A legacy branch is an older, still-supported software line that continues to receive security fixes after newer versions are released. Vendors keep these branches alive so devices that cannot upgrade immediately can still be protected, even if they no longer get the newest features.
In cyber security, legacy branches matter because they often become the weak point in mixed environments. Attackers look for systems that are behind on updates, since an older branch may miss hardening improvements or remain exposed longer if patching is delayed. Defenders need to track versions per branch, not just per product name, and verify that older devices are included in patch management, compliance checks, and asset inventories. A fleet can appear current while one legacy branch still carries risk. Supporting legacy branches reduces exposure, but only if organizations install the fixes consistently and retire unsupported versions before they become an easy target.