CVE-2016-9899 in Firefox
Summary
by MITRE
Use-after-free while manipulating DOM events and removing audio elements due to errors in the handling of node adoption. 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
This vulnerability represents a critical use-after-free condition that occurs during the manipulation of dom events and the removal of audio elements within the firefox browser engine. The flaw stems from improper handling of node adoption processes when audio elements are removed from the document object model. When an audio element is processed for removal while associated dom events are still active, the browser fails to properly manage memory references, creating opportunities for attackers to exploit dangling pointers. The vulnerability specifically manifests when the browser attempts to adopt nodes during event processing while simultaneously handling audio element cleanup operations. This memory management failure allows for potential code execution through crafted web content that triggers the specific sequence of operations leading to the use-after-free condition. The issue affects multiple browser versions including firefox 50.1 and earlier, firefox esr 45.6 and earlier, and thunderbird 45.6 and earlier, indicating a widespread impact across the affected software ecosystem.
The technical implementation of this vulnerability involves the interaction between the browser's dom event handling system and its audio element management subsystem. When an audio element is removed from the dom while events are still being processed, the adoption logic fails to properly update reference counts or invalidate pointers to the audio element's memory structures. This creates a window where freed memory can be accessed and potentially overwritten by subsequent operations. The flaw operates at the intersection of memory management and dom manipulation, making it particularly dangerous as it can be triggered through normal web browsing activities. Attackers can craft malicious web pages that create audio elements, attach event listeners, and then remove the elements in a specific sequence that causes the browser to access freed memory locations. The vulnerability is classified as a use-after-free issue which maps to cwe-416 and aligns with attack techniques in the attack pattern taxonomy that target memory corruption vulnerabilities.
The operational impact of this vulnerability extends beyond simple browser exploitation to potentially enable remote code execution on affected systems. When successfully exploited, an attacker can gain control over the browser process and potentially escalate privileges to the user level. The vulnerability affects not only web browsers but also email clients that share the same underlying browser engine, expanding the attack surface significantly. Users visiting malicious websites or opening specially crafted email messages could be compromised without any user interaction beyond normal browsing behavior. The vulnerability is particularly concerning because it can be exploited through standard web content without requiring any special permissions or user actions beyond visiting the malicious page. Security researchers have noted that the exploitation requires precise timing and memory manipulation, but the widespread adoption of affected browser versions means that many users remain at risk. The vulnerability demonstrates the complexity of modern browser security where interactions between different subsystems can create unexpected attack vectors.
Mitigation strategies for this vulnerability focus on immediate software updates and browser configuration changes. The primary recommendation is to upgrade to firefox version 50.1 or later, firefox esr 45.6 or later, and thunderbird 45.6 or later where the vulnerability has been patched. Organizations should implement automated update mechanisms to ensure rapid deployment of security patches across all affected systems. Browser security features such as address space layout randomization and data execution prevention should be enabled to make exploitation more difficult. Additionally, users should be educated about the risks of visiting untrusted websites and opening suspicious email attachments. Network-level protections including web application firewalls and content filtering systems can provide additional layers of defense against exploitation attempts. Security monitoring should be implemented to detect unusual browser behavior that might indicate exploitation attempts. The vulnerability highlights the importance of regular security assessments and penetration testing to identify similar memory corruption issues in complex software systems. Organizations should also consider implementing sandboxing technologies that limit the potential impact of successful exploitation attempts.