CVE-2011-1691 in Chrome
Summary
by MITRE
The counterToCSSValue function in CSSComputedStyleDeclaration.cpp in the Cascading Style Sheets (CSS) implementation in WebCore in WebKit before r82222, as used in Google Chrome before 11.0.696.43 and other products, does not properly handle access to the (1) counterIncrement and (2) counterReset attributes of CSSStyleDeclaration data provided by a getComputedStyle method call, which allows remote attackers to cause a denial of service (NULL pointer dereference and application crash) via crafted JavaScript code.
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Analysis
by VulDB Data Team • 11/02/2021
The vulnerability identified as CVE-2011-1691 represents a critical NULL pointer dereference flaw within the WebCore component of WebKit browser engine, specifically affecting the counterToCSSValue function in CSSComputedStyleDeclaration.cpp. This issue manifests when applications process CSS style declarations through the getComputedStyle method, creating a pathway for remote attackers to exploit memory access violations. The vulnerability affects versions of Google Chrome prior to 11.0.696.43 and other products utilizing WebKit before revision r82222, demonstrating the widespread impact across browser implementations that rely on this particular CSS processing module. The flaw occurs during the handling of counterIncrement and counterReset attributes, which are part of CSS level 2 and level 3 specifications, making it particularly concerning for web applications that utilize CSS counters for document navigation, numbering systems, and visual styling elements.
The technical execution of this vulnerability involves crafting malicious JavaScript code that triggers the counterToCSSValue function with malformed CSSStyleDeclaration data, specifically targeting the counterIncrement and counterReset properties. When the WebCore engine attempts to process these attributes, it fails to properly validate the CSS style declaration objects, leading to a NULL pointer dereference condition. This memory access violation causes the application to crash and terminate unexpectedly, effectively creating a denial of service condition that can be exploited remotely. The vulnerability falls under CWE-476 which specifically addresses NULL pointer dereference conditions, and demonstrates how improper input validation in CSS processing can lead to application stability issues. The attack vector requires only the ability to execute JavaScript within a target browser context, making it particularly dangerous as it can be delivered through malicious websites or web applications that leverage CSS counter functionality.
The operational impact of CVE-2011-1691 extends beyond simple application crashes, as it represents a potential escalation vector for more sophisticated attacks within the context of browser exploitation. This vulnerability can be leveraged by attackers to disrupt service availability for legitimate users, potentially causing cascading failures in web applications that depend on proper CSS rendering. The flaw's presence in WebKit's CSS implementation means that any web application utilizing getComputedStyle with counter properties could be vulnerable, affecting not only Google Chrome but also other browsers and applications that share this codebase. From an ATT&CK framework perspective, this vulnerability maps to T1211 - Exploitation for Defense Evasion and T1499 - Endpoint Denial of Service, as it enables attackers to cause application instability while potentially masking more sophisticated exploitation attempts. The vulnerability demonstrates how CSS processing, often considered a benign part of web rendering, can become a critical attack surface when proper memory management and input validation are not implemented.
Mitigation strategies for CVE-2011-1691 primarily involve immediate patching of affected software versions, with Google Chrome users upgrading to version 11.0.696.43 or later. Organizations should implement browser hardening measures including disabling unnecessary CSS counter functionality where possible, and deploying web application firewalls that can detect and block malicious JavaScript patterns targeting this vulnerability. The fix implemented in WebKit involved proper validation of CSSStyleDeclaration objects before processing counter attributes, ensuring that NULL pointers are handled gracefully rather than causing application crashes. Security teams should monitor for exploitation attempts through network traffic analysis, particularly looking for JavaScript code that manipulates CSS counter properties in unexpected ways. Additionally, implementing Content Security Policy headers can help limit the execution of malicious scripts that might exploit this vulnerability, while regular security assessments of web applications should include checks for proper CSS handling and validation to prevent similar issues in other CSS processing components.