CVE-2016-9900 in Firefox
Summary
by MITRE
External resources that should be blocked when loaded by SVG images can bypass security restrictions through the use of "data:" URLs. This could allow for cross-domain data leakage. This vulnerability affects Firefox < 50.1, Firefox ESR < 45.6, and Thunderbird < 45.6.
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Analysis
by VulDB Data Team • 11/25/2025
The vulnerability identified as CVE-2016-9900 represents a critical security flaw in Mozilla Firefox and Thunderbird browsers that undermines the intended restrictions on external resource loading within Scalable Vector Graphics images. This weakness specifically targets the browser's handling of data URLs and their interaction with SVG image processing mechanisms, creating an avenue for attackers to circumvent security controls that should prevent unauthorized cross-domain data access. The flaw exploits the browser's interpretation of data URLs within SVG contexts, allowing malicious actors to load external resources that would normally be blocked by security policies, thereby enabling potential data leakage across domain boundaries.
The technical implementation of this vulnerability stems from insufficient validation of data URLs when processing SVG images, particularly in how the browser handles the "data:" URL scheme within SVG content. When SVG images contain embedded references to external resources through data URLs, the security restrictions that typically prevent such access are bypassed due to improper URL parsing and validation logic. This occurs because the browser's security model fails to adequately distinguish between legitimate data URLs and potentially malicious ones when they appear within SVG image contexts, creating a pathway for attackers to retrieve content from restricted domains without proper authorization. The vulnerability specifically affects versions where the SVG processing engine does not properly enforce cross-origin restrictions for data URL resources.
The operational impact of CVE-2016-9900 extends beyond simple information disclosure, as it enables sophisticated cross-domain data leakage attacks that can compromise user privacy and system integrity. Attackers can leverage this vulnerability to extract sensitive information from other domains, potentially accessing cookies, session tokens, or other confidential data that should remain isolated within their respective security domains. The vulnerability affects not only web browsing but also email clients through Thunderbird, making it a comprehensive threat that spans multiple application contexts. This cross-domain leakage capability aligns with attack patterns described in the attack tree framework where adversaries seek to expand their attack surface through indirect means, representing a significant risk to enterprise environments where strict cross-domain security policies are expected to be enforced.
Organizations and users affected by this vulnerability should immediately implement mitigations including updating to patched versions of Firefox, Firefox ESR, and Thunderbird where available. The recommended approach involves applying security patches that correct the URL validation logic within SVG processing components, specifically addressing how data URLs are interpreted and validated within SVG contexts. Additional mitigations include implementing network-level restrictions on data URL usage, configuring browser security policies to limit external resource loading in SVG content, and deploying intrusion detection systems that monitor for suspicious data URL patterns. This vulnerability demonstrates the importance of comprehensive URL validation and the need for robust security boundaries even within seemingly isolated content processing contexts. The flaw's classification aligns with CWE-20, which addresses "Improper Input Validation," and represents a specific instance where the security controls for cross-origin resource sharing are bypassed through improper handling of data URL schemes, making it a significant concern for organizations implementing defense-in-depth strategies against cross-site scripting and data leakage attacks.