CVE-2026-117920 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the DOM: HTML Parser component. This vulnerability was fixed in Firefox 159 and Firefox ESR 153.6.
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Analysis
by VulDB Data Team • 10/27/2026
The identified vulnerability resides within the Document Object Model parser, specifically affecting the HTML parsing subsystem of Mozilla Firefox browsers. As a use-after-free flaw, this issue stems from improper memory management where the application continues to reference a block of memory after it has been freed or reallocated by the system. In the context of web browser engines, the DOM parser is responsible for interpreting raw HTML markup and constructing the corresponding tree structure that represents the document in memory. When an attacker can trigger specific parsing conditions, they may cause the underlying C++ objects representing DOM nodes to be deallocated while references to those same objects are still retained by other parts of the rendering or scripting engine. This discrepancy creates a window where the browser attempts to access invalid memory addresses, leading to undefined behavior that can compromise the stability and security of the application.
From a technical perspective, this flaw aligns with CWE-416, which defines use-after-free errors as conditions resulting from dereferencing memory after it has been freed. The exploitation potential is significant because modern browser engines employ complex object lifecycles where DOM nodes are frequently created, modified, and destroyed during page rendering and script execution. If an attacker can craft a malicious HTML document that forces the parser to free a node while keeping a pointer or reference active elsewhere in the codebase, they may achieve arbitrary read access by reading sensitive data from adjacent memory regions. More critically, with precise heap grooming techniques, it is possible to overwrite freed memory with controlled values, leading to arbitrary write primitives. This capability allows for full control over program execution flow, potentially resulting in remote code execution on the victim's system without any further user interaction beyond visiting a compromised webpage or opening a malicious file.
The operational impact of this vulnerability extends beyond simple application crashes, although denial-of-service remains a likely outcome if exploitation fails to achieve memory corruption goals. Successful exploitation would allow an attacker to bypass browser sandboxing mechanisms and execute arbitrary code with the privileges of the current user. This aligns with MITRE ATT&CK technique T1203, which covers Exploitation for Client Execution, as well as techniques related to privilege escalation if the browser runs with elevated permissions or accesses sensitive local resources. The vulnerability affects both standard releases like Firefox 159 and extended support release versions such as Firefox ESR 153.6, indicating that the flaw existed in a widely deployed codebase across different user segments including enterprise environments relying on stable long-term support builds.
Mitigation strategies primarily involve immediate patching to ensure all instances of affected browsers are updated to versions where this memory management error has been corrected by Mozilla engineers. Security teams should enforce automated update policies for Firefox deployments, particularly in corporate networks where ESR versions are prevalent due to their stability requirements. Additionally, users and administrators can employ browser hardening techniques such as disabling JavaScript execution on untrusted sites or using content security policies that restrict the loading of external resources if feasible within their operational constraints. Monitoring network traffic for unusual outbound connections following visits to potentially compromised websites can also aid in detecting potential exploitation attempts post-vulnerability disclosure before patches are fully deployed across all endpoints.