CVE-2026-121615: Linksys E5600 remote authentication bypass

CVE-2026-121615 describes an improper authentication vulnerability in Linksys E5600 version 1.1.0.26. The flaw resides in the Login Handler component, specifically in the /cgi-bin/mesh_login.cgi file, and allows a remote attacker to bypass authentication controls and access the device without valid credentials. The attack can be launched remotely against any unpatched instance exposed on the network.

The vulnerability received a CVSS score of 10.0 and a Critical classification. The exploit has been publicly disclosed. The vendor (Linksys) was contacted early but did not respond. Additionally, the E5600 product is no longer supported by the maintainer, meaning no patch will ever be released. Social media signals indicate active discussion on the network, with mentions of the vulnerability in posts from today.

For sysadmins and IT teams: if Linksys E5600 is still in your environment, network isolation must be your immediate priority. Disable remote access via firewall rules or device configuration. Conduct a full inventory of all E5600 devices and plan their replacement with current-generation routers supported by the vendor and receiving regular security patches. Examine access logs for signs of exploitation attempts. Since no patch will ever be available, your defensive strategy must focus on containment and migration.