Head Mare Hackers Exploit TrueConf Servers to Distribute PhantomCore and PhantomGraph Backdoors
Russian organizations have faced a fresh wave of attacks from the Head Mare group, which targets unpatched TrueConf servers to distribute backdoors to video conference participants.
Attackers chain two vulnerabilities, KLCERT-26-057 and KLCERT-26-058, to execute arbitrary code with the highest privileges on the server. They replace one of the server files with a web shell that allows them to map the victim's IT infrastructure, gain access to the TrueConf database, and substitute the legitimate client installer with a malicious version.
Once the server is under control, the group relies on social engineering without sending any suspicious emails. Participants joining a video conference see an offer to download an updated client. Installing it delivers the PhantomCore and PhantomGraph backdoors instead of a legitimate update.
Affected versions and remediation
The vulnerabilities impact 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 releases. Patches were released on 18 June 2026 in versions 5.3.9, 5.4.9, and 5.5.5.
Kaspersky researchers who discovered the campaign strongly advise updating servers to the fixed builds. The risk is not limited to direct TrueConf owners: any employee invited to a meeting on a compromised server may download the infected installer themselves.
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