CVE-2026-100785 in Firefox
Summary
by MITRE • 09/29/2026
Use-after-free in the DOM: Core & HTML 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 within the Document Object Model core and HTML components of Mozilla Firefox browsers. Specifically classified as an use-after-free condition, this flaw occurs when the application continues to reference a memory block after it has been deallocated or freed by the system. In the context of browser rendering engines, such errors typically arise during complex interactions involving dynamic content manipulation, DOM node removal, and subsequent re-accessing of those nodes through JavaScript execution or event handlers. The vulnerability stems from improper lifecycle management where pointers to internal data structures are not invalidated immediately upon deallocation, allowing stale references to persist in memory maps until they are eventually overwritten by new allocations.
From a technical perspective, this flaw allows for arbitrary code execution under the context of the victim user if an attacker can successfully trigger the specific sequence that leads to the use-after-free condition. By crafting malicious HTML and JavaScript content hosted on a controlled website or embedded within phishing pages, an adversary can manipulate the browser's rendering engine into freeing memory associated with DOM elements while retaining references to those same elements in active execution contexts. When these stale pointers are subsequently accessed, they may point to attacker-controlled data structures that have been allocated in the freed memory space. This mechanism enables the overwriting of function pointers or object vtables, leading to control flow hijacking and ultimately remote code execution on the target system without any user interaction beyond visiting the malicious page.
The operational impact of this vulnerability is severe due to its potential for full system compromise when exploited within a browser sandbox environment that may have limited but non-zero privileges. Although modern browsers employ multiple layers of defense such as site isolation, memory protection techniques like ASLR and DEP, and strict content security policies, sophisticated exploit chains can often bypass these mitigations through heap grooming techniques or by leveraging additional vulnerabilities in adjacent components. The presence of this flaw affects a wide range of user demographics relying on Firefox for secure web browsing, including enterprise environments where ESR versions are mandated for stability and long-term support. Users who do not update their browsers remain exposed to automated exploitation campaigns that target known unpatched vulnerabilities across the internet.
Mitigation strategies primarily involve immediate software updates as indicated by Mozilla's release notes. Administrators managing Firefox Extended Support Release (ESR) environments must ensure deployment of version 153.4, ESR 115.42, or ESR 140.17 depending on the specific branch in use. Standard desktop users should update to Firefox version 157 or later as soon as possible. In addition to patching, organizations can implement network-level controls such as web filtering proxies that block access to known malicious domains and enforce strict Content Security Policies for internal applications hosted within corporate networks. Deploying endpoint detection and response solutions with behavioral analysis capabilities can also help identify anomalous memory access patterns indicative of exploitation attempts in real-time.
This vulnerability aligns with Common Weakness Enumeration identifier CWE-416, which describes use-after-free conditions resulting from improper handling of pointer invalidation after deallocation. In terms of the MITRE ATT&CK framework, this flaw facilitates initial access and execution techniques commonly associated with drive-by downloads or web-based attacks where no user interaction beyond navigation is required. The exploitation path typically involves reconnaissance to identify vulnerable versions followed by delivery via malicious websites that leverage social engineering tactics to lure victims into accessing compromised content. Continuous monitoring of browser telemetry data for unusual memory allocation patterns can provide early warning indicators of active exploitation attempts targeting this specific class of vulnerabilities in production environments.