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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Cling is an IoT botnet whose analyzed sample used STUN-style traffic to register infected devices and deliver commands. Nozomi Networks Labs reported that command packets appeared to come from an IP address resolving to Google’s public STUN service, but assessed UDP source-address spoofing—not Google relaying the commands—as the most likely explanation.
What did Nozomi observe?
In an analysis published October 1, 2026, Nozomi Networks Labs described Cling activity found while monitoring customer telemetry. Researchers observed attempts to exploit CVE-2021-35394, a remote-code-execution flaw in a diagnostic component of the Realtek Jungle SDK, commonly compiled as UDPServer. The vulnerable component has been used in routers, access points, repeaters, and other appliances, according to the report.
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In the captured exploit, the payload began with orf; followed by shell commands. The example used BusyBox wget to download a binary, make it executable, and launch it with an argument specifying a replication method. This is an observed exploit path, not evidence that CVE-2021-35394 is Cling’s only route into devices or that every internet-exposed appliance is vulnerable.
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Nozomi’s analysis focused primarily on MIPS sample 3b0ac6aaabb3bf8058ca14f9c8ccc613cfa3ea71, alongside observations from related samples. That sample contained exploit logic targeting components and vulnerabilities associated with Realtek SDK, LB-LINK, TBK DVR, Linksys, Eir D1000, FiberHome/China Mobile, and MVPower products. These are capabilities found in the analyzed sample, not a claim that every Cling variant uses every exploit.
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The report does not give a victim count or a campaign-wide prevalence estimate. Its findings describe a particular sample and related activity observed by Nozomi, rather than a measurement of how many devices are infected globally.
How can malware hide commands in STUN traffic?
STUN, specified in RFC 8489, helps a device learn its public IP address and the port assigned to it by a network address translator (NAT). STUN is common in real-time communications and other NAT-traversal settings, so devices may generate legitimate STUN traffic as part of normal operation.
In the analyzed Cling sample, the protocol’s recognizable pattern became cover for a separate command channel. The bot used STUN exchanges to learn mapped ports, sent custom messages to register those ports, and then listened for UDP packets carrying commands in the STUN transaction-ID field. The transaction ID is a 12-byte field normally used to associate a STUN request with its response.
- Probe STUN servers: The sample periodically sent Binding Requests to a hardcoded list of 13 STUN servers, approximately every five seconds. It used all-zero transaction IDs in these requests.
- Learn mapped ports: The bot recorded the externally observed ports returned in STUN responses.
- Register with custom datagrams: It sent registration messages containing mapped ports and an infection-method tag. These datagrams were not conforming STUN messages; Nozomi said conforming servers ignored them in its testing.
- Wait for commands: The bot listened for UDP packets sent to mapped ports. In the analyzed sample, operator commands were encoded in the 12 bytes of the STUN transaction-ID field.
Nozomi compared responses from the servers in the sample’s list and found one that returned an all-zero transaction ID rather than echoing the request ID. In a controlled test, researchers sent registration messages advertising different port sets to different servers. Several hours later, a port advertised only to 145.249.115[.]184 received commands. On that basis, Nozomi identified the host as involved in the botnet’s command-and-control activity; the test does not establish broader infrastructure ownership.
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Does the Google-looking address mean Google was involved?
No. Nozomi reported that the source IP on command packets was 74.125.250[.]129, which resolves to stun.l.google.com. That identifies the address the packets appeared to come from; it does not show that Google’s service sent or relayed them.
The researchers said they were unaware of a legitimate mechanism that would cause a STUN server to relay a chosen transaction ID. They also observed consistent TTL differences between legitimate STUN responses and the command packets. Nozomi assessed UDP source-address spoofing as the most likely explanation. The report does not prove the precise source of the spoofed packets, nor does it establish that Google was hacked, knowingly participated, or relayed Cling commands.
What can the analyzed bot do?
The command set in the analyzed sample points to a bot that can be used both to expand access and to direct activity from compromised devices. Nozomi reported observing self-propagation commands and flood instructions during multiple days of monitoring. Those instructions do not, by themselves, establish that attacks against the named targets succeeded.
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| Command capability | What it enables |
|---|---|
| Execute a downloaded payload | Fetch and run additional code on the compromised device. |
| Scan and exploit other systems | Search for potential targets and attempt to propagate. |
| Stop the scanner | Halt the sample’s scanning activity. |
| Start or stop a TCP tunnel | Set up or end a tunnel through the device. |
| Start or stop a proxy relay | Use the device to relay traffic, or stop that relay. |
| Flood a specified target for a set duration | Issue denial-of-service traffic toward a target. |
How can defenders reduce risk and look for Cling?
Nozomi recommends reviewing internet-facing routers, access points, DVRs, and other embedded appliances for vulnerable components, applying vendor patches where available, reducing unnecessary internet exposure, restricting inbound access, and segmenting IoT devices from more sensitive systems. If a device cannot be patched, network isolation and tight traffic restrictions can reduce its opportunity to communicate or spread.
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Review network behavior
- Look for repeated STUN Binding Requests with all-zero transaction IDs, especially when that pattern is not normal for the device.
- Investigate non-STUN UDP datagrams sent to STUN endpoints, since Cling’s custom registration messages were not conforming STUN traffic.
- Compare destinations, ports, and traffic patterns with the device’s normal baseline. An unusual STUN-looking packet alone is not proof of infection.
- Validate indicators against the actual asset and surrounding evidence before blocking them; shared or changing infrastructure can make an isolated indicator misleading.
Check embedded Linux artifacts
Where host access is available, Nozomi’s report identifies these sample-specific persistence clues:
- Copies of the malware at
/root/.clingor/usr/local/bin/.cling. - References added to SysV or BusyBox initialization files.
wget.randwget.pcompanion files. In the analyzed sample’s replacement arrangement, the genuinewgetbinary was moved towget.r, its location recorded inwget.p, and the malware placed in its stead. When invoked, the replacement could re-execute the malware and then call the original utility.
Because embedded appliances may provide limited host telemetry, network monitoring is important as well as device-level inspection. NIST Special Publication 1800-15, published May 26, 2021, describes Manufacturer Usage Description (MUD) as a way to allow an IoT device only the communications required for its intended function and prohibit other traffic. This is general network-control guidance, not a Cling-specific fix, and whether it can be applied depends on the network equipment and device environment.
As Nozomi Networks Labs put it in its October 1, 2026 report: “Cling is notable not because it introduces a new propagation technique, but because it repurposes ordinary STUN behavior into a practical command-and-control channel.”
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