Realtek SDK Vulnerability Attacks Highlight IoT Supply Chain Threats

A pictorial representation of network attack trends such as CVE-2021-35394 featuring a stylized bug on IoT-related products. The Palo Alto Networks and Unit 42 logos are included.

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Executive Summary

Unit 42 researchers review tens of millions of attack records every month, and most months, attacks targeting a single vulnerability do not exceed 10% of the total number of attacks. However, we discovered that between August and October 2022, the number of attacks attempting to exploit a Realtek Jungle SDK remote code execution vulnerability (CVE-2021-35394) accounted for more than 40% of the total number of attacks.

As of December 2022, we’ve observed 134 million exploit attempts in total leveraging this vulnerability, and about 97% of these attacks occurred after the start of August 2022. At the time of writing, the attack is still ongoing.

Many of the attacks we observed tried to deliver malware to infect vulnerable IoT devices. This tells us that threat groups are using this vulnerability to carry out large-scale attacks on smart devices around the world. While the attacks we observed were successfully blocked by our products, it’s important to assess protection of these devices in your environment. Because IoT devices and routers are often not considered as part of an organization’s security posture, many devices and organizations could still be at risk.

CVE-2021-35394 affects almost 190 models of devices from 66 different manufacturers. We believe that this vulnerability attracted so many attackers because supply chain issues can make it difficult for the average user to identify the affected products that are being exploited.

In response to this concerning phenomenon, we dug deep into all the attack records of this vulnerability from when it was disclosed to December 2022 for analysis.

Palo Alto Networks customers receive protections from the vulnerability and malware families mentioned in this post with the Next-Generation Firewall with cloud-delivered security services including WildFire. Advanced URL Filtering and DNS Security can block the command and control (C2) domain and malware hosting URLs. Our IoT Security platform can help identify anomalous network traffic, as well as determining the vendor, model and firmware version of a device to identify specific devices that are vulnerable to the aforementioned CVE.

Related Unit 42 Topics IoT, Vulnerability, Supply Chain, CVE-2021-35394

Table of Contents

Vulnerability Overview
Malware Analysis
Attack Origin Analysis
Indicators of Compromise
Malicious IPs
Callback URLs
RedGoBot Malware Sample
Mirai Samples

Vulnerability Overview

CVE-2021-35394 was disclosed on Aug. 16, 2021. The vulnerability affects UDPServer in Realtek Jungle SDK version 2.0 and later-Realtek Jungle SDK version 3.4.14B. Remote unauthenticated attackers could leverage this vulnerability to achieve arbitrary command execution, leading to devices being taken over.

Realtek chipsets are used by many IoT vendors in a variety of different products. This is a typical supply chain issue in that it can be difficult to identify whether your own devices are impacted, as the chipset may not be visible from the exterior of the device. According to a Shodan scan searching for this vulnerability, we found port 9034 open in over 80 different IoT devices, and these devices belong to 14 unique vendors.In particular, we noted that router models manufactured by several popular networking vendors are affected by CVE-2021-35394. Based on crowdsourced data from enterprise networks monitored by the Palo Alto Networks IoT Security product, the brands in Table 1 have the most popular vulnerable devices in mid-to-large sized deployments. The supply chain vulnerabilities in these products directly contribute to expanding the attack surface of these networks.

(Note that the vendors who produce these devices may have released updated versions or recommended mitigations. However, organizations sometimes continue to use vulnerable models, and threat actors take advantage of such situations.)

Vendor Name Number of Models Vulnerable
D-Link 31
LG 8
Belkin 6
Zyxel 6
Asus 4
Netgear 1

Table 1. Vulnerable IoT networking devices.

Based on the attacks we have seen in the wild, we found the following three types of payloads:

  • A script executes a shell command on the targeted server. This script actively connects to a malicious IP address, and automatically downloads and executes malware (shown in Figure 1). These threats were mostly from the Mirai malware family.
Image 1 is a screenshot of two lines of code. It is the first payload of CVE-2021-35394.
Figure 1. CVE-2021-35394 payload one.
  • An injected command directly writes the binary payload to a file and then executes it (shown in Figure 2).
Image 2 is a screenshot of the second payload of CVE-2021-35394. It is many lines of binary where the injected command writes this binary payload into a file for execution.
Figure 2. CVE-2021-35394 payload two.
  • An injected command directly reboots the targeted server to achieve denial of service (shown in Figure 3).
Image 3 is a screenshot of a code snippet, showing the third payload of CVE-2021-35394. It achieves a denial of service.
Figure 3. CVE-2021-35394 payload three.

Malware Analysis

Unit 42 researchers conducted analysis of malware samples that are delivered through the exploitation of CVE-2021-35394. Based on the attacks we have seen in the wild, most of the malware samples are from well-known malware families like Mirai, Gafgyt and Mozi. We also observed a new distributed denial-of-service (DDoS) botnet developed in Golang, called RedGoBot (SHA256: 26e96945ee32199536d4c85124a24c28e853b557eb31f3907d19f08b9798dff4)

RedGoBot’s first campaign was first observed in early September 2022. The threat actor tries to deliver a shell script downloader from 185.216.71[.]157 utilizing wget.

The script downloads the following files:

  • hxxp://185.216.71[.]157/Bins_Bot_hicore_amd64
  • hxxp://185.216.71[.]157/Bins_Bot_hicore_arm64
  • hxxp://185.216.71[.]157/Bins_Bot_hicore_arm
  • hxxp://185.216.71[.]157/Bins_Bot_hicore_mips
  • hxxp://185.216.71[.]157/Bins_Bot_hicore_mips64
  • hxxp://185.216.71[.]157/Bins_Bot_hicore_ppc64
  • hxxp://185.216.71[.]157/Bins_Bot_hicore_ppc64le
  • hxxp://185.216.71[.]157/Bins_Bot_hicore_s390x
  • hxxp://185.216.71[.]157/Bins_Bot_hicore_mipsle
  • hxxp://185.216.71[.]157/Bins_Bot_hicore_mips64le

The second wave of the RedGoBot campaign was observed in November 2022, when the threat actor switched its malware host to 185.246.221[.]220.

In this campaign, the shell script utilizes wget and curl to download the following botnet clients to accommodate different processor architectures:

  • hxxp://185.246.221[.]220/Bins_Bot_hicore_s390x
  • hxxp://185.246.221[.]220/Bins_Bot_hicore_ppc64le
  • hxxp://185.246.221[.]220/Bins_Bot_hicore_ppc64
  • hxxp://185.246.221[.]220/Bins_Bot_hicore_mipsle
  • hxxp://185.246.221[.]220/Bins_Bot_hicore_mips
  • hxxp://185.246.221[.]220/Bins_Bot_hicore_arm64
  • hxxp://185.246.221[.]220/Bins_Bot_hicore_arm
  • hxxp://185.246.221[.]220/Bins_Bot_hicore_amd64

The botnet client can accept the following command and control (C2) channel commands.

Command Description
exec Remote OS command execution
attack Launch DDoS attacks
kill-bot Terminate bot client execution
update-bot No related function for updating

Table 2. RedGoBot C2 commands.

After receiving the attack command from the threat operator, the RedGoBot can perform DDoS attacks on HTTP, ICMP, TCP, UDP, VSE and OpenVPN protocols.

Protocol Method Description
HANDSHAKE TCP Handshake Flood
HOLD TCP Hold Flood
OpenVPN N/A OpenVPN Flood (UDP)

Table 3. RedGoBot supported DDoS types.

Attack Origin Analysis

From August 2021 to December 2022, we have observed 134 million exploit attempts in total, targeting CVE-2021-35394, with 97% of these attacks occurring after the start of August 2022. More than 30 international regions were involved as the attack origins, with the United States being the largest source of attacks at 48.3% of the total. Vietnam, Russia, The Netherlands, France, Luxembourg and Germany were also found to be in the top seven countries from which we observed threat actors taking part in these attacks (shown in Figure 4). However, we recognize that the attackers might leverage proxy servers and VPNs located in those countries to hide their actual physical locations.

Image 4 is a chart showing the percentage, by country, of the attack origin distribution, with the United States at 46.4% followed by Vietnam and Russia.
Figure 4. Attack origin distribution.

Figure 5 shows these attack counts broken down by month from August 2021 to December 2022, for top attack origins.

Image 5 is a chart that details attack trends over time by country origin, starting in October 2021 to December 2022. The attacks peak sharply in July through the end of October.
Figure 5. Attack trends over time.

Starting from August 2022, the number of attacks targeting this vulnerability began to increase, and reached a peak in September and October. In November, the attack attempts drastically decreased but were still in a pretty high state.

It's also notable that around 95% of the attacks leveraging CVE-2021-35394 that originated from Russia were targeting organizations in Australia.

Over 98% of attacks originating from Vietnam in this time period were against CVE-2021-35394, and the majority of attacks from Russia and The Netherlands also leveraged this vulnerability.

According to the latest data from November and December, attacks against CVE-2021-35394 suddenly appeared in large numbers from Luxembourg (accounting for 94.6% of the total attacks in the region). Attacks targeting this vulnerability from Vietnam, which accounted for a high proportion in previous months, decreased significantly in November and December (accounting for 3% of the total number of attacks in the region). This could mean that the attacker is changing the proxy while continuing to exploit this vulnerability.

Figure 6 shows the percentage of attacks targeting CVE-2021-35394 out of the total overall attacks, from August to December 2022. The countries the attacks are coming from are sorted from the highest total number of attacks exploiting CVE-2021-35394 to the least.

Image 6 is a stacked bar chart comparing the total overall attacks to attacks targeting CVE-2021-35394.
Figure 6. Percentage of attacks targeting CVE-2021-35394 out of overall attacks.

The following IP addresses were the top 12 originators of attacks targeting CVE-2021-35394.

IP Attack source region # of attacks (million)
199.195.251[.]190 United States 39.8
172.81.41[.]196 United States 17.2
103.149.137[.]124 Vietnam 10.6
103.149.137[.]138 Vietnam 10.5
46.249.32[.]181 The Netherlands 9.9
37.44.238[.]148 France 1.7
37.44.238[.]185 France 1.3
37.44.238[.]217 France 1.2
69.67.150[.]36 United States 1.2
37.44.238[.]144 France 1.2
103.149.137[.]192 Vietnam 1.2
185.122.204[.]30 Russia 1.1

Table 4. Top attacker IP addresses.

We also analyzed and decoded all the attack payloads, which allowed us to summarize the malicious payload that connects to other malware hosting sites that we mentioned earlier. We then counted the callback URLs that appeared more frequently, which are shown in the table below. The attack source region in each cell is listed from most total attacks to least.

From this data, we can see that when launching a campaign, attackers could host the malware on multiple sites. This allows them to send malicious attack payloads from different regions, either by using the machines in the specific region or by compromising a device in the region and commanding it to spread the threat.

Callback URLs Attack source region # of Attacks (million)
hxxp://185[.]205[.]12[.]157/trc/TRC[.]mpsl United States, The Netherlands, Canada, 31.4
hxxp://172[.]81[.]41[.]196/trc/TRC[.]mpsl The, United States, Germany 10.6
hxxp://135[.]148[.]104[.]21/mipsel Vietnam, Kenya, France, United States, Singapore, India 8.1
hxxp://199[.]195[.]251[.]190/trc/TRC[.]mpsl United States, Canada, Germany, 2.0
hxxp://37[.]44[.]238[.]178/d/xd[.]mpsl France, Kenya, United States, 1.7
hxxp://176[.]97[.]210[.]135/assailant[.]mpsl France, United States 1.2
hxxp://198[.]98[.]56[.]129/trc/TRC[.]mpsl United States, Saint Kitts and Nevis, 1.2
hxxp://141[.]98[.]6[.]249/billy[.]sh United States, Kenya, 1.1
hxxp://185[.]216[.]71[.]157/Bins_Bot_hicore_mipsle United States, Vietnam, Kenya, France 0.7
hxxp://45[.]140[.]141[.]205/bins/sora[.]mpsl United States, Germany, The Netherlands, 0.5

Table 5. Top 10 callback URLs


The surge of attacks leveraging CVE-2021-35394 shows that threat actors are very interested in supply chain vulnerabilities, which can be difficult for the average user to identify and remediate. These issues can make it difficult for the affected user to identify the specific downstream products that are being exploited.

Having robust security protections in place can help you block the malicious traffic we’ve described. If you confirm that a device has been affected by the malware referenced in this post, it is necessary to apply a factory reset on the device and reinstall the latest version of its software.

At home, if you have IoT or network devices from the aforementioned vendor list and have not recently looked for software updates or patches, now is a good time to do so. If you are able to determine that these devices are running slow or sending out a large amount of traffic to contact unknown domains, this could be a sign that your device has been affected by attackers.

We strongly recommend regularly applying patches and upgrades on smart devices as well as traditional desktops and mobile devices whenever possible, to ensure the best protections.

Palo Alto Networks customers receive protections from the vulnerability and malware referenced in this post through the following products and services:

  • Next-Generation Firewalls with a Threat Prevention security subscription can block the attacks with Best Practices via Threat Prevention signatures 91535.
  • WildFire can stop the malware referenced in this article with static signature detections.
  • Advanced URL Filtering and DNS Security are able to block the C2 domain and malware hosting URLs.
  • The Palo Alto Networks IoT security platform can leverage network traffic information to identify the vendor, model and firmware version of a device and identify specific devices that are vulnerable to the aforementioned CVE.
  • In addition, IoT Security has an inbuilt machine learning-based anomaly detection that can alert the customer if a device exhibits non-typical behavior, such as a sudden appearance of traffic from a new source, an unusually high number of connections or an inexplicable surge of certain attributes typically appearing in IoT application payloads.

Indicators of Compromise


Malicious IPs

  • 199[.]195[.]251[.]190
  • 172[.]81[.]41[.]196
  • 103[.]149[.]137[.]124
  • 103[.]149[.]137[.]138
  • 46[.]249[.]32[.]181
  • 69[.]67[.]150[.]36
  • 103[.]149[.]137[.]192
  • 45[.]125[.]236[.]14
  • 173[.]247[.]227[.]66
  • 173[.]247[.]227[.]70
  • 185[.]122[.]204[.]30
  • 45[.]95[.]55[.]188
  • 2[.]58[.]113[.]79
  • 45[.]95[.]55[.]24
  • 45[.]95[.]55[.]218
  • 45[.]95[.]55[.]189
  • 193[.]142[.]146[.]35
  • 37[.]139[.]129[.]11
  • 78[.]135[.]85[.]70
  • 45[.]137[.]21[.]166
  • 195[.]178[.]120[.]183
  • 195[.]133[.]81[.]29
  • 5[.]253[.]246[.]67
  • 45[.]61[.]184[.]133
  • 45[.]61[.]184[.]118
  • 149[.]5[.]173[.]33
  • 163[.]123[.]143[.]226
  • 45[.]61[.]188[.]148
  • 103[.]207[.]38[.]165
  • 45[.]13[.]227[.]115
  • 176[.]97[.]210[.]147
  • 163[.]123[.]143[.]200
  • 185[.]44[.]81[.]62
  • 38[.]22[.]109[.]7
  • 147[.]182[.]132[.]144
  • 205[.]185[.]126[.]88
  • 209[.]141[.]51[.]43
  • 198[.]98[.]52[.]213
  • 45[.]95[.]55[.]185
  • 20[.]249[.]89[.]181
  • 3[.]235[.]28[.]168

Callback URLs



RedGoBot Malware Sample


Mirai Samples