CVE-2026-100781 in Firefox
Summary
by MITRE • 09/29/2026
Sandbox escape due to incorrect boundary conditions in the Graphics: WebRender 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 security flaw resides within the Graphics subsystem of Mozilla Firefox, specifically targeting the WebRender component which is responsible for hardware-accelerated rendering of web content using WebGL and other graphics APIs. This vulnerability stems from incorrect boundary condition handling during memory access operations or array indexing logic within the renderer's internal data structures. In complex graphical environments where multiple layers, textures, and compositing effects are processed simultaneously, precise management of buffer limits is critical to prevent out-of-bounds reads or writes. The failure to properly validate these boundaries allows an attacker who controls the web content being rendered to manipulate memory access patterns in ways that bypass expected constraints. This type of error typically arises when input values derived from maliciously crafted HTML, CSS, or JavaScript are not rigorously checked against allocated buffer sizes before being used as indices for array accesses or pointer arithmetic within the rendering pipeline.
From a technical perspective, this flaw represents a classic memory safety issue often categorized under CWE-125 Out-of-bounds Read or CWE-787 Out-of-bounds Write depending on whether the exploit reads sensitive data from adjacent memory regions or overwrites critical control structures and pointers. The WebRender architecture relies heavily on efficient GPU communication and CPU-side management of rendering commands. When boundary checks are omitted or implemented incorrectly, an attacker can trigger undefined behavior in the browser process. This may lead to information disclosure if out-of-bounds reads expose internal state variables, stack contents, or heap metadata that were not intended for public access. Alternatively, if the vulnerability allows for arbitrary memory writes, it could facilitate code execution by overwriting function pointers or return addresses within the Firefox process space. Such scenarios align with MITRE ATT&CK techniques related to privilege escalation and sandbox escape, where an adversary leverages a browser-based initial compromise to break out of the restricted environment provided by the web container model.
The operational impact of this vulnerability is significant because it undermines the fundamental security isolation that modern browsers provide between untrusted web content and the host operating system. By exploiting incorrect boundary conditions in WebRender, a remote attacker could potentially execute arbitrary code with the privileges of the user running Firefox. This effectively neutralizes the sandbox protections designed to limit the damage caused by visiting malicious websites or loading compromised third-party scripts. The risk is particularly acute for users who engage in high-value activities such as online banking, accessing corporate intranets, or handling sensitive documents via web applications. Successful exploitation could lead to full system compromise, allowing attackers to install malware, steal credentials, exfiltrate personal data, or use the infected machine as part of a botnet. Given that WebRender is enabled by default in most recent Firefox versions and handles complex rendering tasks for many modern websites, the attack surface remains broad unless patched promptly.
Mitigation strategies primarily involve applying the vendor-provided software updates immediately upon availability. Mozilla has addressed this issue in specific releases including Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17. Organizations should enforce automated update policies to ensure all endpoints are running these patched versions or later. In environments where immediate patching is not feasible due to compatibility concerns, temporary mitigations might include disabling hardware acceleration in the browser settings if possible, although this may degrade performance for legitimate users and does not eliminate all risks associated with other rendering paths. Additionally, deploying network-level security controls such as web application firewalls or content filtering solutions can help block access to known malicious domains that attempt to host exploit code targeting this specific vulnerability. Regular monitoring of browser telemetry and endpoint detection systems should also be enhanced to identify anomalous behavior indicative of exploitation attempts, such as unusual memory allocation patterns or unexpected process spawns originating from the Firefox executable.