CVE-2015-4518 in Firefox
Summary
by MITRE
The Reader View implementation in Mozilla Firefox before 42.0 has an improper whitelist, which makes it easier for remote attackers to bypass the Content Security Policy (CSP) protection mechanism and conduct cross-site scripting (XSS) attacks via vectors involving SVG animations and the about:reader URL.
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Analysis
by VulDB Data Team • 06/26/2022
The vulnerability identified as CVE-2015-4518 resides within the Reader View feature of Mozilla Firefox versions prior to 42.0, representing a critical security flaw that undermines the browser's content security protections. This issue stems from an inadequate whitelist implementation that fails to properly restrict access to potentially malicious content. The Reader View functionality, designed to simplify web page presentation by stripping away advertisements and clutter, inadvertently creates a security loophole when processing certain types of content through the about:reader URL scheme. The improper whitelist allows remote attackers to exploit this mechanism and bypass the Content Security Policy protections that are fundamental to preventing cross-site scripting attacks. This vulnerability specifically impacts the browser's ability to maintain secure boundaries between trusted and untrusted content, creating opportunities for malicious actors to inject harmful scripts into web pages.
The technical flaw manifests through the manipulation of SVG animation elements within the Reader View context, where the overly permissive whitelist fails to adequately validate or sanitize incoming content. When users navigate to pages that utilize the about:reader URL, the browser's security mechanisms become compromised due to the flawed whitelist implementation. This allows attackers to craft malicious SVG content that contains embedded JavaScript or other dangerous elements, which would normally be blocked by CSP policies but are permitted through the vulnerable Reader View implementation. The vulnerability operates at the intersection of web browser security architecture and content filtering mechanisms, exploiting a gap in the security model that should prevent unauthorized access to potentially harmful content. This represents a classic case of insufficient input validation and access control, where the security boundaries established by CSP are effectively circumvented through improper whitelist management.
The operational impact of this vulnerability extends beyond simple XSS exploitation, as it provides attackers with a sophisticated vector for bypassing security protections that are fundamental to modern web browsing. Remote attackers can leverage this flaw to execute malicious scripts against users who have the Reader View feature enabled, potentially leading to session hijacking, data theft, or further compromise of the user's browsing environment. The vulnerability affects users across multiple operating systems and browser configurations, as it resides in the core browser implementation rather than in specific extensions or add-ons. This makes it particularly dangerous because it can be exploited against any Firefox user who visits compromised websites, regardless of their security awareness or additional protective measures. The attack surface is significantly expanded due to the widespread use of Firefox and the common practice of enabling Reader View features, which makes this vulnerability particularly attractive to threat actors seeking broad impact.
Mitigation strategies for CVE-2015-4518 primarily involve upgrading to Firefox version 42.0 or later, where the whitelist implementation has been properly corrected to maintain appropriate security boundaries. Organizations should implement proactive security measures including regular browser updates, network monitoring for suspicious activity, and user education about the risks of visiting untrusted websites. Security administrators should also consider implementing additional content filtering measures and monitoring for exploitation attempts targeting this specific vulnerability. The fix addresses the underlying CWE-20 issue of improper input validation by ensuring that the whitelist properly restricts access to only trusted content sources while maintaining the intended functionality of the Reader View feature. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation and defense evasion, as attackers can bypass security controls to execute malicious code. Organizations should also consider implementing web application firewalls and CSP enforcement mechanisms as additional layers of protection to mitigate the risk of exploitation. Regular security assessments and penetration testing should include verification that the Reader View functionality properly enforces content security policies to prevent similar vulnerabilities from emerging in future implementations.