CVE-2026-100815 in Firefox
Summary
by MITRE • 09/29/2026
Use-after-free in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox ESR 153.4 and Firefox 157.
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Analysis
by VulDB Data Team • 09/29/2026
The identified vulnerability represents a critical memory management error within the Cascading Style Sheets parsing and computation subsystem of Mozilla Firefox, specifically classified as an use-after-free condition. In software engineering, this type of flaw occurs when a program continues to use a pointer after it has been freed from dynamic memory allocation. Within the context of browser architecture, the CSS engine is responsible for interpreting style rules and computing final presentation values for web content. When parsing complex or malformed stylesheets, if the internal data structures representing these styles are deallocated prematurely while references to them remain active in other parts of the rendering pipeline, subsequent access attempts will target invalid memory addresses. This scenario creates a window where an attacker can manipulate the state of freed memory to control program execution flow or cause unintended side effects during page rendering.
From a technical perspective, this flaw likely stems from improper reference counting or lifecycle management within the CSS object model. When a style rule is removed or updated, the associated objects must be carefully managed to ensure no dangling pointers exist before deallocation occurs. If the synchronization between the parser and the compositor or layout engine fails, an object may be freed while still being referenced by an ongoing animation, transition, or script-driven DOM manipulation. This race condition allows for arbitrary read operations if the attacker can predict memory layouts, potentially leaking sensitive information such as credentials stored in browser memory or internal application structures. More critically, it enables arbitrary write capabilities when heap grooming techniques are employed to place controlled data at the freed address location.
The operational impact of this vulnerability is severe due to its potential for remote code execution. An attacker could craft a malicious webpage containing specially designed CSS rules and JavaScript that triggers the use-after-free condition during rendering. Upon visiting such a page, the browser would execute arbitrary code with the privileges of the user running Firefox. This effectively compromises the integrity and confidentiality of the entire system, allowing full control over the victim's machine. The vulnerability affects both standard releases like Firefox 157 and extended support release versions like ESR 153.4, indicating that the flaw existed in core components shared across different distribution channels used by enterprise and individual users alike.
In terms of industry classification standards, this issue aligns with CWE-416, which defines use-after-free errors as a subset of memory corruption vulnerabilities. It also maps to MITRE ATT&CK technique T1203, specifically the exploitation phase where attackers leverage software flaws for execution. The attack vector is typically classified under remote code execution via browser-based attacks, often involving social engineering or drive-by download tactics where users are lured into visiting compromised websites. Security researchers and threat intelligence platforms monitor such vulnerabilities closely as they provide a direct path to initial access in many advanced persistent threat campaigns targeting enterprise environments running Firefox ESR versions.
Mitigation strategies primarily involve immediate patching of the affected software versions. Administrators should ensure that all endpoints update to Firefox 157 or later, including any subsequent point releases for the ESR branch which incorporate these security fixes. Until patches are applied, network-level controls such as web filtering proxies can be configured to block access to known malicious domains hosting exploit code. Additionally, enabling strict content security policies and disabling JavaScript on untrusted sites reduces the attack surface by preventing the script execution necessary to trigger the memory corruption conditions required for successful exploitation. Regular vulnerability scanning and automated patch management systems are essential to maintain a secure posture against such high-severity browser vulnerabilities.