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Introduction:
The convergence of legacy IoT devices and unpatched vulnerabilities has given rise to a new generation of botnets. Attackers are actively exploiting CVE-2024-3721 in TBK DVRs to deploy the Mirai variant known as Nexcorium, which propagates via old exploits and default credentials, persists on compromised devices, and launches large-scale DDoS attacks. Additionally, end-of-life (EoL) TP-Link routers are being targeted through known flaws, expanding the botnet’s reach.
Learning Objectives:
- Understand the mechanics of CVE-2024-3721 and how Nexcorium propagates across vulnerable IoT devices.
- Detect signs of Mirai variant infection on DVRs and routers using network and host-based indicators.
- Implement hardening measures, including credential management, access controls, and network segmentation to mitigate botnet recruitment.
You Should Know:
1. Understanding CVE-2024-3721 and Nexcorium Propagation
The Nexcorium botnet exploits CVE-2024-3721, a remote command injection vulnerability in TBK DVR devices (firmware versions prior to 4.0.0). Attackers send crafted HTTP requests to the device’s web interface, often using default credentials like `admin:admin` or root:12345. Once inside, they download and execute the Nexcorium binary, which then scans for other vulnerable devices using pre-configured exploit modules.
Step-by-step guide to test for exposure (authorized testing only):
1. Identify TBK DVRs on your network using Nmap:
nmap -p 80,8080,554 --open -sV --script http-default-accounts 192.168.1.0/24
2. Check for default credential access using `curl`:
curl -X POST http://<DVR-IP>/cgi-bin/webauth.cgi -d "username=admin&password=admin"
3. Simulate the exploit payload (CVE-2024-3721) to verify vulnerability:
curl "http://<DVR-IP>/cgi-bin/command.cgi?cmd=id" --header "Authorization: Basic YWRtaW46YWRtaW4="
If the response returns system output (e.g., uid=0(root)), the device is vulnerable.
Mitigation: Immediately change default credentials, disable remote web access, and apply vendor firmware updates if available. For EoL devices, isolate them in a dedicated VLAN with no internet egress.
2. Identifying Compromised Devices on Your Network
Nexcorium-infected devices exhibit unusual outbound traffic patterns, including DNS queries to known DDoS command-and-control (C2) domains and high-volume UDP/ICMP floods. Use network monitoring to detect these anomalies.
Step-by-step detection using Linux tools:
- Capture traffic to identify Mirai-style scanning (port 23, 2323, 7547):
sudo tcpdump -i eth0 'tcp dst port 23 or tcp dst port 2323 or tcp dst port 7547' -n -c 100
- Check for active botnet processes on suspected DVRs (via SSH or telnet):
ps aux | grep -E 'nexcorium|mirai|pnscan'
3. Use `netstat` to spot established C2 connections:
netstat -antp | grep ESTABLISHED | grep :<suspicious-port>
Windows equivalent (for router/DVR management hosts):
Get-NetTCPConnection | Where-Object { $<em>.State -eq 'Established' -and ($</em>.RemotePort -eq 23 -or $_.RemotePort -eq 2323) }
Shodan query to find exposed TBK DVRs globally:
"TBK DVR" "Server: lighttpd" port:80
If your device appears, assume compromise and perform factory reset.
3. Hardening TBK DVRs and Legacy IoT Devices
Most Mirai infections succeed because of default credentials and unnecessary open ports. Apply the following hardening steps to any TBK DVR or similar IoT device.
Step-by-step hardening guide:
- Change default credentials – Use strong, unique passwords (12+ chars, alphanumeric + symbols).
2. Disable telnet and SSH if not required:
On device (if shell accessible) systemctl disable telnet.socket systemctl stop telnet.socket
3. Restrict web interface access using firewall rules (on a Linux gateway or router):
sudo iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT sudo iptables -A INPUT -p tcp --dport 80 -j DROP
4. Apply access control lists (ACLs) on managed switches to block inter-VLAN communication from IoT subnet.
5. Monitor outbound connections – Block known malicious IPs using threat intelligence feeds:
curl -s https://raw.githubusercontent.com/firehol/blocklist-ipsets/master/blocklist_net_ips.txt | sudo ipset add blacklist sudo iptables -I FORWARD -m set --match-set blacklist dst -j DROP
For Windows-based DVR management software, enforce application whitelisting via AppLocker and disable unused services.
- Detecting Mirai/Nexcorium Malware Activity via Logs and Network Flows
Nexcorium leaves forensic artifacts. It modifies `crontab` for persistence and drops binaries in `/tmp` or /var/run. On the network, it generates specific User-Agent strings and performs SYN scans.
Step-by-step log analysis (Linux DVR or syslog server):
1. Check for unusual cron entries:
grep -r "wget|curl|chmod" /var/spool/cron/crontabs/
2. Search for downloaded binaries:
find /tmp /var/run -type f -executable -mtime -2
3. Analyze web server logs for exploit attempts (CVE-2024-3721 pattern):
grep "cmd=id" /var/log/httpd/access.log
Wireshark filter to detect Mirai scanning behavior:
tcp.flags.syn==1 and tcp.flags.ack==0 and (tcp.dstport==23 or tcp.dstport==2323)
Snort rule to alert on Nexcorium C2 beaconing:
alert tcp $HOME_NET any -> $EXTERNAL_NET 9999 (msg:"Nexcorium C2 Beacon"; flow:to_server,established; content:"|00 01 02|"; depth:3; sid:20243721;)
Deploy a honeypot on port 23 to capture botnet scanners: use `cowrie` or t-pot.
5. Mitigating DDoS Attacks Launched from Compromised Devices
If your network hosts infected DVRs, they may participate in volumetric DDoS attacks. Immediate containment is critical.
Step-by-step mitigation:
- Rate-limit outgoing UDP and ICMP from IoT subnet using tc (Linux):
sudo tc qdisc add dev eth0 root handle 1: htb default 30 sudo tc class add dev eth0 parent 1: classid 1:1 htb rate 1mbit sudo tc filter add dev eth0 protocol ip parent 1:0 u32 match ip protocol 17 0xff flowid 1:1
- Blackhole routing for the infected device’s IP at the border router:
ip route add blackhole <infected-IP>/32
- Use Cloudflare or similar WAF with DDoS protection – enable “I’m Under Attack” mode during an active flood.
- Implement BGP RTBH (Remotely Triggered Black Hole) if you control an ASN:
route 192.0.2.1/32 next-hop 192.0.2.254
- Deploy fail2ban to block repeated exploit attempts from internal IPs:
/etc/fail2ban/jail.local [dvr-exploit] enabled = true filter = dvr-exploit logpath = /var/log/nginx/access.log maxretry = 3 bantime = 3600
Create filter `/etc/fail2ban/filter.d/dvr-exploit.conf` with regex for CVE-2024-3721.
6. Securing End-of-Life TP-Link Routers from Known Flaws
The Nexcorium campaign also targets EoL TP-Link routers (e.g., TL-WR841N, TL-WR740N) via unpatched vulnerabilities such as CVE-2019-7406 (command injection). Since no firmware updates exist, you must either replace or aggressively isolate them.
Step-by-step guide for legacy routers:
- Disable remote management (WAN-side access) in the router admin panel.
- Change default LAN IP to a non-standard subnet (e.g., 10.10.10.0/24) to reduce scanning exposure.
- Set up a separate VLAN for the EoL router and block all inbound traffic from WAN except required ports.
- Use a Pi-hole or DNS filter to block known Mirai C2 domains:
echo "0.0.0.0 nexuscorium-c2.net" >> /etc/pihole/blacklist.txt pihole restartdns
- Monitor for suspicious outbound connections from the router’s WAN IP using ntopng or Zeek.
- Replace the router immediately if it controls any critical infrastructure – cost of replacement is lower than DDoS liability.
For advanced users, flash OpenWrt if hardware supports it; otherwise, physically disconnect from the internet.
7. Incident Response Steps for IoT Botnet Infections
When you confirm a TBK DVR or TP-Link router is part of Nexcorium, follow a structured IR process.
Step-by-step IR procedure:
- Isolate – Unplug the device from the network or disable its switch port.
- Image memory and storage (if forensics needed) using `dd` over network:
nc -l -p 9000 | dd of=dvr_image.bin On device: dd if=/dev/mtdblock0 | nc <forensics-IP> 9000
- Factory reset the device via physical button or web interface.
- Update firmware (if available) or manually harden as described in Section 3.
- Change all passwords – not just DVR but any shared credentials on the network.
- Review firewall logs for lateral movement – look for internal scans from the infected IP.
- Report the incident to your ISP and national CERT with indicators (C2 IPs, malware hashes).
- Deploy continuous monitoring using a SIEM rule that alerts on TBK DVRs contacting external IPs on non-standard ports.
What Undercode Say:
- Key Takeaway 1: The resurgence of Mirai variants like Nexcorium proves that default credentials and neglected patching cycles remain the single biggest entry vector for IoT botnets. Organizations must enforce credential rotation and network segmentation as non-negotiable baselines.
- Key Takeaway 2: EoL devices are ticking time bombs. Without vendor support, the only responsible action is to decommission or air-gap them. Relying on “it still works” mentality fuels the next DDoS wave.
Analysis: The Nexcorium campaign blends old-school exploit chaining (CVE-2024-3721 + default creds) with modern persistence mechanisms, making it highly resilient. Defenders often overlook DVRs and routers because they are set-and-forget, but these devices now serve as ideal bot nodes. The lack of logging capabilities on many IoT platforms means detection relies on network traffic analysis alone. Furthermore, the targeting of EoL TP-Link routers indicates attackers have pre-computed vulnerability databases for abandoned hardware. Until IoT manufacturers are legally required to provide security updates for a minimum period, botnets will continue to commoditize these devices. AI-driven anomaly detection on network flows can help, but only if organizations deploy it on internal segments—not just the perimeter.
Prediction: Over the next 12 months, we will see an escalation of hybrid botnets that combine Mirai-like spreading with AI-generated evasion techniques, such as polymorphic payloads and C2 domain generation algorithms. Law enforcement takedowns will become less effective as attackers shift to decentralized command using DHT or blockchain. Additionally, insurance carriers will start mandating IoT-specific hardening audits, and failure to secure DVRs and routers will lead to policy voidance. The most disruptive attacks will not come from zero-days but from recycled exploits against forgotten, unpatched devices sitting in home offices and small businesses.
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Reported By: Hackermohitkumar Attackers – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
