CVE-2025-9866: Google Chrome Auth Bypass Vulnerability

CVE-2025-9866 Overview

CVE-2025-9866 is a high-severity vulnerability in the Extensions component of Google Chrome prior to version 140.0.7339.80. This inappropriate implementation flaw allows a remote attacker to bypass Content Security Policy (CSP) protections via a crafted HTML page. CSP bypass vulnerabilities are particularly dangerous as they undermine a critical browser security mechanism designed to prevent cross-site scripting (XSS) and other code injection attacks.

Critical Impact

Remote attackers can bypass Content Security Policy protections, potentially enabling the execution of malicious scripts and compromising the integrity of web applications that rely on CSP as a defense mechanism.

Affected Products

• Google Chrome versions prior to 140.0.7339.80
• Chromium-based browsers using the affected Extensions component
• All desktop platforms running vulnerable Chrome versions

Discovery Timeline

• 2025-09-03 - CVE-2025-9866 published to NVD
• 2025-09-04 - Last updated in NVD database

Technical Details for CVE-2025-9866

Vulnerability Analysis

This vulnerability stems from an inappropriate implementation within Chrome's Extensions framework (CWE-693: Protection Mechanism Failure). The flaw allows attackers to craft malicious HTML pages that circumvent Content Security Policy restrictions, which are designed to specify which dynamic resources (scripts, stylesheets, etc.) are permitted to load and execute within a web page context.

When exploited, this vulnerability can defeat CSP protections that web applications rely upon to prevent code injection attacks. The attack requires user interaction—specifically, a victim must navigate to or load a malicious HTML page crafted by the attacker. The impact is significant, affecting the confidentiality, integrity, and availability of the browser session and potentially any web applications the user interacts with.

Root Cause

The root cause is classified as CWE-693 (Protection Mechanism Failure). The Extensions component in affected Chrome versions improperly handles certain conditions that should be governed by Content Security Policy enforcement. This inappropriate implementation creates a path through which attackers can bypass the intended security restrictions, effectively nullifying the CSP protections that web developers rely upon to secure their applications.

Attack Vector

The attack is network-based and requires user interaction. An attacker must convince a victim to visit a malicious webpage containing specially crafted HTML content. The crafted page exploits the Extensions implementation flaw to bypass CSP restrictions, potentially allowing:

• Execution of inline scripts that should be blocked by CSP
• Loading of external resources from unauthorized origins
• Circumvention of script-src, object-src, or other CSP directives
• Potential for secondary attacks such as session hijacking or credential theft

The vulnerability mechanism involves the Extensions API improperly processing or validating content in a way that bypasses CSP enforcement. For full technical details, refer to the Chromium Issue Tracker Entry.

Detection Methods for CVE-2025-9866

Indicators of Compromise

• Unusual script execution patterns that violate the expected CSP policy on protected web pages
• Browser console warnings or errors related to CSP violations that succeed when they should fail
• Unexpected network requests to external domains from pages with strict CSP policies
• Signs of malicious browser extensions or suspicious extension behavior

Detection Strategies

• Monitor browser version inventory across the organization to identify systems running Chrome versions prior to 140.0.7339.80
• Implement endpoint detection rules to flag execution of Chrome versions vulnerable to CVE-2025-9866
• Review web application logs for anomalous CSP violation reports that indicate bypass attempts
• Deploy network monitoring to detect connections to known malicious domains that may host exploit pages

Monitoring Recommendations

• Enable Chrome's CSP violation reporting to capture and analyze policy bypass attempts
• Configure endpoint detection and response (EDR) solutions to monitor for suspicious browser behavior
• Implement web content filtering to block access to known malicious pages attempting exploitation
• Establish baseline browser version policies and alert on non-compliant installations

How to Mitigate CVE-2025-9866

Immediate Actions Required

• Update Google Chrome to version 140.0.7339.80 or later immediately across all managed endpoints
• Enable automatic updates for Chrome to ensure timely deployment of security patches
• Notify users of the vulnerability and instruct them to verify their browser version
• Prioritize updating systems with elevated exposure, such as those used for financial or sensitive operations

Patch Information

Google has addressed this vulnerability in Chrome version 140.0.7339.80. The fix corrects the inappropriate implementation in the Extensions component to properly enforce Content Security Policy restrictions. For detailed release information, see the Google Chrome Release Update.

Organizations should deploy this update through their standard patch management processes, prioritizing internet-facing and user-interactive systems. The Chromium Issue Tracker Entry provides additional technical context about the fix.

Workarounds

• Limit browsing to trusted websites until the patch can be applied
• Disable or restrict browser extensions that are not essential to reduce attack surface
• Implement network-level URL filtering to block known malicious domains
• Consider using browser isolation solutions to contain potential exploitation attempts

# Verify Chrome version on Linux/macOS
google-chrome --version
# Expected output: Google Chrome 140.0.7339.80 or later

# Force Chrome update check (requires restart)
# Navigate to: chrome://settings/help
# Or launch with: google-chrome --check-for-update-interval=1