CVE-2026-117947 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the XML component. This vulnerability was fixed in Firefox 159 and Firefox ESR 153.6.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 10/27/2026
The identified vulnerability represents a critical memory management error classified as an out-of-bounds write, specifically manifesting through a use-after-free condition within the browser's Extensible Markup Language processing engine. This flaw arises when the application attempts to access or manipulate a data structure that has already been deallocated from memory by another part of the system. In the context of web browsers, XML parsing is often invoked during the rendering of complex documents, handling of RSS feeds, or execution of certain JavaScript operations involving DOM manipulation. When an object is freed but references to it remain active in other parts of the code, subsequent access can lead to undefined behavior. This typically results in writing data to arbitrary memory locations, which compromises the integrity and confidentiality of the process state.
From a technical perspective, this vulnerability falls under CWE-416, Use After Free, which is widely recognized as one of the most dangerous classes of software defects due to its potential for remote code execution. The exploitation vector likely involves crafting malicious XML content that triggers specific parsing paths within Firefox's engine. An attacker could construct a specially designed webpage or lure a user into opening a compromised document containing this malformed XML. Upon rendering, the browser would allocate memory for an object, free it prematurely due to a logic error in reference counting or lifecycle management, and then proceed to write data through a dangling pointer. This out-of-bounds write allows the attacker to overwrite adjacent heap metadata or function pointers, effectively gaining control over program execution flow.
The operational impact of this vulnerability is severe, as successful exploitation can lead to arbitrary code execution with the privileges of the user running the browser. Modern browsers employ various mitigations such as Address Space Layout Randomization and Data Execution Prevention, but sophisticated attackers often chain multiple vulnerabilities or use heap spraying techniques to bypass these protections. If exploited, an attacker could install malware, steal sensitive information including passwords and cookies, or pivot into a broader network attack. The vulnerability affects both the standard release channel of Firefox version 159 and the Extended Support Release branch at version 153.6, indicating that it was present in stable builds used by enterprise environments and general consumers alike for a significant period before remediation.
In terms of threat intelligence frameworks, this type of memory corruption vulnerability is frequently associated with ATT&CK technique T1203, Exploitation for Client Execution, where the browser serves as the initial attack vector to execute malicious payloads on the victim's system. The lack of immediate sandboxing isolation in certain parsing contexts can exacerbate the risk, allowing the exploit code to interact more directly with underlying operating system resources. Security teams should recognize that use-after-free bugs are often difficult to detect through static analysis alone and typically require dynamic debugging or fuzzing techniques during development phases to identify properly.
To mitigate this specific vulnerability, users must ensure their browsers are updated to Firefox 159 or later for standard releases, or version 153.6 or later for Extended Support Release versions. These updates contain the necessary patches that correct the memory management logic within the XML component, ensuring that objects are not accessed after they have been freed and that reference counts are accurately maintained throughout the parsing lifecycle. Organizations relying on Firefox ESR should prioritize applying this update to their endpoint fleets immediately to close the window of exposure. Additionally, implementing strict content security policies can help limit the impact if a similar vulnerability were present in other components by restricting script execution from untrusted sources. Regular patch management and monitoring for new browser updates remain essential practices in maintaining a secure browsing environment against memory corruption exploits.