Microsoft’s August patch-Tuesday bundle fixes 120 vulnerabilities including two under active exploitation- one of them over two years old.

Weighing in at 120 fixes, the August 2020 Patch Tuesday is the third largest ever released by Microsoft.  Of particular interest are 17 critical updates and two zero-day exploits which are being actively attacked.

IE 11 Remote Code Execution vulnerability

The first zero-day (CVE-2020-1380) affects the scripting engine in Internet Explorer 11 and it is a memory corruption vulnerability which can lead to remote code execution.  The vulnerability can be exploited using a specially crafted website or ActiveX control embedded within a Microsoft Office document.  Even if IE11 is not your default browser, Office will use it to play an embedded video in a Word document for example.

According to Microsoft:

In a web-based attack scenario, an attacker could host a specially crafted website that is designed to exploit the vulnerability through Internet Explorer and then convince a user to view the website. An attacker could also embed an ActiveX control marked “safe for initialization” in an application or Microsoft Office document that hosts the IE rendering engine. The attacker could also take advantage of compromised websites and websites that accept or host user-provided content or advertisements. These websites could contain specially crafted content that could exploit the vulnerability.

Windows Installer Spoofing Vulnerability

The second zero-day (CVE-2020-1464) was first reported to Microsoft two years ago and has remained unpatched yet publicly disclosed until now.

The flaw in Microsoft’s Authenticode signing verification means an attacker can append their own Java executable to a valid signed Windows Installer (.MSI) file and Windows will not notice the tampering and still consider the Installer file to have a valid code signature.  This could allow attackers to smuggle malicious code into systems as some security solutions rely on the validity of the code signatures.

Microsoft’s SigCheck tool from the SysInternals team has identified this attack vector since October 2018 by reporting that the file size does not match the signature.