CVE-2016-3298

What Is the Internet Explorer Messaging API?

Microsoft Internet Explorer uses the Windows Internet Messaging API (also known as the Windows MAPI or Messaging Application Programming Interface bridge) to interact with certain messaging-related objects in memory. This API handles structured data related to messaging functions accessible through Internet Explorer's object model. When IE processes web content that interacts with this API — through JavaScript, ActiveX, or other browser mechanisms — it may access or expose information about the local system that should be restricted to the local context only.

Information disclosure vulnerabilities in browser components have outsized importance compared to their base CVSS score because they function as attack prerequisites: they break Address Space Layout Randomization (ASLR), enabling subsequent memory corruption exploits that would otherwise be blocked by security mitigations.

Overview

CVE-2016-3298 is an information disclosure vulnerability in Internet Explorer's handling of Internet Messaging API objects in memory. A remote attacker who persuades a user to visit a malicious web page can detect the presence (or absence) of specific files on the victim's local filesystem. While file presence detection sounds benign in isolation, it enables powerful attack chains: confirming which security software, browsers, or system versions are installed informs targeted exploitation; disclosing memory layout information breaks ASLR. Microsoft patched CVE-2016-3298 in MS16-126 (October 11, 2016). CISA added it to the KEV catalog in May 2022.

Affected Versions

Technical Details

Root Cause: Improper Object Handling in Internet Messaging API

CVE-2016-3298 is an information disclosure vulnerability (CWE-200) arising from how Internet Explorer handles Internet Messaging API objects in memory. When IE processes certain web content, it may improperly access or expose memory that includes information about local filesystem paths or file existence state. The specific mechanism allows JavaScript running in a malicious web page to determine whether specific files exist on the victim's local disk by observing timing differences, error states, or explicit data returned through the IE API when it attempts to reference Messaging API resources.

File Presence Detection as an Attack Enabler

File presence detection — knowing whether a specific file exists at a specific path — enables attackers to:

• Enumerate installed security software: Confirm presence of specific AV/EDR products by checking for known installation paths, then select evasion techniques accordingly
• Identify OS version and patch level: Registry-accessible information or version-specific files reveal whether the target is running an older, more vulnerable configuration
• Detect enterprise monitoring tools: Confirm presence of SIEM agents, endpoint detection tools, or forensic utilities
• Target chained exploits: Select the appropriate exploit for the confirmed browser version and patch state

ASLR Bypass Relevance

This vulnerability is particularly significant as an ASLR (Address Space Layout Randomization) bypass enabler. ASLR randomizes the memory locations of loaded modules, making it difficult for attackers to reliably target memory addresses in exploitation. If an information disclosure vulnerability leaks module load addresses or filesystem structure that correlates with memory layout, it reduces or eliminates the randomness that ASLR provides — allowing subsequent memory corruption exploits (use-after-free, buffer overflows) to achieve reliable code execution. This is why information disclosure vulnerabilities in browsers are actively weaponized in exploit chains despite their relatively low standalone CVSS score.

Attack Characteristics

Discovery

Identified by Microsoft security researchers; patched in MS16-126 (October 2016 Internet Explorer Security Update), which addressed multiple IE information disclosure and memory corruption vulnerabilities.

Exploitation Context

• Exploit kit integration: Information disclosure vulnerabilities like CVE-2016-3298 were integrated into exploit kits (Angler, Neutrino, RIG) as ASLR-bypass prerequisites enabling reliable exploitation of IE memory corruption bugs; the kit first used the info disclosure to locate modules in memory, then used the disclosed addresses in the shellcode for a paired RCE exploit
• Targeted attack use: Nation-state actors conducting targeted IE exploitation routinely chained information disclosure vulnerabilities with RCE bugs; CVE-2016-3298's file detection capability specifically enables reconnaissance of the victim system before the main exploit payload is delivered
• IE's declining but still large user base: In October 2016, IE still represented a significant portion of enterprise browser usage due to compatibility requirements with legacy intranet applications; this extended the attack surface despite the broader shift to Chrome and Firefox
• CISA KEV (2022): Added May 2022, reflecting documented exploitation in targeted attack chains

Remediation

1. Apply MS16-126 — install the October 2016 Internet Explorer Security Update via Windows Update, WSUS, or MECM. All subsequent IE cumulative updates include this fix.

2. Migrate from Internet Explorer — Microsoft ended mainstream support for IE 11 in June 2022; migrate users to Microsoft Edge, which includes substantially stronger security architecture. IE mode in Edge provides compatibility for legacy intranet sites without the IE security surface.

3. Disable Internet Explorer entirely — on systems where IE is not required, disable it via Group Policy (Computer Configuration → Administrative Templates → Windows Components → Internet Explorer → "Prevent running Internet Explorer") or via Windows Optional Features.

4. Apply cumulative Windows security updates — all current Windows cumulative updates include IE security patches; maintain monthly patching to ensure all known IE vulnerabilities are addressed.