CVE-2017-7844 in Firefox
Summary
by MITRE
A combination of an external SVG image referenced on a page and the coloring of anchor links stored within this image can be used to determine which pages a user has in their history. This can allow a malicious website to query user history. Note: This issue only affects Firefox 57. Earlier releases are not affected. This vulnerability affects Firefox < 57.0.1.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 03/22/2023
This vulnerability represents a sophisticated tracking mechanism that exploits the intersection of external resource loading and browser history disclosure capabilities. The flaw exists in how Firefox handles external svg images that contain anchor link coloring information, creating a covert channel for history-based fingerprinting. When a user visits a webpage that references an external svg image, and that image contains anchor links with specific color properties, the browser's rendering engine can inadvertently expose information about the user's browsing history through the coloration patterns of these links. This occurs because Firefox's implementation of svg rendering processes anchor link colors in a way that can reveal whether certain pages have been visited, effectively allowing malicious actors to construct a profile of a user's browsing behavior through carefully crafted svg content.
The technical exploitation of this vulnerability relies on the browser's handling of external svg resources and its color processing algorithms for anchor elements. When Firefox loads an external svg image containing anchor links, the browser's rendering engine processes these links and applies color schemes based on the user's history state. The vulnerability stems from the fact that visited links in Firefox typically receive different color treatments than unvisited links, and when these color variations are embedded within svg content, they can be detected and analyzed by malicious websites. This mechanism operates through the browser's internal color processing pipeline, where the specific color values of anchor links within svg images can be measured and interpreted to infer whether particular URLs have been accessed by the user.
The operational impact of this vulnerability extends beyond simple tracking to represent a significant privacy breach that can enable extensive behavioral profiling. Attackers can construct queries that systematically test a user's browsing history by referencing different svg images containing varying anchor link color patterns. This allows for the creation of detailed user profiles that can include information about visited websites, browsing patterns, and potentially sensitive personal information. The vulnerability is particularly concerning because it operates at the browser level and can be exploited without user interaction, making it a stealthy method for persistent tracking. The issue affects not just individual privacy but also represents a potential vector for more sophisticated attacks that could leverage the discovered history information for targeted phishing or social engineering campaigns.
This vulnerability aligns with several cybersecurity frameworks and threat models, including the CWE catalog under CWE-200, which addresses "Information Exposure," and the ATT&CK framework's T1566 category for "Phishing with Malicious Attachments" and T1592 for "Gather Victim Identity Information." The flaw demonstrates how seemingly benign browser features can be exploited to create covert information channels that bypass traditional privacy protections. The vulnerability's classification as a history disclosure issue highlights the importance of considering how browser rendering engines process external resources and the potential for unintended information leakage through visual artifacts. Organizations should be aware that this type of vulnerability can be particularly challenging to detect and mitigate, as it often requires browser-level security updates and may not be apparent through standard network monitoring or security scanning tools.
The mitigation strategy for this vulnerability requires immediate application of Firefox version 57.0.1 or later, which contains the necessary patches to address the svg rendering behavior that enables this tracking mechanism. Browser vendors should implement stricter controls over how external svg content is processed and rendered, particularly regarding the handling of anchor link coloring information that could reveal user history. Users should be advised to keep their browsers updated and to exercise caution when visiting websites that reference external svg resources, especially those that appear suspicious or untrusted. Security teams should monitor for potential exploitation attempts and consider implementing network-level controls that restrict external resource loading from untrusted domains, though this approach may impact legitimate web functionality. The vulnerability underscores the importance of comprehensive browser security testing and the need for continuous monitoring of rendering engine behaviors for potential information leakage mechanisms.