CVE-2016-1692 in Chrome
Summary
by MITRE
WebKit/Source/core/css/StyleSheetContents.cpp in Blink, as used in Google Chrome before 51.0.2704.63, permits cross-origin loading of CSS stylesheets by a ServiceWorker even when the stylesheet download has an incorrect MIME type, which allows remote attackers to bypass the Same Origin Policy via a crafted web site.
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Analysis
by VulDB Data Team • 08/22/2022
The vulnerability described in CVE-2016-1692 represents a critical security flaw in the Blink rendering engine that powers Google Chrome and other web browsers. This issue specifically affects the handling of CSS stylesheet loading within the context of ServiceWorker implementations, creating a significant bypass of fundamental web security mechanisms. The vulnerability exists in the StyleSheetContents.cpp file within the WebKit source code, which is responsible for managing CSS stylesheet content and its associated loading behaviors. When a ServiceWorker attempts to load CSS resources from cross-origin sources, the browser should enforce strict security policies to prevent unauthorized access to resources that could compromise user privacy or system integrity.
The technical flaw stems from an improper validation of MIME types during the CSS stylesheet loading process. Normally, browsers enforce strict MIME type checking to ensure that resources are loaded according to their intended format and origin. However, in this case, the Blink engine failed to properly validate the MIME type of CSS resources when they were being loaded through ServiceWorker contexts. This oversight allowed attackers to manipulate the loading process by presenting CSS files with incorrect MIME type headers while still maintaining the ability to execute cross-origin resource loading. The vulnerability specifically impacts versions of Chrome prior to 51.0.2704.63, where the security controls were insufficient to prevent this particular bypass scenario.
The operational impact of this vulnerability is substantial as it enables remote attackers to circumvent the Same Origin Policy, which is one of the fundamental security mechanisms protecting web applications. The Same Origin Policy prevents scripts from one origin from accessing resources from another origin, thereby protecting against cross-site scripting attacks and unauthorized data access. When this policy is bypassed through the ServiceWorker mechanism, attackers can load CSS resources from malicious domains and potentially execute code or access sensitive information. This represents a serious threat to web application security as it allows attackers to perform actions that should be restricted by origin-based security controls, potentially leading to data exfiltration, session hijacking, or other malicious activities.
The vulnerability aligns with CWE-345: Insufficient Verification of Data Authenticity, which addresses the lack of proper validation mechanisms for data integrity and authenticity. It also maps to ATT&CK technique T1059.001: Command and Scripting Interpreter - PowerShell, as the attack vector could enable similar bypass techniques. The flaw demonstrates a classic case of insufficient input validation in web browser security contexts, where the system fails to properly verify the authenticity and integrity of loaded resources. Organizations and security professionals should note that this vulnerability represents a critical risk for any system running affected versions of Chrome, as it provides attackers with a direct pathway to bypass core security mechanisms that protect against cross-origin attacks.
The mitigation strategy for this vulnerability involves updating to Chrome version 51.0.2704.63 or later, which includes proper MIME type validation and improved ServiceWorker security controls. Additionally, administrators should implement strict content security policies that limit the ability of ServiceWorkers to load external resources, and consider monitoring for unusual ServiceWorker activity that might indicate exploitation attempts. The fix addresses the root cause by implementing proper MIME type checking within the CSS loading process, ensuring that resources are validated according to their intended format and origin. This represents a standard security patch that resolves the specific validation gap that allowed cross-origin resource loading despite incorrect MIME type headers.
This vulnerability highlights the importance of proper security validation in web browser implementations, particularly in complex systems like ServiceWorker that provide powerful but potentially dangerous capabilities. The issue demonstrates how seemingly minor implementation flaws in security controls can have significant impacts on overall system security, emphasizing the need for comprehensive testing and validation of security mechanisms in browser engines. Organizations should maintain current browser versions and implement appropriate security monitoring to detect and prevent exploitation attempts targeting such vulnerabilities. The remediation process requires careful attention to ensure that updates are properly applied and that existing security policies are reviewed to address any potential gaps that might still exist in the updated system configuration.