PhantomCore

CVE-2026-72529 and CVE-2026-72530 — a pre-auth chain on TrueConf Server's port 4307 reaches SYSTEM, and the operators use it to replace the client installer the server hands to everyone who joins a meeting

CISA added both halves of this chain to its Known Exploited Vulnerabilities catalogue on 2026-08-20, and ENISA's EU Vulnerability Database independently records each as exploited since the same date. The chain is entirely pre-authentication and sits behind one port. CVE-2026-72529 is a missing-authentication flaw: "An unauthorized attacker can connect to TrueConf server versions 5.3.X before 5.3.9, 5.4.X before 5.4.9, 5.5.X before 5.5.5, as well as earlier versions (our internal analysis showed that all TrueConf server versions released since 2022 are vulnerable) via port 4307/TCP (open by default, according to TrueConf documentation) and execute a malicious script on the server by calling an undocumented function" (Kaspersky ICS CERT, 2026-08-12). That parenthesis matters more than the version list around it: the vendor-stated affected ranges understate the real exposure, so an older build outside the published ranges is not safe by omission.

The script lands inside a sandbox that deliberately withholds the dangerous standard libraries, which is where CVE-2026-72530 comes in — a flaw in the sandbox's own code-generation logic. Kaspersky describes the join: "The uploaded malicious script is executed in an isolated environment, where potentially hazardous libraries (io, os, etc.) are unavailable. However, attackers can exploit another vulnerability (assigned internal Kaspersky identifier KLCERT-26-058) to execute arbitrary code on the server with NT AUTHORITY\SYSTEM privileges" (Kaspersky ICS CERT, 2026-08-12). Both were fixed together on 2026-06-18 in 5.3.9, 5.4.9 and 5.5.5 — roughly two months before the exploitation listing, which is the usual and uncomfortable shape: the patch was available throughout the observed campaign.

What the operators do with it. Kaspersky attributes the activity to Head Mare, and records a classification change worth carrying because it sets expectations about intent: "Previously, we classified them as hacktivists, but now we define them as an APT group due to the sophistication of their TTPs and the absence of destructive activity (encryption, wiping) in the targeted infrastructures" (Kaspersky Securelist, 2026-08-11). With SYSTEM in hand they overwrite a PHP file that sits in the server's public script directory with a web shell — the file's extension is .php even though the directory is named for JavaScript, which matters for anyone building the hunt — and delete the corresponding TrueConf event-log entries, then work from the shell — fingerprinting the environment over PowerShell, reaching the server database with elevated privileges, dumping LSASS memory through the built-in COM services library, and standing up an SSH reverse tunnel (Kaspersky Securelist, 2026-08-11).

Then the step that changes who is exposed: "Most importantly, during the attack, the attackers replace the TrueConf client distribution file located at C:\Program Files\TrueConf Server\ClientInstFiles\trueconf_windows_client_x64.exe" (Kaspersky ICS CERT, 2026-08-12). Meeting participants who accept the resulting update prompt install the genuine client alongside the PhantomCore backdoor, which unpacks into the user's local application-data tree under a filename mimicking a Windows C-runtime component and auto-launches from a registry class registration. Kaspersky notes the replaced installers carried no valid signature — "The malicious distributions we detected did not have a valid digital signature" (Kaspersky Securelist, 2026-08-11) — which is the cheapest available check for anyone who has taken a server-offered client update recently. Kaspersky states the consequence directly: "Even if your organization does not use a TrueConf server, your employees may connect to compromised TrueConf servers of contractors to participate in online meetings and download infected installation packages."

On the server the group also installs PhantomGraph, a two-module Windows-service backdoor Kaspersky describes as a backup channel — the primary control path stays the web shell and remote PowerShell — which, in Kaspersky's words, uses "an account on Microsoft OneDrive cloud storage as their command-and-control (C2) server". Neither cited source says whether that account was compromised or attacker-registered, so this entry does not either. Kaspersky assesses the two service installs were split across separate encoded PowerShell commands deliberately, "to make it harder to detect using EDR tools". On \*nix TrueConf servers the group installs two distinct backdoors: a rootkit that hides its own files and intercepts TrueConf's network functions to receive commands smuggled inside the TrueConf protocol itself, and a separate backdoor that uses GitHub as its command-and-control channel. Kaspersky's detection names for the pair are PhantomHook and PhantomReact; no source states which name belongs to which implant, so this entry does not assert the mapping. Its published artefact list also shows both persisting as systemd units under attacker-chosen service names, which is the \*nix counterpart to the Windows service persistence above.

All observed victims are Russian organisations, in instrument manufacturing, electronics, transportation, energy, IT and software development; no source reports targeting elsewhere. This entry is here for the exposure path rather than the victim list — a European organisation is reachable through the contractor-meeting vector regardless of whose servers the operators are currently choosing.

Detection concepts, telemetry class first. In file-write telemetry on the server, an unexpected write to the public script directory or to the client-installer path outside a vendor-update window is the highest-fidelity signal, and Kaspersky's own published rule names key on exactly those two events plus an unsigned installer. In egress telemetry, a videoconferencing server process making outbound HTTPS calls to a consumer cloud-storage service, or a \*nix TrueConf host calling code-hosting API endpoints, is anomalous for the appliance's role. In endpoint process telemetry, watch for the client installer's process tree loading a library from the user's local TrueConf directory whose name mimics a system component, and for new class registrations under the current user's registry hive. In credential-access telemetry, the COM-services LSASS dump technique and new outbound SSH sessions originating from the server are both worth alerting on.