CVE-2026-84145 in Firefox
Summary
by MITRE • 09/01/2026
Internally found bugs present in Firefox 154, Firefox ESR 153.1, Firefox ESR 140.14 and Firefox ESR 115.39. Some of these bugs showed evidence of memory corruption or another security-relevant defect and we presume that with enough effort some of these could have been exploited. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, and Firefox ESR 153.2.
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Analysis
by VulDB Data Team • 09/01/2026
The identified vulnerabilities reside within the core architecture of Mozilla Firefox across multiple release channels, specifically affecting versions 154, ESR 153.1, ESR 140.14, and ESR 115.39. These issues were discovered through internal security testing and code analysis rather than external reporting, indicating a proactive approach to identifying latent defects before they can be weaponized by threat actors. The presence of these bugs in both the standard release track and the Extended Support Release (ESR) tracks highlights that the underlying engine components are shared across different deployment scenarios used by individual consumers and enterprise environments respectively. This widespread exposure underscores the critical nature of maintaining up-to-date browser versions to mitigate risks associated with complex web rendering engines and scripting interpreters.
The technical nature of these defects is characterized primarily as memory corruption vulnerabilities, which represent one of the most severe classes of software security flaws. Memory corruption typically involves improper handling of computer memory allocation or deallocation, leading to conditions such as buffer overflows, use-after-free errors, or heap overflow scenarios. In the context of a web browser, this often occurs when processing untrusted input from maliciously crafted websites or embedded content. The exploitation potential lies in the ability of an attacker to manipulate these corrupted memory states to execute arbitrary code with the privileges of the victim process. Although direct evidence of active exploitation was not observed during internal testing, the presumption is that sufficient effort by a determined adversary could translate these defects into functional exploits capable of achieving remote code execution or privilege escalation on the affected system.
From an operational perspective, the impact of successfully exploiting these vulnerabilities extends beyond simple data theft to full compromise of the user's environment. An attacker leveraging memory corruption in Firefox can bypass modern browser security mitigations such as sandboxing and address space layout randomization if the vulnerability allows for control over execution flow or arbitrary write primitives. This could lead to the installation of malware, establishment of persistent backdoors, or use of the compromised machine as a pivot point within a larger network infrastructure. For enterprise users relying on ESR versions, which are designed for stability and long-term support rather than rapid feature iteration, the delay in patching can increase the window of exposure significantly if updates are not applied promptly upon release.
The remediation strategy involves upgrading to patched versions immediately available as Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, and Firefox ESR 153.2. These releases contain the necessary code fixes that address the memory management errors identified in previous builds. Organizations should prioritize patching their ESR deployments to ensure continuity of security posture without sacrificing stability. Additionally, enforcing automatic update policies can reduce the risk window by ensuring that end-users receive patches as soon as they are validated and distributed through Mozilla's release channels. Security teams should also monitor for any indicators of compromise related to memory corruption exploits in network traffic or endpoint logs to detect potential attempts to leverage these vulnerabilities before full patch deployment is achieved across all assets.
These vulnerabilities align with Common Weakness Enumeration categories such as CWE-120 Buffer Copy without Checking Size of Input and CWE-416 Use After Free, which are prevalent in complex software systems involving manual memory management or intricate garbage collection mechanisms. In terms of the MITRE ATT&CK framework, these flaws facilitate techniques associated with Initial Access via Spearphishing Link if delivered through malicious web content, followed by Execution commands that leverage browser-based code execution capabilities. Understanding this mapping helps security operations centers prioritize detection rules and incident response procedures tailored to browser-based attack vectors. Continuous monitoring for anomalous behavior in processes spawned by the Firefox executable can provide an additional layer of defense against exploitation attempts that may bypass static patching measures due to zero-day characteristics or delayed update adoption.