CVE-2026-124861 is a critical sandbox escape vulnerability caused by a use-after-free flaw in the Audio/Video cubeb component of Firefox. This vulnerability allows an attacker to potentially break out of the browser's sandbox environment, leading to full system compromise. The issue was fixed in Firefox 160 and Firefox ESR versions 115.45 and 153.7. The vulnerability has a high CVSS score of 9.6, indicating severe impact on confidentiality, integrity, and availability.