The flaw in question, CVE-2018-4878, is a use-after-free bug that Adobe patched on February 6, following reports that North Korean hackers had been exploiting the vulnerability in attacks aimed at South Korea.
The threat group, tracked as APT37, Reaper, Group123 and ScarCruft, has been expanding the scope and sophistication of its campaigns. After Adobe patched the security hole, which allows remote code execution, other malicious actors started looking into ways to exploit CVE-2018-4878.
Morphisec said it spotted a campaign on February 22, which had been using a version of the exploit similar to the one developed by APT37. However, researchers pointed out that the exploit in the malspam campaign, unlike the one used in the original attacks, did not have a 64-bit version. The attack starts with a spam email containing a link to a document stored on safe-storage[.]biz. Once downloaded and opened, the document informs users that an online preview is not available and instructs them to enable editing mode in order to view the content.
If users comply, the Flash vulnerability is exploited and the Windows command prompt is executed. The associated cmd.exe file is then injected with malicious shellcode that connects to the attacker’s domain. A DLL file is then downloaded by the shellcode and executed using the Microsoft Register Server (regsvr32) utility. The legitimate tool is abused in an effort to bypass whitelisting products.
The malicious documents and the Flash exploit were only detected by a few security solutions based on their signature at the time of Morphisec’s analysis. Since the URLs included in the spam emails were created using Google’s URL shortening service, researchers determined that each of the different links delivered in this campaign had been clicked tens and even hundreds of times within 3-4 days of being created. Users clicked on the links from various browsers and email services, including Outlook, Gmail and Aruba.it.
“As expected and predicted, adversaries have quickly adopted the Flash exploit, which is easily reproducible,” Morphisec’s Michael Gorelik explained in a blog post. “With small variations to the attack, they successfully launched a massive malspam campaign and bypassed most of the existing static scanning solutions once again.”