CVE-2017-5396 in Firefox
Summary
by MITRE
A use-after-free vulnerability in the Media Decoder when working with media files when some events are fired after the media elements are freed from memory. This vulnerability affects Thunderbird < 45.7, Firefox ESR < 45.7, and Firefox < 51.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability described in CVE-2017-5396 represents a critical use-after-free condition within the media decoding subsystem of Mozilla's browser and email applications. This flaw occurs when the Media Decoder component processes media files and handles certain event notifications that are triggered after the underlying media elements have already been deallocated from memory. The fundamental issue arises from improper memory management where the system continues to reference freed memory locations, creating opportunities for malicious actors to exploit this behavior for arbitrary code execution.
This vulnerability specifically impacts Thunderbird versions prior to 45.7, Firefox Extended Support Release versions before 45.7, and standard Firefox versions before 51. The affected software components operate within the context of media processing pipelines where multimedia content is decoded and rendered, making this a particularly dangerous flaw given the widespread use of these applications for handling various media formats. The use-after-free condition manifests when asynchronous event handling occurs after memory deallocation, creating a temporal window where attackers can manipulate the system state to achieve code execution.
The technical implementation of this vulnerability involves the Media Decoder's event handling mechanism failing to properly validate whether referenced media elements still exist in memory before processing associated events. When media files are processed and subsequently freed, the system maintains references to these objects in event queues or callback mechanisms. If an event is fired after the memory deallocation has occurred, the application attempts to access freed memory locations, potentially allowing attackers to control the execution flow through memory corruption. This behavior aligns with CWE-416, which specifically addresses use-after-free vulnerabilities where memory is accessed after it has been freed, and represents a classic example of improper memory management in event-driven systems.
The operational impact of this vulnerability extends beyond simple privilege escalation as it can be exploited through carefully crafted media content delivered via email attachments or web pages. Attackers can construct malicious media files that trigger the vulnerable code path when processed by affected applications, potentially leading to complete system compromise. The exploitability is enhanced by the fact that these applications commonly process multimedia content from untrusted sources, making the attack surface particularly broad. This vulnerability also maps to ATT&CK technique T1059.007 for command and control through application-specific protocols and T1203 for exploitation for persistence, as successful exploitation can provide attackers with persistent access to compromised systems.
Mitigation strategies for CVE-2017-5396 require immediate application of security patches provided by Mozilla, specifically updating to versions 45.7 for Thunderbird and Firefox ESR, and 51 for standard Firefox releases. Organizations should implement additional defensive measures including email filtering for suspicious multimedia attachments, web content filtering to prevent access to potentially malicious media files, and regular security assessments of media processing workflows. Network administrators should consider implementing sandboxing mechanisms for media processing components and monitoring for unusual memory access patterns that might indicate exploitation attempts. The vulnerability demonstrates the critical importance of proper memory management in event-driven systems and highlights the need for comprehensive testing of asynchronous event handling mechanisms in multimedia processing pipelines.