CVE-2019-1429

What is the Internet Explorer Scripting Engine?

Internet Explorer's scripting engine (jscript.dll/jscript9.dll) parses and executes JavaScript/JScript embedded in web pages. Even though IE has been superseded by Edge, its scripting engine remains present on nearly all Windows installations and is still reachable through legacy applications and Office documents that render HTML content, making memory-corruption bugs here durable attack surface for years after IE's peak usage.

Overview

CVE-2019-1429 is a memory corruption vulnerability in the Internet Explorer scripting engine that can lead to remote code execution in the security context of the current user. An attacker who convinces a victim to visit a specially crafted website — or a compromised site delivering malicious content (a "watering hole" pattern common to IE exploit campaigns) — can corrupt memory such that arbitrary code executes as that user. Microsoft rated it Critical/Important with a CVSS 3.1 base score of 7.5, requiring user interaction (visiting a page) but no privileges.

Technical Details

The underlying flaw is a use-after-free (CWE-416): the scripting engine accesses an object in memory after it has been freed, typically due to a script triggering a callback or garbage-collection event that invalidates a reference the engine continues to use. Attackers who can control the freed memory's replacement content can hijack execution flow, typically combined with a separate memory-disclosure primitive to defeat ASLR before achieving full code execution. If the victim is browsing with administrator rights, the attacker gains the same level of control as the user.

Discovery

This vulnerability was resolved in Microsoft's November 2019 Patch Tuesday release. Public details on discovery attribution are limited; use-after-free bugs in the IE/JScript engine were a recurring theme in 2019, several of which (including a separate high-profile IE zero-day, CVE-2019-1367) were reported by external researchers and observed in targeted watering-hole campaigns that year.

Exploitation Context

CISA's KEV catalog entry (added November 2021) confirms this bug has been exploited in the wild. Legacy IE scripting-engine vulnerabilities remain attractive to attackers because IE/jscript.dll is still present on most Windows systems even where Edge is the default browser, and it can be reached indirectly through Office documents and other applications that host the MSHTML/JScript rendering components.

Remediation

1. Apply the November 2019 cumulative Windows update or any subsequent monthly rollup, which includes the fix for CVE-2019-1429.
2. Where feasible, retire Internet Explorer entirely and migrate legacy line-of-business apps off the MSHTML/JScript engine.
3. Disable ActiveX and legacy scripting engines via Group Policy where IE is not business-critical.
4. Use application control/browser isolation to sandbox any remaining legacy IE usage from sensitive systems.
5. Monitor for anomalous IE process behavior (unexpected child processes spawned from iexplore.exe) as an indicator of exploitation attempts.