CVE-2026-12309 is a memory safety bug in Mozilla Firefox identified and fixed in Firefox 152 and Firefox ESR 140.12. It is one of multiple memory safety vulnerabilities addressed simultaneously, some of which showed evidence of memory corruption that could allow arbitrary code execution with sufficient effort. The vendor advisories (MFSA 2026-57 and MFSA 2026-58) confirm that these vulnerabilities affect Firefox 151, Firefox ESR 140.11, and earlier versions, and were remediated in the stated fixed versions. No active exploitation has been reported. The fixes are included in the official Mozilla Firefox and Firefox ESR releases.

The vulnerability involves memory safety issues that could lead to memory corruption. Such corruption may allow attackers to execute arbitrary code within the context of the affected Firefox browser. This could compromise user security and privacy. However, no known exploits in the wild have been reported for CVE-2026-12309 specifically. The impact is considered moderate based on Mozilla's assessment.

Mozilla has released official fixes for this vulnerability in Firefox 152 and Firefox ESR 140.12. Users and administrators should update affected Firefox installations to these versions or later to remediate the issue. Since the vulnerability is fixed in these releases, no additional mitigation steps are required beyond applying the official update.