CVE-2017-5438 in Firefox
Summary
by MITRE
A use-after-free vulnerability during XSLT processing due to the result handler being held by a freed handler during handling. This results in a potentially exploitable crash. This vulnerability affects Thunderbird < 52.1, Firefox ESR < 45.9, Firefox ESR < 52.1, and Firefox < 53.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability identified as CVE-2017-5438 represents a critical use-after-free condition that occurs during XSLT processing within Mozilla's web browser engine. This flaw manifests when the result handler is maintained in memory while the associated handler object has already been freed, creating a scenario where subsequent memory access operations can lead to unpredictable behavior and potential code execution. The vulnerability specifically impacts the processing of Extensible Stylesheet Language Transformations which are used to transform xml documents into other formats such as html or text. The issue stems from improper memory management during the XSLT transformation lifecycle where the system fails to properly track handler object references, leading to a situation where freed memory can still be accessed by the result handler component.
The technical exploitation of this vulnerability occurs when maliciously crafted XSLT content is processed by affected browsers, triggering the use-after-free condition through improper memory deallocation and subsequent access patterns. This type of vulnerability falls under the CWE-416 category of Use After Free, which is classified as a critical memory safety issue that can lead to arbitrary code execution. The flaw is particularly dangerous because it can be triggered through web content processing, making it a remote code execution vulnerability that affects web browsers' ability to safely handle XML transformations. During normal operation, when XSLT processing occurs, the system allocates memory for handler objects to manage the transformation process, but fails to properly synchronize the deallocation of these objects with their subsequent usage by result handlers, creating a race condition that can be exploited by attackers.
The operational impact of this vulnerability extends across multiple browser versions and platforms, affecting Thunderbird email clients and various Firefox browser releases including ESR versions and regular releases. This widespread impact means that users across different environments are potentially exposed to remote exploitation through web-based attacks. The vulnerability can be leveraged by attackers to execute arbitrary code on affected systems, potentially leading to complete system compromise. Attackers can craft malicious web pages or email content that when processed by affected browsers triggers the memory corruption, allowing for privilege escalation, data theft, or system control. The exploitation requires no user interaction beyond visiting a malicious webpage or opening a specially crafted email, making it particularly dangerous in phishing campaigns or drive-by download scenarios. The vulnerability affects both desktop and mobile browser implementations, with the specific versions mentioned indicating that the fix was implemented in Firefox 52.1 and Firefox ESR 45.9, while Thunderbird received updates in version 52.1.
Mitigation strategies for CVE-2017-5438 primarily focus on immediate software updates and patches provided by Mozilla to address the memory management issue in XSLT processing. Organizations should prioritize updating all affected browser installations to the latest versions that contain the memory safety fixes, which typically include proper reference counting and memory deallocation synchronization mechanisms. Additionally, implementing network-based security controls such as web application firewalls and content filtering systems can help reduce the risk of exploitation by blocking malicious XSLT content. Security teams should also consider deploying browser hardening measures including disabling XSLT processing for untrusted content and implementing strict content security policies. From an ATT&CK framework perspective, this vulnerability maps to techniques involving exploitation of memory corruption vulnerabilities and remote code execution, with potential lateral movement opportunities if successful exploitation occurs. The vulnerability demonstrates the importance of proper memory management in security-critical applications and highlights the need for comprehensive code review processes, particularly for components handling complex data transformations and memory-intensive operations.