A recently observed targeted malware campaign against South Korean users was using Hangul Word Processor (HWP) documents as the infection vector, Talos researchers reveal.
Active between November 2016 and January 2017, the campaign was targeting a limited number of people using the Hancom-developed alternative to Microsoft Office, mainly because of its popularity among South Korean users. The malicious documents were written in Korean, allegedly written by the Korean Ministry of Unification.
For an increased sense of legitimacy, the documents attempted to download a file from an official Korean government website: kgls.or.kr (Korean Government Legal Service). The file in question was a binary masquerading as a jpeg file, which was meant to be executed as part of the infection.
Talos researchers suspect the website was compromised specifically to legitimize the attack. They also suggest that a sophisticated actor was behind the campaign, because compromised sites were cleaned or removed after the attack and the final payload was nowhere to be found. Further, the attackers didn’t use the same infrastructure for more than a few days and never returned to used infrastructure.
“Due to these elements it’s likely that this loader has been designed by a well-funded group in order to target public sector entities in South Korea. Many of these techniques fit the profile of campaigns previously associated with attacks by certain government groups,” Talos says.
While uncommon, the use of HWP files for infection makes sense in the context, as the software is widely used within Korea, including by the South Korean government. Furthermore, because this is a regional file format, security devices might not be equipped to process HWP files, providing the attacker with a vector less likely to be detected.
Titled “Analysis of ‘Northern New Year’ in 2017,” the document includes the logo of the Ministry Of Unification, which is working towards the reunification of North & South Korea. The document features details about the North Korean celebration of New Year and includes two links to additional documents at the end, informing users they should double click the links to access these documents.
After opening the decoy document, the binaries execute wscript.exe and inject shellcode into the process. The shellcode, embedded in a resource called ‘BIN’, unpacks a second PE32 in the legitimate wscript.exe process and executes it.
This information might have been used for reconnaissance, to determine if the final payload was to be sent or not. The analyzed sample attempted to connect to an index.php file first, and then to a .jpg file, which might have been automatically generated by the index.php file based on the collected data. The content of the jpg file is saved as ‘officepatch.exe‘ and executed.
Because the infrastructure was down during analysis, the security researchers couldn’t analyze the payload directly. However, they were able to find four C&C servers used by the actor, three located in South Korea and a fourth in the Netherlands. The actors used a MalDoc with multiple droppers for infection and C&C communication to obtain the final payload, along with decoy documents, which reveals that they wanted to use a social engineering / enticement aspect in the attack.
“This campaign has clearly targeted at a specific group of users, this rings true with the use of such specific file formats. Steps were clearly taken to limit the ability of security products to detect the threat as well as adherence to a strict timeline to prevent the malicious files from being discovered. The attackers were careful to remove their malicious payloads and not re-use their infrastructure,” Talos says. The attackers are believed to have attempted “to gain a foothold into assets which can be deemed extremely valuable.”
Responding to a SecurityWeek inquiry via email, a Talos representative said they couldn’t attribute the attacks to a specific actor: “The attackers had access to native Korean speakers and have a high degree of sophistication. However, any conjecture about what specific group or nation state might be behind the attack is pure speculation as the patterns are consistent with a few groups”.
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