Users of Microsoft's Windows operating system have grown accustomed to a regular, predictable cadence for patches—on the first Tuesday of every month.
On Dec. 19, Microsoft broke that cadence with an emergency out-of-band update for its Internet Explorer (IE) web browser. The reason is simple: Attackers are actively exploiting a zero-day vulnerability, putting millions of users around the world at immediate risk.
"We released a security update for Internet Explorer after receiving a report from Google about a new vulnerability being used in targeted attacks," the Microsoft Security Response Center (MSRC) wrote in a media advisory.
The flaw impacts multiple versions of IE, ranging from IE 9 on Windows Server 2008 all the way up to IE 11 running on Windows 10. IE has been Microsoft's primary web browser, but with the launch of Windows 10, Microsoft introduced the Edge web browser, which is intended to be the successor to IE. The new flaw apparently is limited to IE and so does not impact Edge.
The zero-day flaw has been designated as CVE-2018-8653 and is identified by Microsoft as a scripting engine memory corruption vulnerability.
"A remote code execution vulnerability exists in the way that the scripting engine handles objects in memory in Internet Explorer," Microsoft warned in its security advisory. "The vulnerability could corrupt memory in such a way that an attacker could execute arbitrary code in the context of the current user."
The memory corruption could have enabled an attacker to have the same access to a system as the logged-in user. Microsoft stated that if a victim browser was logged in as an administrator, the attacker could then get administrative access to a vulnerable system. In that scenario, attackers could do whatever they wanted on a compromised system, including deleting data, installing new programs or even adding new user accounts.
In terms of how the CVE-2018-8653 issue could be exploited, Microsoft stated that in a web-based attack scenario, an attacker could host a specially crafted website that is designed to exploit the vulnerability through IE and then persuade a user to view the website, for example, via a phishing email.
Neither Microsoft nor Google has yet to publicly disclose the actual attacks where the CVE-2018-8653 issue is currently being used, though Microsoft did disclose that Google alerted it that the flaw is being used in targeted attacks.
For Microsoft Windows users, patches for the CVE-2018-8653 issue are now available via the standard Windows update mechanism. Patches are also available on the MSRC issue page for CVE-2018-8653.
There are other measures that enterprises can take to help reduce the risk of the CVE-2018-8653 vulnerability. Multiple Intrusion Prevention Systems (IPS) including the open-source Snort project, which is run by Cisco, have provided updates to help identify and block the issue. By identifying and blocking the issue at the network layer, enterprises can limit the risk of the vulnerability being used against unpatched endpoints.
In its own advisory on the issue, Symantec recommends that users simply not run IE in administrative mode.
"To reduce the impact of latent vulnerabilities, always run nonadministrative software as an unprivileged user with minimal access rights," Symantec advised.
Sean Michael Kerner is a senior editor at eWEEK and InternetNews.com. Follow him on Twitter @TechJournalist.