Google Chrome V8 JIT Compiler Input Validation Vulnerability leading to Remote Code Execution (CVE-2021-21220)

1. Vulnerability Overview & Root Cause

This vulnerability (CVE-2021-21220) exists in the V8 JavaScript and WebAssembly engine of Google Chrome, specifically within the JIT (Just-In-Time) compiler pipeline known as TurboFan. The root cause is insufficient validation of untrusted input during the 'Typer' phase. When processing specific bitwise operations (such as XOR), the compiler incorrectly predicts the range of the resulting variable. This type confusion leads the compiler to erroneously optimize away critical array bounds checks, assuming they are redundant when they are actually required.

2. Exploitation Mechanics & Attack Scenario

An attacker exploits this vulnerability by hosting a specially crafted HTML page containing malicious JavaScript designed to trigger the JIT compiler's type mismatch. When a victim visits the page, TurboFan compiles the script and optimizes out the bounds check for an array. This grants the script Out-of-Bounds (OOB) read and write access to the engine's memory space. By manipulating the corrupted heap structures, the attacker can construct primitives to execute arbitrary shellcode within the context of the sandboxed renderer process.

3. Infrastructure & Business Impact

As a highly critical vulnerability with an active exploit history in the wild, this flaw is cataloged in CISA's Known Exploited Vulnerabilities (KEV) list. Successful exploitation allows attackers to compromise the integrity of the user's endpoint. Although the renderer sandbox provides a layer of defense, combining this vulnerability with a sandbox escape exploit can lead to full system compromise, unauthorized data exfiltration, and potential lateral movement within corporate networks.