the NSIS (Nullsoft Scriptable Install
System) loader, which leverages the PROPagate injection technique to
inject shellcode into explorer.exe. This shellcode executes the next
payload, which downloads and executes the Monero miner. The flow chart
for the attack chain is shown in Figure 1.
Figure 1: Attack chain flow chart
Exploit Kit Analysis
When the user visits a compromised site that is injected with an
iframe, the iframe loads the landing page. The iframe injected into a
compromised website is shown in Figure 2.
Figure 2: Injected iframe
The landing page contains three different JavaScripts snippets, each
of which uses a different technique to deliver the payload. Each of
these are not new techniques, so we will only be giving a brief
overview of each one in this post.
JavaScript 1
The first JavaScript has a function, fa, which returns a VBScript
that will be executed using the execScript function, as shown by the
code in Figure 3.
Figure 3: JavaScript 1 code snippet
The VBScript exploits
which allows it to download the payload and execute it using the code
snippet seen in Figure 4.
Figure 4: VBScript code snippet
JavaScript 2
The second JavaScript contains a function that will retrieve
additional JavaScript code and append this script code to the HTML
page using the code snippet seen in Figure 5.
Figure 5: JavaScript 2 code snippet
This newly appended JavaScript exploits
which utilizes a vulnerability in JSON.stringify. This script
obfuscates the call to JSON.stringify by storing pieces of the exploit
in the variables shown in Figure 6.
Figure 6: Obfuscation using variables
Using these variables, the JavaScript calls JSON.stringify with
malformed parameters in order to trigger CVE-2015-2419 which in turn
will cause native code execution, as shown in Figure 7.
Figure 7: Call to JSON.Stringify
JavaScript 3
The third JavaScript has code that adds additional JavaScript,
similar to the second JavaScript. This additional JavaScript adds a
flash object that exploits
as shown in Figure 8.
Figure 8: JavaScript 3 code snippet
Once the exploitation is successful, the shellcode invokes a command
line to create a JavaScript file with filename u32.tmp, as shown in
Figure 9.
Figure 9: WScript command line
This JavaScript file is launched using WScript, which downloads the
next-stage payload and executes it using the command line in Figure 10.
Figure 10: Malicious command line
Payload Analysis
For this attack, the actor has used multiple payloads and
anti-analysis techniques to bypass the analysis environment. Figure 11
shows the complete malware activity flow chart.
Figure 11: Malware activity flow chart
Analysis of NSIS Loader (SmokeLoader)
The first payload dropped by the RIG EK is a compiled NSIS
executable famously known as SmokeLoader. Apart from NSIS files, the
payload has two components: a DLL, and a data file (named ‘kumar.dll’
and ‘abaram.dat’ in our analysis case). The DLL has an export function
that is invoked by the NSIS executable. This export function has code
to read and decrypt the data file, which yields the second stage
payload (a portable executable file).
The DLL then spawns itself (dropper) in SUSPENDED_MODE and injects
the decrypted PE using process hollowing.
Analysis of Injected Code (Second Stage Payload)
The second stage payload is a highly obfuscated executable. It
consists of a routine that decrypts a chunk of code, executes it, and
re-encrypts it.
At the entry point, the executable contains code that checks the OS
major version, which it extracts from the Process Environment Block
(PEB). If the OS version value is less than 6 (prior to Windows
Vista), the executable terminates itself. It also contains code that
checks whether the executable is in debugged mode, which it extracts
from offset 0x2 of the PEB. If the BeingDebugged flag is set,
the executable terminates itself.
The malware also implements an Anti-VM check by opening the registry
key HKLM\SYSTEM\ControlSet001\Services\Disk\Enum with value 0.
It checks whether the registry value data contains any of the
strings: vmware, virtual, qemu, or xen. Each of these strings is
indictative of virtual machines
After running the anti-analysis and environment check, the malware
starts executing the core code to perform the malicious activity.
The malware uses the
injection method to inject and execute the code in a targeted
process. The PROPagate method is similar to the SetWindowLong
injection technique. In this method, the malware uses the SetPropA
function to modify the callback for UxSubclassInfo and cause the
remote process to execute the malicious code.
This code injection technique only works for a process with lesser
or equal integrity level. The malware first checks whether the
integrity of the current running process is medium integrity level
(2000, SECURITY_MANDATORY_MEDIUM_RID). Figure 12 shows the code snippet.
Figure 12: Checking integrity level of
current process
If the process is higher than medium integrity level, then the
malware proceeds further. If the process is lower than medium
integrity level, the malware respawns itself with medium integrity.
The malware creates a file mapping object and writes the dropper
file path to it and the same mapping object is accessed by injected
code, to read the dropper file path and delete the dropper file. The
name of the mapping object is derived from the volume serial number of
the system drive and a XOR operation with the hardcoded value (Figure 13).
File Mapping Object Name = “Volume Serial Number” + “Volume Serial
Number” XOR 0x7E766791
Figure 13: Creating file mapping object name
The malware then decrypts the third stage payload using XOR and
decompresses it with RTLDecompressBuffer. The third stage payload is
also a PE executable, but the author has modified the header of the
file to avoid it being detected as a PE file in memory scanning. After
modifying several header fields at the start of decrypted data, we can
get the proper executable header (Figure 14).
Figure 14: Injected executable without
header (left), and with header (right)
After decrypting the payload, the malware targets the shell process,
explorer.exe, for malicious code injection. It uses GetShellWindow and
GetWindowThreadProcessId APIs to get the shell window’s thread ID
(Figure 15).
Figure 15: Getting shell window thread ID
The malware injects and maps the decrypted PE in a remote process
(explorer.exe). It also injects shellcode that is configured as a
callback function in SetPropA.
After injecting the payload into the target process, it uses
EnumChild and EnumProps functions to enumerate all entries in the
property list of the shell window and compares it with UxSubclassInfo
After finding the UxSubclassInfo property of the shell window, it
saves the handle info and uses it to set the callback function through SetPropA.
SetPropA has three arguments, the third of which is data. The
callback procedure address is stored at the offset 0x14 from the
beginning of data. Malware modifies the callback address with the
injected shellcode address (Figure 16).
Figure 16: Modifying callback function
The malware then sends a specific message to the window to execute
the callback procedure corresponding to the UxSubclassInfo property,
which leads to the execution of the shellcode.
The shellcode contains code to execute the address of the entry
point of the injected third stage payload using CreateThread. It then
resets the callback for SetPropA, which was modified by malware during
PROPagate injection. Figure 17 shows the code snippet of the injected shellcode.
Figure 17: Assembly view of injected shellcode
Analysis of Third Stage Payload
Before executing the malicious code, the malware performs
anti-analysis checks to make sure no analysis tool is running in the
system. It creates two infinitely running threads that contain code to
implement anti-analysis checks.
The first thread enumerates the processes using
CreateToolhelp32Snapshot and checks for the process names generally
used in analysis. It generates a DWORD hash value from the process
name using a custom operation and compares it with the array of
hardcoded DWORD values. If the generated value matches any value in
the array, it terminates the corresponding process.
The second thread enumerates the windows using EnumWindows. It uses
GetClassNameA function to extract the class name associated with the
corresponding window. Like the first thread, it generates a DWORD hash
value from the class name using a custom operation and compares it
with the array of hardcoded DWORD values. If the generated value
matches any value in the array, it terminates the process related to
the corresponding window.
Other than these two anti-analysis techniques, it also has code to
check the internet connectivity by trying to reach the URL: www.msftncsi[.]com/ncsi.txt.
To remain persistent in the system, the malware installs a scheduled
task and a shortcut file in %startup% folder. The scheduled task is
named “Opera Scheduled Autoupdate {Decimal Value of GetTickCount()}”.
The malware then communicates with the malicious URL to download the
final payload, which is a Monero miner. It creates a MD5 hash value
using Microsoft CryptoAPIs from the computer name and the volume
information and sends the hash to the server in a POST request. Figure
18 shows the network communication.
Figure 18: Network communication
The malware then downloads the final payload, the Monero miner, from
the server and installs it in the system.
Conclusion
Although we have been observing a decline in Exploit Kit activity,
attackers are not abandoning them altogether. In this blog post, we
explored how RIG EK is being used with various exploits to compromise
endpoints. We have also shown how the NSIS Loader leverages the lesser
known PROPagate process injection technique, possibly in an attempt to
evade security products.
FireEye MVX and the FireEye Endpoint Security (HX) platform detect
this attack at several stages of the attack chain.
Acknowledgement
We would like to thank Sudeep Singh and Alex Berry for their
contributions to this blog post.
Source: RIG Exploit Kit Delivering Monero Miner Via PROPagate Injection Technique (http://)