CVE-2026-5911: Google Chrome Policy Bypass Vulnerability

CVE-2026-5911 Overview

CVE-2026-5911 is a policy bypass vulnerability in the ServiceWorkers implementation of Google Chrome prior to version 147.0.7727.55. This security flaw allows a remote attacker to bypass Content Security Policy (CSP) protections via a crafted HTML page. The vulnerability is classified under CWE-693 (Protection Mechanism Failure), indicating a fundamental weakness in the browser's security boundary enforcement.

Critical Impact

Remote attackers can circumvent Content Security Policy protections through maliciously crafted HTML pages, potentially enabling cross-site scripting attacks or unauthorized script execution that would otherwise be blocked by CSP directives.

Affected Products

• Google Chrome versions prior to 147.0.7727.55
• Chromium-based browsers using vulnerable ServiceWorker implementations
• Desktop platforms running affected Chrome versions (Windows, macOS, Linux)

Discovery Timeline

• 2026-04-08 - CVE-2026-5911 published to NVD
• 2026-04-09 - Last updated in NVD database

Technical Details for CVE-2026-5911

Vulnerability Analysis

This vulnerability represents a Protection Mechanism Failure in Google Chrome's ServiceWorker API. ServiceWorkers are powerful scripts that run in the background and can intercept network requests, enabling features like offline functionality and push notifications. The flaw allows attackers to craft malicious HTML pages that exploit improper policy enforcement within the ServiceWorker context, effectively bypassing Content Security Policy restrictions.

The attack requires user interaction—specifically, the victim must navigate to an attacker-controlled or compromised webpage containing the crafted payload. Once triggered, the vulnerability enables the attacker to execute actions that would normally be blocked by the site's CSP headers, undermining the security model that web applications rely on to prevent content injection attacks.

Root Cause

The root cause is classified under CWE-693: Protection Mechanism Failure. The ServiceWorker implementation in Chrome failed to properly enforce Content Security Policy directives in certain scenarios. This indicates an insufficient validation or enforcement gap in how CSP rules are applied when ServiceWorkers handle or intercept requests, allowing policy-violating content to be processed or executed.

Attack Vector

The attack is network-based and requires minimal complexity but does require user interaction. An attacker would:

1. Create a malicious HTML page containing specially crafted content designed to exploit the ServiceWorker policy bypass
2. Lure a victim to visit the malicious page through phishing, malvertising, or other social engineering techniques
3. The crafted page exploits the ServiceWorker vulnerability to bypass CSP restrictions
4. Content or scripts that should be blocked by the target site's CSP are allowed to execute

The vulnerability affects the integrity of web applications by allowing unauthorized modifications or script execution. For detailed technical information, refer to the Chromium Issue Tracker Entry.

Detection Methods for CVE-2026-5911

Indicators of Compromise

• Unusual ServiceWorker registrations from untrusted origins or unexpected scripts
• CSP violation reports showing blocked content that subsequently executes
• Network traffic to suspicious domains initiated from ServiceWorker contexts
• Browser console errors related to ServiceWorker policy enforcement anomalies

Detection Strategies

• Monitor browser version inventories to identify instances running Chrome versions prior to 147.0.7727.55
• Implement CSP reporting endpoints (report-uri or report-to directives) to detect policy bypass attempts
• Review ServiceWorker registrations across organizational endpoints for unauthorized entries
• Deploy endpoint detection solutions capable of monitoring browser process behavior

Monitoring Recommendations

• Enable Chrome Enterprise logging to capture ServiceWorker registration events
• Configure web application firewalls to detect and alert on crafted HTML payloads targeting ServiceWorker vulnerabilities
• Establish baseline ServiceWorker behavior and alert on deviations
• Monitor for unusual outbound connections from browser processes

How to Mitigate CVE-2026-5911

Immediate Actions Required

• Update Google Chrome to version 147.0.7727.55 or later immediately
• Enable automatic updates for Chrome across all managed endpoints
• Audit currently registered ServiceWorkers and remove any unauthorized entries
• Review and strengthen Content Security Policy headers as a defense-in-depth measure

Patch Information

Google has addressed this vulnerability in Chrome version 147.0.7727.55. The fix corrects the policy enforcement mechanism in the ServiceWorker implementation to properly honor Content Security Policy directives. Organizations should prioritize this update across all endpoints.

For official patch details, see the Google Chrome Update Announcement.

Workarounds

• Consider temporarily disabling ServiceWorkers via Chrome policies if immediate patching is not possible
• Implement strict CSP headers with nonce-based script allowlisting as an additional defense layer
• Use browser isolation solutions to contain potential exploitation attempts
• Block access to known malicious domains that may host exploit pages

# Chrome Enterprise policy to force minimum version requirement
# Add to Chrome policy configuration (Windows Registry example)
reg add "HKLM\SOFTWARE\Policies\Google\Chrome" /v MinimumVersionRequired /t REG_SZ /d "147.0.7727.55" /f

# Verify current Chrome version on endpoint
"C:\Program Files\Google\Chrome\Application\chrome.exe" --version