CVE-2011-2635 in Web Browser
Summary
by MITRE
The Cascading Style Sheets (CSS) implementation in Opera before 11.10 allows remote attackers to cause a denial of service (application crash) via vectors involving use of the :hover pseudo-class, in conjunction with transforms, for a floated element.
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Analysis
by VulDB Data Team • 11/14/2021
The vulnerability identified as CVE-2011-2635 represents a significant denial of service flaw within Opera's CSS rendering engine that existed prior to version 11.10. This weakness specifically targets the browser's handling of cascading style sheets and manifests through a particular combination of CSS pseudo-classes and layout properties that can trigger application instability. The flaw demonstrates how seemingly benign CSS declarations can be exploited to cause complete browser crashes, highlighting the complexity and potential danger of CSS parsing implementations in web browsers.
The technical exploitation of this vulnerability occurs when the Opera browser processes CSS rules that combine the :hover pseudo-class with CSS transforms applied to floated elements. This specific combination creates a parsing scenario where the browser's CSS engine encounters an unexpected state during rendering operations. The :hover pseudo-class when used in conjunction with transform properties on floated elements creates a condition where the browser's internal rendering pipeline fails to properly handle the element's state transitions, leading to memory corruption or invalid memory access patterns that ultimately result in application termination.
From an operational perspective, this vulnerability presents a substantial risk to users who may encounter maliciously crafted web pages designed to trigger the specific CSS combination. Attackers can construct web pages containing carefully crafted CSS that, when loaded in vulnerable Opera versions, will cause the browser to crash immediately upon page rendering. This denial of service can be particularly problematic in environments where browser stability is critical, as users may lose access to their browsing session and potentially lose unsaved work or data. The vulnerability affects the core browser functionality rather than individual web applications, making it a systemic risk that impacts all websites and web content accessed through the affected browser.
The vulnerability aligns with CWE-121, which addresses buffer overflow conditions in heap-based memory management, as the browser's CSS engine likely encounters memory management issues when processing the specific element combination. This flaw also relates to the broader category of memory safety issues that can be exploited through browser rendering engines, similar to patterns found in the ATT&CK framework under the T1203 technique for legitimate program execution. The issue demonstrates how CSS parsing engines, which are often overlooked in security assessments, can contain critical flaws that enable denial of service attacks without requiring complex exploitation techniques.
Organizations and users should immediately upgrade to Opera version 11.10 or later to remediate this vulnerability, as no effective workarounds exist for the specific CSS combination that triggers the flaw. Browser vendors should implement more robust input validation and memory management in CSS parsing components, particularly when handling pseudo-classes in combination with layout properties. Security teams should monitor for any similar vulnerabilities in other browser rendering engines and consider the broader implications of CSS-based attacks on web application security, as this vulnerability shows how seemingly simple styling elements can be weaponized to compromise application availability. The remediation process should include comprehensive testing of CSS rendering components to ensure that similar combinations of pseudo-classes and layout properties do not create similar instability conditions in other browser implementations.