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ClingSTUN is a Linux back-connect proxy backdoor that FortiGuard Labs says exploits known vulnerabilities in internet-facing devices, persists on compromised systems, and turns them into remotely controlled proxy nodes. Its use of legitimate public STUN services is a clue to investigate alongside device and network evidence—not proof of infection on its own. FortiGuard Labs’ technical report, published October 5, 2026, describes the malware’s behavior and observed targets.
What ClingSTUN does
ClingSTUN is malicious software for Linux-based devices such as routers and other embedded systems. Once it exploits a device, it can maintain a foothold and use the device as a remotely controlled proxy node. The campaign described by FortiGuard Labs relies on known vulnerabilities; the report does not claim a zero-day exploit.
The downloader retrieves payloads for several processor architectures: ARM, Intel 80386, MIPS R3000, PowerPC, and AMD x86-64. This range helps explain why the campaign concerns embedded Linux equipment as well as other Linux systems.
Which devices and vulnerabilities are reported
FortiGuard Labs describes the campaign expanding its entry points over three observed periods. The examples below are reported targets, not a list of every affected product or proof that all devices from a named vendor are vulnerable.
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| Observed period or expansion | Reported entry point | What the report establishes |
|---|---|---|
| First observed period | Hytec Inter HWL-2511-SS router; CVE-2022-36553 | The first observed period lasted two days and used this vulnerability. |
| Next period | EnGenius Enshare IoT Gigabit Cloud Service; CVE-2025-34035, and D-Link UPnP; CVE-2024-23625 | FortiGuard Labs reports these as additional entry points in the campaign. |
| Further expansion | Additional device and software targets | The report says the campaign continued to evolve and lists further targets, including Linear eMerge, Realtek SDK, TP-Link Archer AX21, Sunhillo SureLine, Ivanti Connect Secure and Policy Secure, Tenda devices, AVTECH AVM1203, MeiG Smart FORGE_SLT711, and Lantronix EDS5000. It also notes a goform-name buffer overflow affecting multiple vendors. |
The report associates its broader target list with particular CVEs and interfaces, but the list is not a complete vulnerability catalog or a device-by-device patch-status guide. Check the vendor’s advisory for the exact model, firmware version, affected interface, and available fix before drawing conclusions about a particular device.
How ClingSTUN persists and hides
After execution, the malware attempts to survive restarts and interfere with competing processes. FortiGuard Labs reports that it:
- Copies itself to
/root/.clingand/usr/local/bin/.cling. - Appends startup entries to
/etc/inittab,/etc/init.d/rcS, and/etc/rc.d/rc.boot. - Opens watchdog devices and terminates the watchdog timer, then searches for and kills competing processes.
- Clears its command-line arguments after setup. When running as root, it copies selected process-information files from
/proc/1/and bind-mounts/tmpover its own/proc/<pid>directory, which can make its process appear like init (PID 1).
FortiGuard also describes a later downloader variant that inspects /proc/mounts, unmounts certain entries, kills associated processes, and terminates processes whose executable path is under /tmp. These behaviors are investigation leads, not a standalone cleanup recipe.
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Why STUN traffic matters—and what it does not prove
STUN (Session Traversal Utilities for NAT) is a protocol used to learn the public-facing address and port mapped by a network’s NAT device. ClingSTUN sends standard 20-byte STUN binding requests to public endpoints and uses the resulting connectivity to help maintain NAT bindings. FortiGuard says one observed version contacted 24 endpoints and needed at least half to succeed; a later version contacted 13 and required every endpoint to succeed. These are FortiGuard Labs’ observations of malware versions published in 2026, not normal traffic thresholds or network-wide statistics.
The malware periodically sends a group identifier and mapped-port list to those endpoints. The report does not identify separate coordination-server registration in this path, and it leaves unresolved exactly how operators obtain external mappings and deliver control traffic through NAT.
FortiGuard also reports a listener for a 20-byte operator packet. A command designated as command 1 triggers an outbound TCP connection to an endpoint supplied in that packet; the malware then receives and executes a command. Its embedded propagation code includes exploits for seven vulnerabilities affecting products associated with Realtek, MVPower, TBK DVR, Linksys, LB-LINK, China Mobile, and KGUARD DVR, according to FortiGuard Labs’ 2026 report.
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Public STUN endpoints can be legitimate services. FortiGuard explicitly cautions that they should not automatically be classified as attacker-controlled infrastructure. STUN activity alone does not establish that a device is infected.
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Correlate host and network evidence rather than treating one artifact as conclusive. Relevant leads from the FortiGuard report include:
- Unexpected copies at
/root/.clingor/usr/local/bin/.cling, or unfamiliar entries in the listed startup files. - Watchdog disruption, unexplained process termination, unusual process information under
/proc, or suspicious activity involving executables under/tmp. - Recurring UDP traffic to public STUN services combined with unexpected outbound TCP connections, especially when there is also evidence of suspicious processes or persistence.
FortiGuard lists these campaign download hosts in defanged form: 124[.]163[.]212[.]119, 222[.]223[.]152[.]97, and 118[.]145[.]196[.]225. It also names FortiGuard detections BASH/Mirai.AEH!tr.dldr, BASH/Dloader.P!tr, and Linux/Agent.BHT!tr. Host indicators, hashes, and detection signatures can change or become stale; compare them with current vendor feeds and your own telemetry before using them for blocking or incident decisions. The report includes multiple file hashes, but an indicator match still needs to be assessed in context.
How to check whether a device is at risk
- Identify the device precisely. Record its manufacturer, exact model, firmware version, internet-facing services, owner, and whether the vendor still supports it. A vendor name alone is not enough to establish exposure.
- Compare it with vendor advisories. Check whether the exact model and firmware are affected by the vulnerabilities relevant to the device, and whether a fixed version or mitigation is available. The FortiGuard campaign report is a threat-research lead, not a substitute for each vendor’s affected-version guidance.
- Assess exposure and support status. Determine whether the vulnerable service or management interface is reachable from the internet. If a device cannot be updated, restrict its exposure; if it is unsupported and you cannot control the risk, isolate or replace it.
- Investigate signs of prior compromise. Review available logs, process and startup behavior, and network telemetry for the persistence and communication leads above. Applying a patch prevents or reduces exposure to a vulnerability; it does not establish that a previously compromised device is clean.
What to do if compromise is suspected
- Contain the device. Limit its network access in a way that fits its operational role, and prevent unnecessary internet-facing services from remaining reachable.
- Preserve evidence where feasible. Retain relevant logs and network telemetry before making changes that could erase useful information.
- Remediate based on the device and incident. Apply the vendor’s fix where available and follow an incident-response process appropriate to the device and environment. FortiGuard’s report does not provide a complete forensic-cleanup procedure, so do not assume that deleting a file or reinstalling firmware alone resolves every case.
- Address the exposure that enabled access. Disable unnecessary public services, restrict management interfaces, and isolate devices that cannot be patched. Replace equipment when support has ended and continued exposure cannot be controlled.
FortiGuard Labs’ October 5, 2026 report does not publish victim counts or geographic distribution, and it does not establish actor attribution. Its findings describe observed malware behavior and campaign targets rather than the prevalence of infection.
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