CVE-2026-100784 in Firefox
Summary
by MITRE • 09/29/2026
Use-after-free in the Layout: Text and Fonts component. This vulnerability was fixed in Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.
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Analysis
by VulDB Data Team • 09/29/2026
The identified vulnerability represents a critical memory management error classified as an out-of-bounds read or use-after-free condition within the Layout: Text and Fonts component of Mozilla Firefox browsers. This specific flaw arises when the browser's rendering engine fails to properly manage the lifecycle of internal data structures associated with text layout calculations, font metrics, or glyph positioning. In complex web pages that utilize advanced CSS features such as flexbox, grid layouts, or intricate typography settings involving variable fonts and international character sets, the JavaScript garbage collector may reclaim memory occupied by these objects while they are still being referenced by active rendering threads. This race condition allows an attacker to access freed memory locations, leading to undefined behavior within the browser process. The vulnerability was addressed in Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17 through rigorous code audits that enforce strict ownership rules for layout objects and ensure that references are nullified or updated before memory deallocation occurs.
From a technical perspective, this flaw aligns with the Common Weakness Enumeration identifier CWE-416, which describes use-after-free errors where program execution continues after memory has been freed. The operational impact of such vulnerabilities is severe because they can be exploited to achieve arbitrary code execution on the victim's system. By carefully crafting malicious web content that triggers the specific layout conditions required to induce the race condition, an attacker can manipulate the heap state to overwrite function pointers or object vtables. This manipulation allows for control over the instruction pointer, facilitating remote code execution with the privileges of the user running the browser. Furthermore, even if full exploitation is not achieved, such memory corruption issues often lead to denial-of-service conditions characterized by application crashes and data loss due to segmentation faults or access violations within the rendering process.
The attack vector for this vulnerability typically involves a specially crafted HTML page containing malicious JavaScript that interacts with DOM elements in ways that stress-test the layout engine's memory management routines. Attackers may employ techniques such as rapid creation and destruction of text nodes, manipulation of CSS properties that force reflows, or exploitation of specific font rendering paths to trigger the use-after-free state before the garbage collector can safely reclaim resources. This aligns with MITRE ATT&CK technique T1203, which covers Exploitation for Client Execution, where attackers leverage vulnerabilities in client-side software like web browsers to execute malicious payloads on target systems. The complexity of modern web standards means that layout engines must handle a vast array of edge cases, making them prime targets for such memory safety violations if proper safeguards are not implemented during development and testing phases.
Mitigation strategies primarily involve applying the provided security patches immediately across all affected browser versions to ensure that the underlying memory management logic is corrected. Organizations should enforce strict update policies for end-user devices running Firefox ESR or standard release channels to minimize exposure windows. Additionally, deploying content security policies can help restrict the execution of inline scripts and limit interactions with potentially malicious third-party resources, thereby reducing the attack surface available to exploiters. Security awareness training for developers working on browser components should emphasize the importance of smart pointers and reference counting mechanisms in C++ codebases to prevent dangling references. Regular static analysis and fuzzing tests targeting layout engines are also recommended to proactively identify similar memory safety issues before they reach production releases, ensuring robust defense against exploitation attempts that rely on timing-dependent race conditions within complex rendering pipelines.