CVE-2018-8373

What is the Internet Explorer Scripting Engine?

Internet Explorer's scripting engine parses and executes JavaScript/JScript on webpages, and remains present and reachable on Windows systems even on machines that primarily use a different default browser, since many applications and Office components embed IE's rendering and scripting components. Memory-safety bugs in this engine have historically been one of the most common routes for browser-based remote code execution against Windows users.

Overview

CVE-2018-8373 is a remote code execution vulnerability caused by memory corruption in how Internet Explorer's scripting engine handles objects in memory. An attacker who convinces a victim to visit a malicious webpage — or view content that invokes the engine, including certain Office documents — can trigger the flaw to execute arbitrary code with the privileges of the logged-on user.

Technical Details

The bug is classified as CWE-787 (Out-of-Bounds Write): a memory corruption condition arises from improper handling of object lifetime or bounds during script execution, allowing an attacker to write attacker-controlled data outside the intended memory region. This is exploitable through crafted JavaScript running in the context of the scripting engine, and, like most IE scripting-engine bugs of this era, requires user interaction (visiting a page or opening a document) but no prior authentication.

Discovery

The vulnerability was addressed through Microsoft's routine security update process as part of the August 2018 Patch Tuesday release.

Exploitation Context

CISA added CVE-2018-8373 to the KEV catalog in March 2022 based on confirmed exploitation. Internet Explorer scripting-engine vulnerabilities from this era were frequently incorporated into commercial and criminal exploit kits and used in both broad drive-by campaigns and more selective spear-phishing operations, since IE and its embedded rendering components remained widely present on enterprise Windows fleets well after IE ceased to be most users' primary browser.

Remediation

1. Apply the August 2018 Windows security update (or later cumulative update) addressing CVE-2018-8373.
2. Retire Internet Explorer as an active browsing tool wherever possible — it has since reached end of support on most supported Windows versions.
3. Restrict or disable ActiveX and legacy scripting features in any application that still embeds IE's rendering engine.
4. Enforce Office Protected View and disable automatic loading of embedded objects that could trigger the vulnerable engine.
5. Monitor endpoint detection tooling for exploitation indicators associated with legacy IE scripting-engine memory-corruption bugs.