STUN traffic is not, by itself, evidence that a router is infected. But in a Cling MIPS malware sample analyzed by Nozomi Networks Labs and reported on October 1, 2026, STUN-like exchanges helped the malware register infected devices and receive commands hidden in UDP packet transaction IDs. The case matters because ordinary-looking network protocols can be repurposed; defenders need to look for the sample’s unusual behavior and corroborate it with device evidence.
What STUN does—and what it does not tell you
STUN, or Session Traversal Utilities for NAT, helps an endpoint discover the public IP address and port mapping that a server sees through a network address translator. It is also used in legitimate real-time communications, including systems that use ICE or TURN. The IETF’s RFC 8489 defines the protocol and its normal request-and-response behavior.
That means a router or network making STUN connections is not automatically suspicious. The useful question is whether the traffic behaves like ordinary STUN. Nozomi’s report concerns one analyzed Cling sample, not all STUN use, all routers, or every Cling variant.
How the reported Cling sample turned STUN-like traffic into command-and-control
In a normal STUN Binding exchange, a client sends a request and receives a response that reports the public-facing address and port observed by the server. Nozomi described the following sequence in its analyzed sample:
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- The sample periodically sent Binding Requests to a hard-coded list of 13 STUN servers, approximately every five seconds. Its requests used an all-zero transaction ID, unlike the random value expected by the protocol.
- It recorded the public IP address and mapped port reported in Binding Success Responses.
- It sent custom UDP registration datagrams to the contacted endpoints. These included mapped ports and an infection-method tag, but did not conform to STUN; conforming servers ignored them.
- It listened for UDP packets sent to the mapped ports and interpreted data in the STUN transaction-ID field as commands.
Nozomi reported that one endpoint returned an all-zero transaction ID rather than echoing the request’s ID. In a controlled validation, the researchers advertised different port sets to different endpoints, then received commands on a port advertised to the suspect endpoint. They inferred that endpoint was involved in the botnet’s command infrastructure.
The researchers also said command packets appeared to come from an IP address associated with stun.l.google.com, and proposed source-address spoofing as the likely explanation. That observation is not evidence that Google operated the command server.
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How the reported behavior differs from ordinary STUN
| Signal | Ordinary STUN behavior | Behavior reported in the Cling sample |
|---|---|---|
| Transaction ID | A request uses a random transaction ID, which the response uses to match the exchange (IETF RFC 8489). | Binding Requests used an all-zero ID; command data was carried in the transaction-ID field (Nozomi Networks Labs, October 1, 2026). |
| Traffic purpose | Binding exchanges help an endpoint learn its public-facing address and port mapping. | Custom UDP registration datagrams advertised mapped ports and an infection-method tag; the sample also listened for commands on mapped ports (Nozomi Networks Labs, October 1, 2026). |
| Response pattern | A response corresponds to the request’s transaction ID. | Nozomi reported one endpoint returning an all-zero ID instead of echoing the request’s ID (Nozomi Networks Labs, October 1, 2026). |
These are behavioral clues, not a rule that every deviation proves malware. Network equipment, applications, and measurement conditions can affect what defenders observe; investigate the full exchange and the device involved.
Why a compromised router matters
The commands Nozomi identified in the sample could download and execute payloads, scan for and exploit other systems, start or stop a TCP tunnel, start or stop a proxy relay, and launch a denial-of-service flood. A compromised router could therefore serve as an initial foothold, relay, tunnel endpoint, or botnet node—not just as a target whose own traffic is affected.
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Nozomi also reported attempts to exploit CVE-2021-35394, a remote-code-execution flaw in the Realtek Jungle SDK diagnostic component commonly compiled as UDPServer. In its observed example, a UDP datagram beginning with orf; was followed by shell commands that downloaded and ran malware. Realtek SDK components appear in various embedded products, but that does not mean every device using a Realtek component is vulnerable or was compromised.
The analyzed sample contained exploit logic associated with other products and vulnerabilities, including Realtek SDK, LB-LINK routers, TBK DVRs, Linksys, Eir D1000 routers, FiberHome SR1041F/China Mobile HG6543C4, and MVPower CCTV DVRs. The presence of exploit code is not proof of successful exploitation or of a vulnerability in every product from a named vendor.
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What router owners and defenders can investigate
Look for a combination of network behavior and device-side evidence. A single STUN connection—or any one indicator below—does not establish infection.
Network clues
- Repeated STUN Binding Requests with all-zero transaction IDs.
- Custom UDP registration datagrams sent to endpoints contacted for STUN.
- Responses that fail to echo a request’s transaction ID.
- Unexpected inbound UDP packets arriving at ports learned through the STUN exchanges.
Device clues from the analyzed sample
- Copies of the malware at
/root/.clingor/usr/local/bin/.cling. - References to the malware appended to init-related files.
- A replaced
wgetexecutable, with the original preserved and its location recorded; Nozomi lists companion pathswget.randwget.pas hunting clues. - Use of port
33957for the sample’s single-instance check.
These artifacts are leads specific to the analyzed sample, not guaranteed indicators for every variant. Nozomi recommends reviewing exposed routers, access points, DVRs, and other embedded appliances for relevant vulnerable components, and using indicators and technique mappings to guide investigation. CISA and partner agencies’ broader communications-infrastructure guidance also supports keeping device and firmware inventories, using secure authentication, centralizing logs, and establishing baselines for normal network behavior.
What to do if the signals appear
- Identify the device precisely. Record the manufacturer, exact model, hardware revision, firmware version, and whether its management or diagnostic services are reachable from the internet.
- Check the vendor’s guidance for that exact model and firmware. Confirm whether the relevant component and vulnerability apply, and follow the vendor’s remediation instructions. The reporting cited here does not provide a complete model-by-model patch matrix.
- Preserve useful evidence. If you manage the network, retain relevant firewall, DNS, and flow logs and document suspicious destinations, ports, timestamps, and device identifiers. Avoid treating a STUN destination alone as proof.
- Escalate suspected compromise. For an organization, involve its incident-response team or service provider; for a home router, contact the device vendor or internet provider for model-specific recovery advice. If a device cannot be supported or secured, ask the provider about a supported replacement rather than assuming a factory reset alone resolves the vulnerability.
Nozomi reported a spike in observed attempts and analyzed one MIPS sample, but did not establish a population-wide infection count or prevalence rate. The report’s server count and request interval describe that sample, not how common the activity is across routers.
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