Recently, Google’s Threat Analysis Group discovered a set of zero-day vulnerabilities in Adobe Flash and the Microsoft Windows kernel that were already being actively used by malware attacks against the Chrome browser. Google alerted both Adobe and Microsoft of the discovery on October 21, and Adobe issued a critical fix to patch its vulnerability last Friday. But Microsoft has yet to patch a critical bug in the Windows kernel that allows these attacks to work—which prompted Google to publicly announce the vulnerabilities today.

“After 7 days, per our published policy for actively exploited critical vulnerabilities, we are today disclosing the existence of a remaining critical vulnerability in Windows for which no advisory or fix has yet been released,” wrote Neel Mehta and Billy Leonard of Google’s Threat Analysis Group.”This vulnerability is particularly serious because we know it is being actively exploited.”

The bug being exploited could allow an attacker to escape from Windows’ security sandbox. The sandbox, which normally allows only user-level applications to execute, lets programs execute without needing administrator access while isolating what it can access on the local system through a set of policies.

But by using a specific type of call to a legacy support Windows system library generally used for the graphics subsystem—win32k.sys—malicious code can escalate its privileges and execute outside of the sandbox, allowing it to execute code with full access to the Windows environment. Win32k.sys has been a problem before: Microsoft issued a warning back in June about a similar privilege escalation problem that had not yet been exploited, and another arrived in August.

Google Chrome already blocks this sort of an attack on Windows 10 using a modification to the Chromium sandbox called “Win32k lockdown.” That approach explicitly denies access to the library. However, code executed in other Web browsers or otherwise launched outside Chrome could still exploit the vulnerability.