The more serious of the zero-day vulnerabilities is CVE-2018-8174, a critical issue that allows attackers to remotely execute arbitrary code on all supported versions of Windows.
The existence of the flaw was revealed last month by Chinese security firm Qihoo 360, which reported that a known advanced persistent threat (APT) actor had been exploiting the vulnerability via Internet Explorer and specially crafted Office documents.
Microsoft has credited Qihoo 360 and Kaspersky Lab for reporting this vulnerability. Both companies say the flaw has been exploited in targeted attacks, but no information is currently available on the threat group.
According to Microsoft, the security hole exists due to the way the VBScript engine handles objects in memory. The weakness can be exploited through Internet Explorer by getting the targeted user to visit a malicious website (including via malvertising) or by embedding an ActiveX control marked “safe for initialization” in an application or an Office document that hosts the Internet Explorer rendering engine.
Kaspersky has described it as a use-after-free (UAF) bug. In the attacks observed by the company, the attackers delivered malicious documents set up to download a second-stage payload, specifically a malicious HTML page. The code in this web page triggers the UAF and a shellcode that downloads a malicious payload is executed.
“This technique, until fixed, allowed criminals to force Internet Explorer to load, no matter which browser one normally used – further increasing an already huge attack surface,” explained Anton Ivanov, the Kaspersky Lab researcher credited by Microsoft for reporting this flaw. “Fortunately, proactive discovery of the threat has led to the timely release of the security patch by Microsoft. We urge organizations and private users to install recent patches immediately, as it won't be long before exploits to this vulnerability make it to popular exploit kits and will be used not only by sophisticated threat actors, but also by standard cybercriminals.”
The second zero-day vulnerability patched on Tuesday by Microsoft is CVE-2018-8120, a privilege escalation weakness in Windows. The flaw, related to how the Win32k component handles objects in memory, allows an attacker to execute arbitrary code in kernel mode, but exploitation requires authentication.
Microsoft says the vulnerability only affects Windows 7 and Windows Server 2008 – newer versions of the operating system do not appear to be impacted. An ESET researcher has been credited for reporting this flaw to Microsoft, but the antivirus firm has yet to share any details about the attacks involving CVE-2018-8120.
The May 2018 updates also resolve two Windows vulnerabilities whose details have been made public. The flaws have been rated “important” and they can lead to privilege escalation (CVE-2018-8170) and information disclosure (CVE-2018-8141).
Nearly 20 other issues addressed this month have been rated “critical.” They include memory corruptions in the Edge and Internet Explorer scripting engines and remote code execution flaws in Hyper-V.
Adobe has also released Patch Tuesday updates, but it has only addressed five security bugs in Flash Player, Creative Cloud and Connect.