CVE-2021-35394

Realtek Jungle SDK Flaw CVE-2021-35394 Fuels Cling Botnet Spread Across Routers

Researchers at Nozomi Networks have observed a sharp rise in exploitation attempts against CVE-2021-35394, a critical remote code execution vulnerability in the Realtek Jungle SDK. The flaw, rated 9.8 on the CVSS scale and disclosed five years ago, is being used to deploy the Cling botnet on routers and video recorders. The affected SDK is embedded in products from multiple vendors, leaving large numbers of devices exposed because firmware updates are rarely applied. Cling carries exploits for seven distinct vulnerabilities targeting Realtek, Linksys, MVPower, TBK, LB-LINK, FiberHome and China Mobile hardware. Once installed, the malware performs recursive scanning, spreads like a worm, manipulates TCP tunnels and proxies, and participates in DDoS attacks. Its command-and-control channel hides instructions inside STUN protocol transaction IDs, impersonating legitimate responses from Google public STUN servers. FortiGuard Labs has confirmed the findings and tracks the variant as ClingSTUN.