CVE-2011-2631 in Web Browser
Summary
by MITRE
The Cascading Style Sheets (CSS) implementation in Opera before 11.11 does not properly handle the column-count property, which allows remote attackers to cause a denial of service (infinite repaint loop and application hang) via a web page, as demonstrated by an unspecified Wikipedia page.
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Analysis
by VulDB Data Team • 11/14/2021
The vulnerability identified as CVE-2011-2631 represents a critical flaw in Opera's CSS rendering engine that specifically targets the column-count property implementation. This issue affects Opera versions prior to 11.11 and demonstrates how seemingly benign CSS properties can be exploited to create significant operational disruptions. The vulnerability operates through a sophisticated mechanism that leverages the browser's handling of multi-column layouts, where the column-count property controls how content flows across multiple columns. When malformed or maliciously constructed CSS code utilizes this property, it triggers an infinite repaint loop that consumes excessive system resources and ultimately causes the browser application to hang completely.
The technical exploitation of this vulnerability occurs when a web page contains CSS code that manipulates the column-count property in a manner that creates a feedback loop within the browser's rendering engine. This specific flaw falls under CWE-129, which addresses improper validation of input data, as the CSS parser fails to properly validate or sanitize the column-count property values. The vulnerability is particularly dangerous because it can be triggered through legitimate web content, making it difficult for users to identify malicious pages. The infinite repaint loop phenomenon creates a condition where the browser continuously attempts to re-render the page content, consuming CPU cycles and memory resources until the application becomes unresponsive or crashes entirely.
From an operational perspective, this vulnerability presents a significant risk to users who browse the web with affected Opera versions, as it can be exploited through any web page that contains malicious CSS code. The impact extends beyond simple inconvenience to potentially compromising the availability of the browser application itself, which can be particularly problematic in enterprise environments where browser stability is crucial. The demonstration of this vulnerability through an unspecified Wikipedia page indicates that even reputable sites could be compromised through supply chain attacks or compromised content delivery networks. This type of vulnerability aligns with ATT&CK technique T1203, which covers exploitation of software vulnerabilities for denial of service attacks, and represents a classic example of how browser-based exploits can be used to achieve system-level disruptions.
The mitigation strategy for CVE-2011-2631 requires immediate upgrading to Opera version 11.11 or later, which contains the necessary patches to address the column-count property handling. Users should also implement additional security measures such as browser hardening configurations that limit CSS processing capabilities and enable security features like content security policies to prevent execution of potentially malicious CSS code. Organizations should consider deploying web application firewalls and implementing strict content filtering to prevent access to pages that might contain malicious CSS constructs. The vulnerability serves as a reminder of the importance of keeping browser software updated and highlights the need for comprehensive security testing of CSS rendering engines, particularly those handling complex layout properties like column-count that can create intricate rendering scenarios.