FortiGuard Labs reports that attackers are exploiting 24 known vulnerabilities in internet-facing network and IoT equipment to install ClingSTUN, a Linux back-connect proxy backdoor. The report describes a campaign snapshot—not a new Linux kernel flaw or a count of infected devices. For administrators, the priority is to check exact device models and firmware, reduce unnecessary internet exposure, and investigate multiple indicators together. A connection to a public STUN service by itself does not prove infection.
What is ClingSTUN?
ClingSTUN is malware that gives attackers persistent remote access to compromised Linux-based network and IoT devices, and can turn those devices into proxy nodes. FortiGuard Labs published its technical analysis on October 5, 2026, describing three campaign periods and the vulnerabilities known to its researchers at publication. Its list may change as researchers collect additional vulnerabilities and update detection signatures.
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The reported 24 vulnerabilities affect products from multiple vendors. They are known flaws used for initial access, not one newly discovered vulnerability shared by all Linux devices. FortiGuard’s examples include CVE-2022-36553 affecting Hytec Inter HWL-2511-SS, CVE-2025-34035 in EnGenius EnShare, and CVE-2024-23625 in D-Link UPnP, alongside reported flaws involving Linear eMerge, Realtek SDK, TP-Link Archer AX21, AVTECH AVM1203, Sunhillo, Ivanti, Tenda, MeiG, and Lantronix. A vendor name alone does not establish that every product or firmware version is affected; check the exact model and software against its advisory. FortiGuard Labs’ October 5 analysis provides the detailed vulnerability and entry-point list.
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How STUN helps the backdoor—and what it does not prove
STUN (Session Traversal Utilities for NAT) is a legitimate protocol used by applications such as VoIP and WebRTC to discover the public IP address and port mapped by a network address translation (NAT) device. FortiGuard says ClingSTUN uses public STUN services to discover externally mapped addresses and ports and maintain connectivity through NAT.
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Because legitimate applications also use public STUN infrastructure, a STUN connection—or recurring STUN traffic—does not by itself identify an infected device or show that a public STUN server is attacker-controlled. It is more meaningful when correlated with suspicious processes, unexpected UDP traffic, altered startup files, or other indicators. ThaiCERT’s October 6 summary also cautions that legitimate STUN use can occur. ThaiCERT’s ClingSTUN advisory summarizes the campaign and the STUN caveat.
What the 24 vulnerabilities mean
The number 24 is the reported vulnerability count for the campaign’s initial-access set as described by FortiGuard as of October 5, 2026. ThaiCERT separately reports seven additional vulnerabilities embedded in the malware for propagation. These are two distinct findings: do not add the seven to the 24 as if they were one initial-access list, or assume every listed flaw affects every targeted device.
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The reporting does not provide a total number of compromised devices, name victim organizations, or identify the operators. HackRead notes those disclosure limits in its October 5 account. The count therefore describes vulnerabilities reported in the campaign, not its reach or attribution. HackRead’s account also carries a comment from Jason Soroko, senior fellow at Sectigo: “A device does not need to hold sensitive data to be useful to an attacker,” and “ClingSTUN lets attackers relay traffic through compromised devices and run commands on them.”
How ClingSTUN persists and operates
FortiGuard documents downloads for several processor architectures: ARM, Intel 80386, MIPS R3000, PowerPC, and AMD x86-64. This breadth reflects the variety of processor platforms found in embedded and network equipment; it does not mean a specific device is infected.
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The report describes persistence through startup files including /etc/inittab, /etc/init.d/rcS, and /etc/rc.d/rc.boot, with copies placed at /root/.cling and /usr/local/bin/.cling. Other reported behaviors include process and watchdog manipulation, concealment, and remote command execution. These are useful investigation leads, but the presence or absence of any single file or behavior should be assessed in the context of the device’s firmware and normal operation.
How administrators should check exposed devices
- Inventory internet-facing equipment. Record exact models, firmware versions, support status, and exposed management interfaces and services. Include routers, cameras, gateways, and other Linux-based network or IoT equipment.
- Check device-specific advisories. Compare each model and firmware version with vendor security guidance for the reported vulnerabilities. Apply available firmware and security updates; confirm whether the specific version is affected or fixed before drawing a conclusion.
- Reduce exposure. Remove direct internet access to management interfaces when it is not required, and disable or restrict services that do not need to be public. Isolate or replace unsupported equipment that cannot receive necessary security updates.
- Correlate investigation signals. Look for suspicious processes, unexpected changes to startup files, unexplained UDP connections, and recurring STUN activity in combination. STUN traffic alone is not confirmation of compromise.
- Use current incident guidance if compromise is suspected. Consult FortiGuard’s original report for indicators and check current vendor guidance. The reporting does not establish one cleanup procedure that applies to every device and firmware combination.
What is known—and what remains unknown
FortiGuard’s October 5, 2026 analysis and ThaiCERT’s October 6 summary establish a campaign involving known device vulnerabilities, a persistent Linux backdoor, and proxy use. They do not establish how many devices are infected, which organizations were affected, or who operates the campaign. Since vulnerability lists, signatures, and vendor fixes can change, decisions about an individual device should rely on current vendor advisories and its exact firmware and exposure—not on the campaign count alone.
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