CVE-2017-5439 in Firefox
Summary
by MITRE
A use-after-free vulnerability during XSLT processing due to poor handling of template parameters. 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-2017-5439 represents a critical use-after-free condition that occurs during XSLT (Extensible Stylesheet Language Transformations) processing within Mozilla's web browser engine. This flaw manifests when the system improperly handles template parameters during stylesheet transformations, creating a scenario where memory previously allocated to a template parameter object is accessed after it has been freed. The technical nature of this vulnerability places it squarely within the realm of memory safety issues, specifically categorized under CWE-416 which describes the use of freed memory condition. The vulnerability affects multiple Mozilla products including Thunderbird and various Firefox versions, indicating a widespread impact across the browser ecosystem.
The operational impact of this vulnerability extends beyond simple crash conditions to potentially enable remote code execution when exploited. During XSLT processing, when a template parameter is processed and subsequently freed from memory, the application continues to reference this freed memory location in subsequent operations. This creates an exploitable condition where an attacker could craft malicious XSLT content that triggers the use-after-free scenario, potentially allowing arbitrary code execution with the privileges of the affected application. The vulnerability's presence in both Firefox ESR and regular Firefox releases demonstrates the severity of the issue and its potential for widespread exploitation across different user bases.
Security researchers have identified this vulnerability as particularly dangerous due to its location within core browser functionality that processes web content. The XSLT processing engine is commonly used in web applications for transforming xml data into formatted documents, making this attack vector accessible through standard web browsing activities. The vulnerability affects versions prior to specific security patches, with Firefox ESR 45.9, Firefox ESR 52.1, and Firefox 53 being the affected releases. This timeline indicates that the vulnerability was present for an extended period, providing potential attackers with ample opportunity to develop exploits before the patches were released. The attack surface is broadened by the fact that XSLT processing can be triggered through various means including embedded content, external style sheets, and dynamic content generation.
Mitigation strategies for CVE-2017-5439 primarily focus on immediate patching of affected systems and implementation of additional security measures. Organizations should prioritize updating all affected Mozilla products to their patched versions, which include Thunderbird 52.1, Firefox ESR 45.9, Firefox ESR 52.1, and Firefox 53. Additionally, implementing content security policies that restrict access to potentially malicious XSLT content can provide additional defense layers. Network-level protections such as web application firewalls and intrusion detection systems can help identify and block suspicious XSLT processing patterns. The vulnerability's classification under ATT&CK technique T1059.007 for command and scripting interpreter indicates that exploitation could involve automated script execution, making layered security approaches essential. Security teams should also consider implementing monitoring for unusual XSLT processing activities and conduct regular vulnerability assessments to identify other potential memory safety issues in similar browser components.