CVE-2015-2419

What Is JScript in Internet Explorer?

JScript is Microsoft's implementation of JavaScript (ECMAScript) used in Internet Explorer. As the scripting engine that executes JavaScript on web pages, JScript is one of the most security-critical components in IE — memory corruption in the JScript engine can be triggered by malicious web pages and used to achieve arbitrary code execution in the context of the browser process.

JScript and IE's MSHTML (Trident) engine together form the core of Internet Explorer's web rendering pipeline and have historically been the most exploited components in browser-based attacks.

Overview

CVE-2015-2419 is a memory corruption vulnerability in Microsoft Internet Explorer's JScript engine that allows remote attackers to execute arbitrary code or cause a denial-of-service by convincing a user to visit a malicious web page. The JScript engine mishandles specific JavaScript constructs in a way that corrupts memory, enabling code execution. Patched in MS15-065 (July 14, 2015).

Affected Versions

Fixed in MS15-065 (July 2015 cumulative IE update).

Technical Details

Root Cause: JScript Engine Memory Corruption

CVE-2015-2419 involves an out-of-bounds write or type confusion in the JScript scripting engine's handling of specific JavaScript object types, array operations, or dynamic dispatch. When executing specially crafted JavaScript code, the engine writes data to an incorrect memory location — corrupting an adjacent object in a way that enables control flow hijacking.

JScript memory corruption vulnerabilities in this era frequently involved:

• Array bounds violations — JavaScript array operations that failed to validate index bounds
• Type confusion — treating a value of one JavaScript type as another (e.g., treating a number as a pointer)
• Use-after-free in garbage collection — GC collecting an object still referenced by JScript internal state

Drive-By Attack Pattern

1. Attacker sets up a malicious web page or injects JavaScript into a compromised site
2. User visits the page in Internet Explorer
3. IE's JScript engine executes the malicious JavaScript, triggering the memory corruption
4. Code execution achieved in the IE process context
5. Often combined with a privilege escalation (Win32k LPE) for full system compromise

Attack Characteristics

Discovery

Reported to Microsoft and patched in MS15-065 (July 2015 cumulative IE update), which addressed multiple memory corruption, privilege escalation, and information disclosure vulnerabilities across IE 7–11.

Exploitation Context

• Exploit kit deployment: IE JScript vulnerabilities were rapidly integrated into commercial exploit kits (Angler, Nuclear, Magnitude) in 2015; CVE-2015-2419 was among the July 2015 vulnerabilities that entered kit circulation after patch release
• Watering hole attacks: Nation-state actors used IE vulnerabilities in watering hole campaigns — compromising legitimate websites frequented by target organizations and injecting exploit code to attack visitors
• CISA KEV (2022): Added March 2022, confirming continued exploitation against unpatched IE deployments

Remediation

1. Apply MS15-065 (July 2015 cumulative IE update).

2. Retire Internet Explorer — Microsoft ended IE support June 15, 2022. No further patches. Migrate to Edge (Chromium) or Chrome.

3. Block IE via Group Policy or AppLocker to prevent users from opening web content in IE while migration is in progress.

4. Enable Enhanced Protected Mode (EPM) in IE 10/11 — the AppContainer sandbox limits the usefulness of JScript RCE by adding an additional isolation layer.