Author：Yuki Chen of Qihoo 360 Vulcan Team
In May 2018, the Advanced Threat Response Team of 360 Core Security Division detected an APT attack exploiting a 0-day vulnerability and captured the world’s first malicious office sample that uses a browser 0-day vulnerability. The sample exploited a use-after-free vulnerability in the VBScript engine which was fixed by Microsoft as CVE-2018-8174 in May 2018 security update, the detailed analysis of the bug could be found at: http://blogs.360.cn/blog/cve-2018-8174-en/ After analyzing the patch for CVE-2018-8174 carefully, we realized that the fix was not so complete and there still exists similar problems which could be leveraged to achieve reliable remote code execution in VBScript engine. We reported the issues we found to Microsoft immediately and Microsoft addressed them with a new fix (CVE-2018-8242) in July 2018 security update. In this blog, we will introduce the details of the new exploitable issues with CVE-2018-8174 patch we found. We will also discuss how we exploit the new bug we found and get reliable remote code execution in an upcoming blog (part II).