CVE-2017-5440 in Firefox
Summary
by MITRE
A use-after-free vulnerability during XSLT processing due to a failure to propagate error conditions during matching while evaluating context, leading to objects being used when they no longer exist. 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-2017-5440 represents a critical use-after-free condition that manifests during XSLT processing within Mozilla's web browser engine. This flaw occurs when the system fails to properly propagate error conditions during the matching phase of XSLT evaluation, creating a scenario where objects remain accessible in memory even after they should have been deallocated. The root cause lies in the improper handling of context evaluation during XSLT transformations, where error states are not adequately communicated to the memory management system, resulting in premature object deallocation while references to those objects still exist within the processing pipeline.
The technical implementation of this vulnerability exploits the fundamental memory management principles that govern how web browsers handle dynamic memory allocation and deallocation. When XSLT processing encounters specific error conditions during context matching, the system should terminate the operation and clean up all associated resources. However, in this case, the error propagation mechanism breaks down, allowing the memory management system to free objects while the XSLT processor continues to reference them. This creates a classic use-after-free scenario where subsequent memory operations may corrupt the freed memory or allow attackers to manipulate the freed object's memory location for exploitation purposes.
The operational impact of CVE-2017-5440 extends beyond simple crash conditions to potentially enable remote code execution in affected browser versions. The vulnerability affects a broad range of Mozilla products including Thunderbird and various Firefox releases, with specific versions impacted including Firefox ESR 45.9, Firefox ESR 52.1, Firefox 53, and Thunderbird versions prior to 52.1. Attackers can craft malicious XSLT documents that trigger this condition when processed by vulnerable browsers, potentially leading to arbitrary code execution with the privileges of the browser process. The exploitation requires the target to process a specially crafted XSLT document, which could occur through various attack vectors including web pages, email attachments, or other content delivery mechanisms that invoke XSLT processing.
This vulnerability maps directly to CWE-416, which defines the use-after-free condition as a critical memory safety issue where memory is accessed after it has been freed. The flaw also intersects with ATT&CK technique T1059.007, which covers the use of scripting languages for execution, as XSLT processing involves script-like operations that can be manipulated to trigger the vulnerability. The affected software components include the Gecko rendering engine's XSLT processor, specifically the context matching and error propagation subsystems that handle the transformation of XML data through XSLT stylesheets. Organizations should prioritize immediate patching of affected versions, as the vulnerability represents a significant risk to user security and system integrity.
Mitigation strategies for CVE-2017-5440 focus primarily on software updates and patch management, with affected organizations required to upgrade to patched versions of Firefox ESR 45.9, Firefox ESR 52.1, Firefox 53, and Thunderbird 52.1. Additional protective measures include implementing content filtering mechanisms that prevent processing of untrusted XSLT content, disabling XSLT processing in web applications when possible, and employing sandboxing techniques to limit the impact of potential exploitation. Network-level protections such as web application firewalls and intrusion detection systems can help identify and block malicious XSLT content, while user education regarding the dangers of processing untrusted web content remains essential. The vulnerability demonstrates the critical importance of proper error handling and memory management in complex software systems, particularly those handling dynamic data transformations.