Written by: Jayson Frost, Bryce Greathouse, Brian Meyer, John Palmisano, Rajesh Rajeshirke, Steve Rasch, John Wolfram Introduction On October 15,…
Written by: Jayson Frost, Bryce Greathouse, Brian Meyer, John Palmisano, Rajesh Rajeshirke, Steve Rasch, John Wolfram Introduction On October 15, BeyondTrust disclosed a vulnerability, CVE-2026-102764, impacting BeyondTrust Remote Support (formerly Bomgar) appliances. BeyondTrust Remote Support is an enterprise secure remote access solution that enables IT teams to securely access and troubleshoot remote endpoints. The successful exploitation of this vulnerability results in the arbitrary upload of files. During an incident response engagement, Mandiant identified evidence that ARMORY CASTLE, a suspected PRC-nexus espionage threat actor (formerly tracked as UNC5221 ), exploited this vulnerability as a zero-day, in late August 2026. This blog describes how ARMORY CASTLE escalated privileges and established persistent access to compromised BeyondTrust Remote Support appliances following exploitation of CVE-2026-102764, and provides actionable guidance for defenders. Organizations should prepare for potential widespread exploitation following this disclosure, as ARMORY CASTLE and other threat actors have historically conducted broad, opportunistic exploitation once the vulnerability becomes public. Exploitation Mandiant identified evidence of CVE-2026-102764 exploitation within the access, syslog, messages, thin-scc, brain and SSL log files, which are only accessible via an LUKS-encrypted partition which contains the /home directory. We strongly recommend that affected organizations immediately apply the security patches provided by BeyondTrust to remediate the vulnerability, and review additional guidance in their blog . Post-Exploitation Following the successful exploitation of CVE-2026-102764, Mandiant observed ARMORY CASTLE leverage multiple distinct TTPs to establish a foothold, escalate privileges, and enable lateral movement. Throughout the intrusion, ARMORY CASTLE leveraged a deep understanding of BeyondTrust appliance internals, as evidenced by their abuse of proprietary CLI utilities, startup configuration templates and local reverse-proxy routing. The threat actor used newly identified custom malware, the SAWFLY webshell and the BARECLAW tunneler to interact with the BeyondTrust device. Additionally, the actor routinely deleted appliance logs and endpoint artifacts to maintain access and…