CVE-2012-5839: Heap-based buffer overflow in the gfxShapedWord::CompressedGlyph::IsClusterStart function in Mozilla Firefo...

Security readout for executives and security teams

CVE-2012-5839 is an old Mozilla client software flaw that could let a remote attacker run code through unspecified input. It affects obsolete Firefox, Firefox ESR, Thunderbird, Thunderbird ESR, and SeaMonkey versions. The business risk is mainly legacy software exposure, especially unmanaged desktops, old Linux images, or forgotten portable installs. Exposure is most likely where unsupported Mozilla-era clients remain installed: legacy Linux desktops, old enterprise images, kiosk systems, lab machines, or archived virtual machines. Current supported browser and mail-client deployments should not be assumed exposed unless inventory confirms affected versions. Prioritize this if the organization still operates legacy desktops, old Linux builds, kiosks, or unmanaged browser/mail software. For modern managed endpoints, verify inventory rather than treating it as an emergency. Mitigation focus: Upgrade Firefox to 17.0 or later, or remove unsupported Firefox installations.; Upgrade Firefox ESR 10.x to 10.0.11 or later.; Upgrade Thunderbird to 17.0 or later, including ESR 10.0.11 or later..

Prepared

Jun 24, 2026, 12:04 UTC (UTC+00:00)

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.