CVE-2017-5402 in Firefox
Summary
by MITRE
A use-after-free can occur when events are fired for a "FontFace" object after the object has been already been destroyed while working with fonts. This results in a potentially exploitable crash. This vulnerability affects Firefox < 52, Firefox ESR < 45.8, Thunderbird < 52, and Thunderbird < 45.8.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability identified as CVE-2017-5402 represents a critical use-after-free condition within Mozilla Firefox and Thunderbird applications that stems from improper memory management during font event processing. This flaw occurs when the application attempts to access memory locations that have already been freed or destroyed, creating opportunities for malicious actors to exploit the system through controlled memory corruption. The vulnerability specifically manifests when FontFace objects are manipulated in scenarios where events are fired after object destruction, leading to potential arbitrary code execution.
The technical implementation of this vulnerability resides in the browser's font handling subsystem where the FontFace object lifecycle management fails to properly track object references during event processing. When a FontFace object is destroyed but subsequent events are still fired against it, the application attempts to dereference memory that has already been deallocated. This memory corruption scenario creates a predictable crash condition that can be leveraged by attackers to execute malicious code with the privileges of the affected application. The vulnerability is classified under CWE-416 as a use-after-free condition, which represents one of the most common and dangerous memory safety issues in software applications.
The operational impact of this vulnerability extends across multiple Mozilla products and versions, affecting Firefox browsers prior to version 52 and Firefox ESR prior to version 45.8, as well as Thunderbird email clients before version 52 and Thunderbird ESR before version 45.8. This widespread exposure means that users across various deployment scenarios remain vulnerable, including enterprise environments running older software versions and individual users who have not updated their applications. The crash condition can be triggered through malicious web content that manipulates font objects during page rendering, making it particularly dangerous in web browsing contexts where users may encounter untrusted content.
Security researchers have documented this vulnerability as part of the broader ATT&CK framework under techniques related to code injection and memory corruption attacks, where adversaries exploit memory safety issues to gain unauthorized system access. The vulnerability demonstrates the importance of proper object lifecycle management in complex applications and highlights the risks associated with asynchronous event handling in browser environments. Organizations should prioritize immediate patching of affected systems and implement additional security controls such as application whitelisting and web content filtering to mitigate potential exploitation attempts.
The remediation approach for this vulnerability requires immediate application of security patches from Mozilla, which address the underlying memory management issues in the font handling components. System administrators should ensure that all affected versions of Firefox and Thunderbird are updated to their respective secure releases, with particular attention to Firefox ESR and Thunderbird ESR versions that maintain long-term support. Additional defensive measures including browser hardening configurations, sandboxing implementations, and monitoring for suspicious memory access patterns can provide additional layers of protection. The vulnerability serves as a reminder of the critical importance of regular security updates and proper memory management practices in preventing exploitation of similar issues in browser applications.