CVE-2015-2502

What Is Internet Explorer?

Internet Explorer was Microsoft's dominant web browser for two decades, handling the vast majority of enterprise and consumer web browsing through the mid-2010s. IE's MSHTML rendering engine and associated scripting engines were the most targeted browser attack surface throughout this period — a zero-day in IE represented potential access to the majority of Windows-based internet users.

Overview

CVE-2015-2502 is a memory corruption zero-day in Microsoft Internet Explorer that was actively exploited in the wild before Microsoft released a patch. Microsoft issued an emergency out-of-band bulletin MS15-093 on August 18, 2015 — one week after August Patch Tuesday — specifically to address this actively exploited zero-day. The pattern mirrors the Hacking Team zero-day patches from July 2015: active exploitation forces an accelerated emergency patch cycle outside the normal monthly cadence.

Affected Versions

Fixed in MS15-093 (August 18, 2015 emergency update).

Technical Details

Root Cause: MSHTML Memory Corruption

CVE-2015-2502 involves out-of-bounds write memory corruption (CWE-787) in Internet Explorer's MSHTML rendering engine. When IE processes a specially crafted web page — containing HTML, CSS, JavaScript, or DOM elements that trigger a specific parsing or rendering path — the engine writes beyond an allocated buffer boundary, corrupting adjacent heap memory.

The corruption can be leveraged via:

1. Heap grooming to place an attacker-controlled object adjacent to the corrupted region
2. ASLR bypass (via a companion information disclosure vulnerability) to determine object locations
3. ROP chain construction to bypass DEP/NX
4. Code execution at the browser process's privilege level

Emergency Patch Cycle

Microsoft's decision to release an out-of-band emergency patch — bypassing the normal monthly Patch Tuesday cycle — reflects the severity of confirmed active exploitation. Emergency IE patches in 2014–2015 (MS14-021, MS15-093, MS15-078) all shared this pattern: a zero-day with confirmed in-the-wild exploitation that created unacceptable risk for the period until the next Patch Tuesday.

Attack Characteristics

Discovery

Identified during analysis of active exploitation in August 2015. Microsoft attributed the zero-day exploitation to targeted attacks and released MS15-093 on an emergency basis to protect users before the next Patch Tuesday (September 8, 2015).

Exploitation Context

• Zero-day exploitation: CVE-2015-2502 was exploited in active targeted attacks before any patch was available — making it particularly dangerous during the window between discovery and the emergency patch
• Targeted attack context: Microsoft's communications around MS15-093 indicated the zero-day was used in targeted attacks against specific organizations, suggesting APT or commercial exploit tool use rather than mass criminal exploitation
• Exploit kit adoption: After the patch, CVE-2015-2502 entered exploit kit rotation as an n-day exploit targeting IE users who had not applied the emergency patch
• CISA KEV (2022): Added April 2022

Remediation

1. Apply MS15-093 (August 18, 2015 emergency IE update). Any IE installation updated after August 2015 includes this fix.

2. Retire Internet Explorer — IE reached end-of-life June 15, 2022 with no further patches. Migrate to Edge or Chrome.

3. Block IE via Group Policy or AppLocker to prevent users from using IE for web browsing while migration is in progress.

4. Enable Enhanced Protected Mode (EPM) in IE 10/11 to limit damage from memory corruption via IE's AppContainer sandbox.