The Head Mare hacktivist group has been exploiting vulnerabilities in unpatched TrueConf video conferencing servers to replace client installers with malicious versions that deliver backdoors.
The exploited vulnerabilities allowed the attacker to execute arbitrary code with the highest level of privileges and deploy the PhantomCore and PhantomGraph backdoors.
TrueConf is a video conferencing tool widely used in Russia, especially in the enterprise and government sectors, as a secure, on-premise alternative to Western tools such as Zoom and Microsoft Teams.
Researchers at cybersecurity company Kaspersky discovered the attack in July. They found that Head Mare hackers used TCP port 4307, which is open by default, to connect to the target TrueConf server without authentication.
They leveraged a vulnerability internally tracked by Kaspersky as KLCERT-26-057 to execute a malicious script within TrueConf's isolated environment, and KLCERT-26-058 to escape the sandbox and run commands on the underlying operating system.
The attacker then increased their privileges to NT AUTHORITY\SYSTEM, and replaced the ‘\public\js\locale.php’ file with a web shell that gave them persistent remote access to the compromised server.
Kaspersky reports that Head Mare uses a web shell to collect sensitive information from the victim’s environment, access the TrueConf database, and replace the legitimate TrueConf Client installer hosted on the server with a malicious version that contains the PhantomCore backdoor.
When members of the organization connect to the local TrueConf server, they receive a trojanized, non-digitally signed client installer as an update.
“Even if your organization does not use the TrueConf server, employees of the organization can connect to compromised counterparty TrueConf servers to participate in online meetings and download infected installation packages,” Kaspersky warns.
Additionally, Head Mare deploys PhantomGraph, a separate backdoor consisting of two DLL files (SysExcSvc.dll and SysReadSvc.dll) that accept commands via a Microsoft OneDrive account, execute them, and return the results.
Observed attacker activity through PhantomGraph included dumping the memory of the Local Security Authority Subsystem Service (LSASS) process to exfiltrate credentials.
The malware also runs commands for reconnaissance activity, such as hostname and whoami, and starts a reverse SSH tunnel.
Kaspersky says it is currently observing multiple active Head Mare campaigns targeting Russian organizations in various sectors: instrumentation, electronics, transportation, energy, IT, and software development.
According to the researchers, the threat actor is using several initial access methods that include phishing, exploiting public-facing web servers, and access via contractors.
TrueConf vulnerabilities
The two flaws Kaspersky saw leveraged in attacks affect TrueConf Server 5.3.x before 5.3.9, 5.4.x before 5.4.9, 5.5.x before 5.5.5, and older versions.
The vendor fixed them in versions 5.3.9, 5.4.9, and 5.5.5, released on June 18.
In April 2026, CheckPoint Research reported that hackers were targeting a zero-day arbitrary file execution flaw in TrueConf, tracked as CVE-2026-3502, compromising users via trojanized client updates.
CheckPoint named the campaign ‘Operation True Chaos,’ and tentatively attributed it to Chinese threat actors behind the Havoc implant, which was used in these attacks.
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