Pierluigi Paganini
March 12, 2025
In mid-2024, Mandiant identified custom backdoors on Juniper Networks’ Junos OS routers, and attributed the attacks to a China-linked espionage group tracked as UNC3886. These TINYSHELL-based backdoors had various capabilities, including active and passive access and a script to disable logging. Mandiant collaborated with Juniper Networks to investigate, finding that the affected Juniper MX routers were running outdated hardware and software, making them vulnerable to exploitation.
UNC3886 is a sophisticated China-linked cyber espionage group that targets network devices and virtualization technologies using zero-day exploits. Its primary focus is on defense, technology, and telecommunications sectors in the US and Asia.
In 2023, the APT group targeted multiple government organizations using the Fortinet zero-day CVE-2022-41328 to deploy custom backdoors.
The group’s latest operation on Juniper Networks’ Junos OS routers demonstrates a deep knowledge of system internals. UNC3886 prioritizes stealth by using passive backdoors and tampering with logs and forensic artifacts to ensure long-term persistence while evading detection.
Mandiant observed UNC3886 using compromised credentials to access Junos OS CLI from terminal servers managing network devices, escalating to FreeBSD shell mode. Junos OS includes a Verified Exec (veriexec) subsystem, adapted from NetBSD Veriexec, to ensure file integrity by preventing unauthorized code execution, including binaries, libraries, and scripts. To deploy malware, the threat actor had to first bypass this security mechanism. UNC3886 bypassed it by injecting malicious code into trusted processes. This allowed them to install six TinyShell-based backdoors named appid, to, irad, jdosd, oemd, and lmpad. Each backdoor was designed for remote access, persistence, and stealth, enabling attackers to evade detection and maintain long-term control.
“Veriexec protection prevents unauthorized binaries from executing. This poses a challenge for threat actors, as disabling veriexec can trigger alerts. However, execution of untrusted code is still possible if it occurs within the context of a trusted process. Mandiant’s investigation revealed that UNC3886 was able to circumvent this protection by injecting malicious code into the memory of a legitimate process.” reads the report published by Mandiant “This specific technique is now tracked as CVE-2025-21590, as detailed in Juniper Network’s security bulletin JSA93446.”
Below are the descriptions of the backdoors provided by Mandiant:
appid – TINYSHELL-based active backdoor, mimicking a legitimate binary named appidd (Application Identification Daemon)to – TINYSHELL-based active backdoor, mimicking a legitimate binary named top (Table of Processes)irad – TINYSHELL-based passive backdoor, mimicking a legitimate binary named irsd (Interface Replication and Synchronization Daemon)lmpad – TINYSHELL-based utility and passive backdoor, mimicking a legitimate binary named lmpd (Link Management Protocol Daemon)jdosd – TINYSHELL-based passive backdoor, mimicking a legitimate binary named jddosd (Juniper DDOS protection Daemon)oemd – TINYSHELL-based passive backdoor, mimicking a legitimate binary named oamd (Operation, Administration and Maintenance Daemon)“China-nexus espionage actors are continuing to compromise networking infrastructure with custom malware ecosystems. While UNC3886 previously focused their operations on network edge devices, this activity demonstrated they’re also targeting internal networking infrastructure, such as Internet Service Provider (ISP) routers. Mandiant observed the threat actor targeting network authentication services, including the Terminal Access Controller Access-Control System (TACACS+), and terminal servers with access to the routers to gain privileged initial access.” concludes the report published by Mandiant. “This privileged access allowed the threat actor to enter Junos OS shell mode and perform restricted operations.”
Mandiant also provided Indicators of Compromise (IoCs) and Yara rules to detect these backdoors.
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