CVE-2021-21220: Google Chrome V8 RCE Vulnerability

CVE-2021-21220 Overview

CVE-2021-21220 is a high-severity heap corruption vulnerability in the V8 JavaScript engine used by Google Chrome and Chromium-based browsers. The flaw stems from insufficient validation of untrusted input in V8, allowing a remote attacker to trigger heap corruption through a crafted HTML page. Successful exploitation leads to arbitrary code execution within the browser renderer process. The vulnerability affects Google Chrome versions prior to 89.0.4389.128 and Chromium-based packages shipped in Fedora 32, 33, and 34. CISA has added CVE-2021-21220 to its Known Exploited Vulnerabilities catalog, confirming active exploitation in the wild. Public exploit code targeting the V8 JIT XOR typer was released shortly after disclosure.

Critical Impact

A remote attacker can achieve arbitrary code execution in the renderer process by luring a user to a malicious web page, with public exploit code available and confirmed in-the-wild exploitation.

Affected Products

• Google Chrome versions prior to 89.0.4389.128
• Fedora 32, 33, and 34 (Chromium packages)
• Gentoo Linux Chromium packages (GLSA 202104-08)

Discovery Timeline

• 2021-04-26 - CVE-2021-21220 published to the National Vulnerability Database
• 2025-10-24 - Last updated in NVD database

Technical Details for CVE-2021-21220

Vulnerability Analysis

The vulnerability resides in the V8 JavaScript engine that powers Chrome and Chromium-derived browsers. V8 applies just-in-time (JIT) compilation to JavaScript, and during type inference the engine assigns range information to numeric operations. The flaw involves the type computation for bitwise operations, where the JIT incorrectly narrows the inferred integer range for certain operations such as XOR. Because the compiler trusts this incorrect range during subsequent optimization passes, bounds checks on array accesses are eliminated. The resulting machine code performs out-of-bounds reads and writes against TypedArray and ArrayBuffer backing stores in the V8 heap, classified under [CWE-787] Out-of-bounds Write.

Root Cause

The root cause is an incorrect typer rule in V8's TurboFan optimizing compiler. The compiler assumes a tighter numeric range than the runtime can actually produce when evaluating specific bitwise expressions. Optimization passes use this faulty range to prove that bounds checks are unnecessary and remove them. At runtime the operation produces values outside the assumed range, enabling indices that escape the array's allocated bounds.

Attack Vector

Exploitation requires only that a victim load attacker-controlled HTML and JavaScript in a vulnerable browser. The crafted page constructs JavaScript objects, triggers JIT compilation of the vulnerable code path, and then executes the optimized function with inputs that produce out-of-bounds memory access. Attackers chain the primitive to corrupt adjacent V8 objects, build arbitrary read/write capabilities, and pivot to shellcode execution inside the renderer sandbox. Combined with a sandbox escape, the chain yields full remote code execution on the host. Public exploits demonstrating this technique are documented in the Packet Storm advisories for Chrome XOR Typer Out-Of-Bounds Access RCE and Chrome V8 JIT XOR Arbitrary Code Execution.

No verified non-public exploitation code is reproduced here. Refer to the linked advisories and the Chrome Bug Report 1196683 for technical details.

Detection Methods for CVE-2021-21220

Indicators of Compromise

• Chrome or Chromium processes spawning unexpected child processes such as cmd.exe, powershell.exe, or shell interpreters from a renderer context.
• Renderer process crashes with access violations in v8.dll or libv8 shortly after visiting an untrusted page.
• Outbound HTTPS connections to newly registered or low-reputation domains immediately following browser activity.
• Browser version strings reporting Chrome builds older than 89.0.4389.128 in endpoint inventory data.

Detection Strategies

• Inventory installed Chrome and Chromium-based browser versions and flag any build below 89.0.4389.128.
• Hunt for renderer-to-shell process lineage in EDR telemetry, where chrome.exe or chromium is the parent of a command interpreter.
• Monitor for unsigned or anomalous memory regions marked executable within Chrome processes.
• Correlate web proxy logs for users visiting unknown sites followed by browser crash events on the endpoint.

Monitoring Recommendations

• Enable browser crash reporting and forward crash dumps to a centralized analysis pipeline.
• Alert on Chrome processes loading non-standard DLLs or injecting into other processes.
• Track endpoints that fail to receive automatic Chrome updates and escalate stale installations.
• Cross-reference CISA Known Exploited Vulnerabilities advisories with patch compliance dashboards.

How to Mitigate CVE-2021-21220

Immediate Actions Required

• Update Google Chrome to version 89.0.4389.128 or later on all managed endpoints.
• Apply Fedora and Gentoo Chromium updates per the Fedora package announcement and Gentoo GLSA 202104-08.
• Update other Chromium-based browsers (Edge, Brave, Opera, Vivaldi) to builds that incorporate the V8 fix.
• Verify patch compliance against the CISA KEV catalog entry, which mandates federal remediation.

Patch Information

Google released the fix in the Chrome Stable Channel update on April 13, 2021, version 89.0.4389.128, documented in the Chrome Releases blog. The corresponding V8 change addresses the incorrect typer behavior that enabled bounds-check elimination. Linux distributions republished the fix through the referenced Fedora and Gentoo advisories.

Workarounds

• Disable JavaScript execution for untrusted sites using browser site settings or enterprise policy until patching is complete.
• Enforce strict site isolation and the Chrome --site-per-process flag to limit the blast radius of renderer compromise.
• Route browser traffic through a secure web gateway that blocks known malicious domains hosting V8 exploits.
• Restrict use of legacy Chromium-based applications that cannot be updated to the patched build.

# Enterprise policy enforcement example for Chrome auto-update on Windows
reg add "HKLM\SOFTWARE\Policies\Google\Update" /v AutoUpdateCheckPeriodMinutes /t REG_DWORD /d 60 /f
reg add "HKLM\SOFTWARE\Policies\Google\Update" /v UpdateDefault /t REG_DWORD /d 1 /f

# Verify installed Chrome version meets the patched baseline
reg query "HKLM\SOFTWARE\Google\Chrome\BLBeacon" /v version