CVE-2026-117939 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the XML component. This vulnerability was fixed in Firefox 159, Firefox ESR 115.44, and Firefox ESR 153.6.
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Analysis
by VulDB Data Team • 10/27/2026
The identified vulnerability represents a critical memory management error classified as an out-of-bounds write resulting from use-after-free conditions within the XML parsing component of Mozilla Firefox browsers. This specific class of flaw occurs when the application continues to reference a block of memory after it has been freed, leading to undefined behavior that can be exploited by malicious actors. The vulnerability was addressed in recent security updates including Firefox version 159 and extended support releases such as ESR 115.44 and ESR 153.6, indicating the severity assigned to this issue by the development team.
From a technical perspective, use-after-free vulnerabilities arise when an object is deallocated but pointers or references to that memory location remain active within the application's runtime environment. In the context of XML processing, complex parsing logic often involves dynamic allocation and deallocation of data structures representing nodes, attributes, or text content. If the parser fails to properly nullify these references after freeing the underlying memory, subsequent operations may read from or write to that stale pointer. This can result in arbitrary code execution if an attacker crafts a malicious XML document designed to trigger this specific sequence during parsing. The exploitation typically involves manipulating the timing of garbage collection or object lifecycle events to ensure the freed memory is reused for controlled data structures before the vulnerable reference is accessed again.
The operational impact of such vulnerabilities extends beyond simple application crashes, although denial-of-service remains a possible outcome if the corruption leads to an immediate fault. More critically, successful exploitation allows remote code execution on the victim's system without user interaction beyond visiting a compromised webpage or opening a malicious file attachment containing crafted XML content. This aligns with common attack patterns observed in web-based exploits where attackers leverage browser vulnerabilities to establish persistence or escalate privileges within the local environment. The presence of this flaw highlights risks associated with complex text processing engines that handle untrusted input, emphasizing the need for rigorous memory safety checks and automated fuzzing during development cycles.
Industry standards categorize this issue under CWE-416 which defines use-after-free as a distinct weakness in software design where pointers are used after they have been freed. This classification underscores the importance of proper resource management practices such as reference counting, smart pointers, or memory-safe languages to prevent stale pointer dereferences. Furthermore, from an offensive security perspective this vulnerability maps to MITRE ATT&CK technique T1203 which covers exploitation for client-side execution through browser vulnerabilities. Security teams should prioritize patching these browsers across all endpoints to mitigate the risk of drive-by attacks that rely on such memory corruption flaws.
Mitigation strategies primarily involve applying the provided software updates immediately to ensure the underlying parsing logic is corrected and references are properly invalidated upon deallocation. Organizations should also implement application whitelisting or browser hardening policies to restrict execution capabilities in untrusted zones, thereby reducing the blast radius if a vulnerability were to be exploited before patching occurs. Regular security awareness training for users regarding suspicious attachments and links remains essential as social engineering often accompanies technical exploits targeting these types of memory corruption flaws. Continuous monitoring for anomalous network traffic originating from browser processes can also aid in detecting potential exploitation attempts in real time.