CVE-2026-117883 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the DOM: HTML Parser 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 within the Document Object Model parser of Mozilla Firefox browsers, specifically categorized as an use-after-free condition in the HTML parsing component. This type of flaw occurs when the browser's rendering engine attempts to access or manipulate a block of memory that has already been deallocated by the system. In the context of web page processing, this typically happens during the complex sequence of tokenizing and constructing DOM nodes from raw HTML source code. When an attacker crafts a malicious webpage containing specific sequences of malformed or specially constructed HTML elements, they can trigger conditions where the parser releases memory associated with certain node structures while other parts of the browser's internal state still hold references to those same addresses. This discrepancy between reference validity and actual memory allocation creates a window for exploitation that undermines the fundamental security assumptions of the browser sandbox.
From a technical perspective, this vulnerability aligns closely with CWE-416, which defines use-after-free errors as accessing memory after it has been freed. The HTML parser is responsible for interpreting markup tags, attributes, and content to build the interactive structure of web pages. During high-load scenarios or when processing deeply nested or irregularly structured documents, race conditions or logic errors in reference counting mechanisms can lead to premature deallocation. An attacker who successfully exploits this flaw could potentially achieve arbitrary code execution within the context of the browser process. This is particularly dangerous because it bypasses many standard web security mitigations such as Cross-Site Scripting filters, allowing for direct interaction with underlying system resources if combined with other vulnerabilities or through sophisticated exploitation techniques that leverage the corrupted memory state to overwrite function pointers or control flow data structures.
The operational impact of this vulnerability extends beyond simple application crashes, although denial-of-service remains a likely outcome for unexploited instances where invalid memory access causes immediate segmentation faults. For exploited cases, the primary risk is remote code execution without user interaction beyond visiting a malicious website. This capability allows threat actors to install malware, steal sensitive data such as cookies and session tokens, or pivot into internal network environments if the victim has broader system privileges. The severity of this issue necessitates urgent attention because it affects core browser functionality that users engage with daily across millions of devices worldwide. Compromised browsers serve as gateways for further attacks including keylogging, credential harvesting, and installation of persistent backdoors on affected systems.
Mitigation strategies primarily involve immediate software updates to the specified versions which contain patches addressing these memory management logic errors in the HTML parser. Users should ensure their Firefox installations are updated to version 159 or later, as well as the Extended Support Release channels at versions 115.44 and 153.6 respectively. These releases include code changes that enforce stricter lifecycle management for DOM nodes during parsing operations, ensuring that memory is only freed when all references have been properly released. Additionally, organizations should employ browser hardening techniques such as disabling JavaScript on untrusted sites where possible or using content security policies to restrict script execution from malicious domains. Security awareness training remains vital since social engineering often accompanies the delivery of exploit kits targeting this specific vulnerability class.
In terms of threat intelligence and classification frameworks, this incident maps directly to MITRE ATT&CK technique T1203 which covers exploitation for client-side code execution via browser vulnerabilities. It also relates to CWE-416 as previously noted and potentially CWE-787 if the memory corruption leads to buffer overflows during subsequent operations on the freed memory region. Security teams should monitor for indicators of compromise associated with exploit kits known to target Firefox HTML parser flaws, including unusual network connections from browser processes or unexpected child process creation patterns. Regular vulnerability scanning and patch management cycles are essential defenses against such zero-day or recently disclosed exploits that leverage deep internal components like parsers which have historically been rich sources of high-severity vulnerabilities in web browsers.