CVE-2025-6556: Google Chrome Auth Bypass Vulnerability

CVE-2025-6556 Overview

CVE-2025-6556 is an insufficient policy enforcement vulnerability in the Loader component of Google Chrome prior to version 138.0.7204.49. This security flaw allows remote attackers to bypass Content Security Policy (CSP) protections through specially crafted HTML pages. CSP is a critical browser security mechanism designed to mitigate cross-site scripting (XSS) and other code injection attacks, making this bypass a significant concern for web application security.

Critical Impact

Attackers can circumvent Content Security Policy protections, potentially enabling script injection and data exfiltration on affected websites that rely on CSP for defense-in-depth security.

Affected Products

• Google Chrome versions prior to 138.0.7204.49
• Chromium-based browsers using vulnerable Loader component
• All platforms (Windows, macOS, Linux)

Discovery Timeline

• 2025-06-24 - CVE-2025-6556 published to NVD
• 2025-07-02 - Last updated in NVD database

Technical Details for CVE-2025-6556

Vulnerability Analysis

The vulnerability resides in Google Chrome's Loader component, which is responsible for fetching and processing web resources. The insufficient policy enforcement allows specially crafted HTML pages to circumvent Content Security Policy restrictions that should normally prevent unauthorized script execution or resource loading.

Content Security Policy serves as a fundamental defense mechanism against XSS attacks by allowing website administrators to specify which sources of content browsers should trust. When this policy enforcement fails in the Loader component, attackers can potentially inject and execute malicious scripts that would otherwise be blocked, compromising the integrity of the affected web pages.

The flaw requires user interaction (visiting a malicious page), but the attack can be executed remotely over the network without requiring authentication or special privileges. The impact includes potential confidentiality and integrity violations, though availability is not affected.

Root Cause

The root cause is classified under CWE-288 (Authentication Bypass Using an Alternate Path or Channel). In this context, the Loader component fails to consistently enforce CSP directives across all resource loading paths, creating an alternate path that bypasses the intended security policy. This inconsistency in policy enforcement allows attackers to craft HTML content that exploits the gap between expected and actual security behavior.

Attack Vector

The attack vector is network-based, requiring a victim to navigate to an attacker-controlled or compromised web page. The attacker constructs a malicious HTML page specifically designed to exploit the policy enforcement gap in the Loader component.

The exploitation flow involves:

1. Attacker crafts an HTML page with content designed to bypass CSP checks
2. Victim navigates to the malicious page using a vulnerable Chrome version
3. The Loader component processes the crafted content without properly enforcing CSP
4. Malicious scripts or resources execute despite the site's CSP restrictions

For technical implementation details, refer to the Chromium Issue Tracker Entry.

Detection Methods for CVE-2025-6556

Indicators of Compromise

• Unusual CSP violation reports in browser console or server logs that indicate blocked content was actually executed
• Web pages loading external scripts or resources from unexpected origins despite restrictive CSP headers
• Anomalous browser behavior where inline scripts execute on pages with strict script-src policies

Detection Strategies

• Monitor CSP report-uri endpoints for unusual patterns or discrepancies between reported violations and actual blocked content
• Implement browser version tracking across endpoints to identify systems running Chrome versions below 138.0.7204.49
• Deploy network monitoring to detect connections to suspicious external resources from pages with restrictive CSP policies

Monitoring Recommendations

• Enable CSP reporting with report-uri or report-to directives to capture potential bypass attempts
• Utilize endpoint detection and response (EDR) solutions to track Chrome browser versions and flag outdated installations
• Configure SentinelOne Singularity to monitor for suspicious web content execution patterns indicative of security policy bypass

How to Mitigate CVE-2025-6556

Immediate Actions Required

• Update Google Chrome to version 138.0.7204.49 or later immediately across all systems
• Enforce automatic Chrome updates through enterprise group policies or MDM solutions
• Review web application CSP configurations to ensure multiple layers of protection against script injection

Patch Information

Google has addressed this vulnerability in Chrome version 138.0.7204.49, released on June 24, 2025. The fix ensures proper enforcement of Content Security Policy directives across all resource loading paths in the Loader component.

For complete details on the security update, refer to the Google Chrome Stable Channel Update Announcement.

Organizations should prioritize deploying this update using their standard browser management processes. The vulnerability affects all Chrome installations below the patched version regardless of operating system.

Workarounds

• Until patching is complete, consider implementing additional server-side input validation and output encoding as defense in depth
• Deploy web application firewalls (WAF) with rules to detect and block common CSP bypass techniques
• Limit access to untrusted websites through browser security policies or proxy filtering where feasible
• Enable Chrome's Enhanced Safe Browsing feature to receive additional protection against malicious web content

# Verify Chrome version via command line (Windows)
"C:\Program Files\Google\Chrome\Application\chrome.exe" --version

# Verify Chrome version via command line (Linux/macOS)
google-chrome --version

# Expected output should show 138.0.7204.49 or later