Attacks exploiting Realtek’s CVE-2021-35394 rose sharply in Palo Alto Networks Unit 42 telemetry beginning in August 2022, but that report covers activity only through December 2022; it does not establish a new increase in 2026. The vulnerability affects a diagnostic component in certain Realtek Jungle SDK versions and can allow unauthenticated remote command execution. Because the SDK is embedded in products from many manufacturers, check your exact device and firmware with its maker rather than relying on its chipset or brand alone.
What CVE-2021-35394 is—and what the attack figures mean
CVE-2021-35394 affects UDPServer, a diagnostic component in Realtek Jungle SDK versions 2.0 through 3.4.14B. Unit 42 says a remote attacker who does not need to authenticate could use the flaw to execute arbitrary commands and take over a vulnerable device. The SDK may be included in products sold under other manufacturers’ names, so a router or IoT device’s retail branding may not reveal whether it contains the affected component.
Unit 42’s report, published in 2023 and based on telemetry through December 2022, counted 134 million exploit attempts from August 2021 through December 2022. About 97% of those attempts occurred after the beginning of August 2022. Those figures count attempts observed by Unit 42—not compromised devices, unique victims, or a worldwide census—and they do not show what happened after December 2022. The report therefore documents a historical surge, not a verified current rise. Read Unit 42’s analysis.
Within Unit 42’s analyzed attack records, more than 40% of attacks during August–October 2022 attempted to exploit CVE-2021-35394. Unit 42 also reported that 48.3% of observed attack origins were in the United States; it cautioned that proxies and VPNs can obscure an attacker’s actual location. These are measurements from that organization’s telemetry, not estimates of all internet attacks or the operators’ physical locations.
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What attackers attempted to do
Unit 42 analyzed payloads that downloaded and ran malware, wrote and executed binaries, or rebooted a target. Its analysis identified Mirai, Gafgyt, Mozi, and a Golang DDoS botnet it named RedGoBot. The presence of these payloads in the analysis does not mean every attempted exploit succeeded or that every device running the SDK was infected.
Which products may be affected
Unit 42 estimated that CVE-2021-35394 affected almost 190 device models from 66 manufacturers. That breadth reflects how software components can travel through a supply chain: Realtek supplies an SDK, while another company may build and support the finished router or device. A Realtek chipset label alone does not establish that a product uses the vulnerable SDK or a vulnerable version.
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For the 2021 vulnerability family, D-Link lists related CVEs CVE-2021-35392 through CVE-2021-35395 and identifies affected SDK families as rtl819x-SDK-v3.2.x, v3.4.x, v3.4T, v3.4T-CT, and rtl819x-eCos-v1.5.x. Its advisory uses a model, hardware-revision, and region table; it does not say every D-Link router or every Realtek-based device is affected. Check the applicable row and guidance in D-Link’s advisory.
How the 2021 flaw differs from later Realtek advisories
Realtek published separate advisories in 2024 and 2025. They concern different CVEs, software components, and attack conditions; neither is evidence that the 2021 campaign newly surged. The version lists can overlap, but that does not make the vulnerabilities interchangeable or establish that a particular finished product is affected.
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| Issue | SDK or component and versions named | Attack condition described | Source |
|---|---|---|---|
| CVE-2021-35394, part of the 2021 Jungle SDK vulnerability family | Unit 42 describes UDPServer in Realtek Jungle SDK 2.0 through 3.4.14B. D-Link’s advisory lists related RTL8xxx issues CVE-2021-35392 through CVE-2021-35395 and the SDK families rtl819x-SDK-v3.2.x, v3.4.x, v3.4T, v3.4T-CT, and rtl819x-eCos-v1.5.x. | Unit 42 describes unauthenticated remote command execution through UDPServer. | Unit 42; D-Link |
| CVE-2023-50381, CVE-2023-50382, and CVE-2023-50383 (2024 AP-Router WPS advisory) | Realtek lists rtl819x SDK v2.x, v3.2.x, v3.4.x, v3.4T, v3.4T-CT, and v3.6.0 series. It describes inadequate input validation in WPS webpage handlers and assigns CVSS 3.1 base score 7.2 (High). | The advisory describes OS command-injection flaws in WPS webpage handlers; it does not characterize them as the 2021 UDPServer issue. | Realtek’s March 19, 2024 advisory |
| xDSL web-interface command injection (2025 advisory) | Realtek names RTL8672, marked end of life, and xDSL SDK Linuxdsl-4.0.1 and 4.0.2. | An attacker with administrative privileges could execute commands through the web management interface. Realtek says remote exploitation is possible if that interface is exposed to untrusted networks. | Realtek’s September 4, 2025 advisory |
Realtek’s 2024 advisory says: “To fix these OS command injection vulnerabilities, the handler must validate the input value carefully and filter out illegal characters.” That guidance concerns the 2024 WPS-handler flaws; it is not a substitute for determining whether a particular device’s firmware addresses CVE-2021-35394.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to check and secure a router or IoT device
- Record the exact device details. Note the manufacturer, full model name, hardware revision, country or region, and installed firmware version. The hardware revision and region matter because vendor support and affected-model listings may differ.
- Check the manufacturer’s official support and security pages. Search by the exact model and revision, then compare the device with the advisory’s affected-product table. For D-Link devices, use the model/revision/region table in its Realtek chipset advisory. Do not infer vulnerability solely from a Realtek chipset or SDK family.
- Install only the applicable vendor firmware. If the manufacturer provides a fixed firmware release for that exact variant, follow its update instructions. Do not install generic firmware or a patch intended for another model, hardware revision, or SDK series.
- Limit exposure of management interfaces. Avoid making router administration or web-management interfaces reachable from untrusted networks. This is particularly relevant to the separate 2025 xDSL issue, for which Realtek identifies exposed management as a condition for remote exploitation.
- Retire unsupported devices when no suitable fix exists. If the vendor marks the device end of life or no longer supports it and provides no applicable update, replacement with a currently supported device is a reasonable security measure. A generic firewall or antivirus product does not patch vulnerable firmware.
If you manage multiple devices
For an organization, inventory IoT devices and their firmware versions, check them against the vendors’ advisories, and review network and security logs for suspicious activity. If compromise is suspected, follow the organization’s incident-response procedures rather than assuming that installing an update alone resolves an active incident.
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