CVE-2011-0681 in Web Browser
Summary
by MITRE
The Cascading Style Sheets (CSS) Extensions for XML implementation in Opera before 11.01 recognizes links to javascript: URLs in the -o-link property, which makes it easier for remote attackers to bypass CSS filtering via a crafted URL.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 10/13/2021
The vulnerability described in CVE-2011-0681 represents a significant security flaw in Opera's handling of CSS extensions for XML documents. This issue specifically affects versions of Opera prior to 11.01 and stems from the browser's improper interpretation of the -o-link CSS property. The vulnerability allows remote attackers to circumvent existing CSS filtering mechanisms by embedding javascript: URLs within CSS stylesheets, creating a potential vector for cross-site scripting attacks. The flaw exploits the browser's failure to properly sanitize or validate URLs referenced through CSS extensions, particularly when these references are embedded in the -o-link property which is part of Opera's proprietary CSS implementation.
The technical nature of this vulnerability aligns with CWE-79, which covers Cross-Site Scripting (XSS) conditions, and CWE-20, which addresses Improper Input Validation. The root cause lies in the browser's insufficient validation of URLs within CSS properties, specifically the -o-link property that Opera uses for its CSS extensions. When Opera processes CSS stylesheets containing javascript: URLs within the -o-link property, it fails to properly filter or reject these potentially malicious references, allowing them to execute within the browser context. This creates a scenario where an attacker can craft malicious CSS content that, when rendered by the vulnerable browser, executes arbitrary javascript code, bypassing traditional CSS filtering protections that would normally prevent such execution.
The operational impact of this vulnerability extends beyond simple XSS attacks, as it represents a fundamental flaw in the browser's security model for handling external CSS content. Attackers can leverage this vulnerability to execute arbitrary code on vulnerable systems, potentially leading to session hijacking, data theft, or complete system compromise. The vulnerability is particularly concerning because it operates at the CSS parsing level, meaning that even if users have proper security measures in place for web content, the malicious CSS content can still execute code when processed by the vulnerable browser. This makes the attack surface broader than typical XSS vectors, as the vulnerability can be triggered through various CSS injection scenarios including malformed external stylesheets, compromised content management systems, or malicious advertising networks.
The implications of this vulnerability are particularly severe in enterprise environments where CSS filtering is commonly implemented as part of security policies. The ability to bypass these protections through CSS extensions undermines the security posture of organizations that rely on CSS-based filtering mechanisms. From an attacker's perspective, this vulnerability provides a stealthy method of executing malicious code without triggering traditional web application firewalls or security scanning tools that might not detect malicious CSS content. The vulnerability also demonstrates the importance of proper input validation and sanitization at multiple levels of the browser rendering stack, as the issue occurs during the CSS parsing phase rather than during traditional web content processing. Organizations should implement immediate mitigations including updating to Opera 11.01 or later, implementing additional network-level filtering, and conducting security reviews of CSS content sources. The vulnerability highlights the need for comprehensive security testing that includes CSS parsing and rendering components, as well as the importance of adhering to security standards such as those outlined in the OWASP Top Ten and NIST cybersecurity guidelines for web application security.