TrueConf Server Flaws Exploited to Replace Client Installers with PhantomCore - The Hacker News

The threat actor known as Head Mare has been observed weaponizing security flaws in unpatched TrueConf servers once again in attacks targeting Russian companies spanning instrumentation, electronics, transport, energy, IT, and software development sectors.

Russian cybersecurity vendor Kaspersky said it detected the attacks in July 2026.

The activity involves exploiting a vulnerability chain in the TrueConf videoconferencing server to replace the original TrueConf client installers with poisoned versions that deliver the PhantomCore backdoor and remote access trojan (RAT) into susceptible systems.

The vulnerabilities, tracked as KLCERT-26-057 and KLCERT-26-058, enable arbitrary code execution with elevated privileges. The attack impacts TrueConf server versions 5.3.x up to 5.3.9, 5.4.x up to 5.4.9, 5.5.x up to 5.5.5, and earlier.