CVE-2017-7801 in Firefox
Summary
by MITRE
A use-after-free vulnerability can occur while re-computing layout for a "marquee" element during window resizing where the updated style object is freed while still in use. This results in a potentially exploitable crash. This vulnerability affects Thunderbird < 52.3, Firefox ESR < 52.3, and Firefox < 55.
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Analysis
by VulDB Data Team • 11/26/2025
This vulnerability represents a classic use-after-free condition that manifests during dynamic web content rendering, specifically when handling marquee elements during window resizing operations. The flaw occurs within the browser's layout engine where memory management fails to properly handle the lifecycle of style objects during asynchronous updates. When a window is resized, the browser attempts to recalculate the layout for marquee elements, but the system frees memory associated with the updated style object while references to that memory remain active in the rendering pipeline. This memory corruption scenario creates a potential exploitation vector that could allow remote attackers to execute arbitrary code through carefully crafted web content.
The technical implementation of this vulnerability stems from improper memory management within the Gecko rendering engine that powers Mozilla Firefox and Thunderbird applications. During window resizing events, the browser's layout computation process encounters a race condition where style objects are deallocated before all references to them are resolved. The marquee element, which is a deprecated html element that creates scrolling text, triggers complex layout recalculations that expose this memory management flaw. When the style object is freed while still being accessed by the rendering engine, subsequent memory access attempts result in undefined behavior that can manifest as crashes or memory corruption.
The operational impact of this vulnerability extends beyond simple application instability to potential remote code execution capabilities, making it particularly dangerous in web browsing contexts. Attackers could craft malicious web pages containing specially formatted marquee elements that, when displayed in affected browsers, would trigger the memory corruption during window resizing operations. The vulnerability affects multiple browser versions including Firefox ESR 52.2 and earlier, Firefox 54 and earlier, and Thunderbird versions prior to 52.3, representing a significant portion of the user base that was vulnerable to this class of exploit. The exploitation potential aligns with attack techniques described in the attack tree model where memory corruption vulnerabilities can be leveraged for privilege escalation and system compromise.
Mitigation strategies for this vulnerability involve both immediate patching and defensive programming approaches. The primary solution requires updating affected browser versions to patched releases that include proper memory management fixes for the layout engine. Organizations should prioritize deployment of security updates for Firefox 52.3, Firefox ESR 52.3, and Thunderbird 52.3 releases. Additionally, defensive measures such as enabling address space layout randomization, implementing stack canaries, and deploying web application firewalls can provide additional layers of protection. The vulnerability demonstrates the importance of memory safety practices and aligns with common weakness enumerations in CWE 416 which catalogs use-after-free conditions. Security teams should also consider implementing browser hardening configurations and monitoring for exploitation attempts through network traffic analysis and endpoint detection mechanisms.