Critical Linksys Router Flaw Exploited by TheMoon Worm: What You Need to Know

Estimated reading time: 7 minutes

Key Takeaways:

  • A critical vulnerability (CVE-2025-34037) affects Linksys E-Series routers and is being actively exploited by TheMoon worm.
  • The vulnerability allows unauthenticated attackers to inject shell commands remotely.
  • Affected users should immediately identify vulnerable devices, apply security patches, and monitor network traffic.
  • Non-technical users should consult technical support and power cycle their routers as a temporary measure.

Table of Contents:

Understanding the Linksys Router Flaw (CVE-2025-34037)

The vulnerability lies in an OS command injection flaw present in various models of Linksys E-Series routers. Specifically, the /tmUnblock.cgi and /hndUnblock.cgi endpoints are susceptible to command injection. This allows unauthenticated attackers to inject shell commands. The core issue is the lack of proper sanitization of the ttcp_ip parameter passed to these CGI scripts, accessible over HTTP port 8080.

Attackers do not require authentication to exploit this flaw. Even though requests may include randomly generated credentials like “admin,” these are not validated. This makes CVE-2025-34037 a zero-click, zero-auth remote command injection vulnerability. It is actively exploited to execute arbitrary shell commands on vulnerable routers.

Affected Models

While a comprehensive list is still being compiled, confirmed vulnerable models include:

  • E4200
  • E3200
  • E3000
  • E2500
  • E2100L
  • E2000
  • E1550
  • E1500
  • E1200
  • E1000
  • E900

Other potentially impacted models include WAG, WAP, WES, and WRT-series Linksys routers, as well as Wireless-N access points.

TheMoon Worm: Exploitation in the Wild

TheMoon worm is actively exploiting CVE-2025-34037. It uses the vulnerability to infect devices and propagate across networks. Named after the benign-looking HTML pages and imagery it uses, TheMoon worm’s attack chain begins when it connects to a router’s port 8080 and requests the /HNAP1/ endpoint.

Attack Chain Details

  1. Initial Contact: The worm connects to the router’s port 8080 and requests the /HNAP1/ endpoint.
  2. Information Gathering: The /HNAP1/ endpoint returns an XML profile containing the router’s model name and firmware version.
  3. Vulnerability Confirmation: The worm uses the model and firmware information to confirm the router is vulnerable.
  4. Exploit Trigger: A second unauthenticated request is sent to trigger the exploit. This executes a small shell script.
  5. Payload Download: The shell script downloads the actual worm – a 2MB ELF MIPS binary.
  6. Infection and Propagation: Once executed, the infected router scans for other potential victims and serves the worm binary at a random low port.

This process results in a distributed infrastructure where each infected router acts as a temporary HTTP server. This setup is used to deliver the payload to new victims. This amplifies the worm’s propagation speed and reach.

Scanning and Target Selection

Infected devices immediately begin scanning the internet for other routers to infect. The malware includes a hardcoded list of approximately 670 IP network blocks, primarily in /21 and /24 subnets, linked to DSL and cable ISPs across multiple countries. This widespread scanning increases the likelihood of finding and compromising vulnerable devices.

Potential Botnet Conversion

The worm exhibits characteristics of a self-replicating threat. However, strings within the binary may suggest the presence of a command and control (C2) channel. This raises concerns about the potential for TheMoon worm to be converted into a botnet, enabling attackers to perform coordinated malicious activities.

Practical Takeaways and Actionable Advice

Exploit targeting Linksys routers via TheMoon worm

Given the severity and active exploitation of CVE-2025-34037, immediate action is required to mitigate the risk. Here’s what technical and non-technical users should do:

Technical Users

  1. Identify Vulnerable Devices: Compile a list of Linksys routers within your network. Compare the models against the confirmed and potentially impacted lists provided earlier.
  2. Check Firmware Version: Access the router’s administration interface and verify the firmware version. Older firmware versions are more likely to be vulnerable.
  3. Monitor Network Traffic:
    • Look for heavy outbound scans on TCP ports 80 and 8080.
    • Detect inbound connection attempts to random ports below 1024.
    • Identify temporary local HTTP servers running on low-numbered ports.
  4. Scan for Vulnerability: Use nc command to check for a vulnerable device, such as: echo "GET /HNAP1/ HTTP/1.1\r\nHost: test\r\n\r\n" | nc routerip 8080. If an XML response is returned, the device may be vulnerable.
  5. Apply Security Patches: Check the Linksys support website for available firmware updates or security patches for your specific router model. Apply any available patches immediately. If no patch is available, consider replacing the router.
  6. Implement Network Segmentation: Segment your network to limit the spread of the worm. Place routers in a separate VLAN or subnet to isolate them from critical systems.
  7. Intrusion Detection/Prevention Systems (IDS/IPS): Configure IDS/IPS to detect and block malicious traffic associated with TheMoon worm, including scans on ports 80 and 8080, and attempts to access the /tmUnblock.cgi and /hndUnblock.cgi endpoints.
  8. Dark web monitoring service: Set up alerts to track mentions of your network or Linksys router models on underground forums and dark web marketplaces, to discover if there are any threat actors planning to target your company.

Non-Technical Users

  1. Identify Your Router Model: Locate the model number on your Linksys router. This is typically found on a sticker on the bottom or back of the device.
  2. Consult Technical Support: Contact your internet service provider (ISP) or a qualified IT professional to check if your router model is affected and to assist with applying security updates or replacing the device.
  3. Power Cycle Your Router: As a temporary measure, power cycle your router by unplugging it from the power outlet for 30 seconds and then plugging it back in. This can disrupt any active worm infections.
  4. Change Default Credentials: If you are able to access the router’s settings, change the default username and password to a strong, unique password.
  5. Enable Automatic Updates: If available, enable automatic firmware updates in the router’s settings. This ensures that the router receives the latest security patches automatically.
  6. Telegram threat monitoring: Monitor cybersecurity-related Telegram channels for any updates or warnings about Linksys router vulnerabilities.

PurpleOps and Router Security

At PurpleOps, we recognize the critical role that network devices like routers play in the overall security posture of an organization. The exploitation of vulnerabilities like CVE-2025-34037 by malware such as TheMoon worm can have serious consequences, ranging from network disruptions to data breaches.

Our services can assist organizations in mitigating risks associated with router vulnerabilities:

  • Cyber Threat Intelligence: Our cyber threat intelligence platform provides real-time ransomware intelligence and information on emerging threats, including details on exploits targeting network devices. This can help organizations stay ahead of potential attacks.
  • Supply-Chain Risk Monitoring: We offer supply-chain risk monitoring to identify vulnerabilities in third-party devices and software used within your network, including routers. This proactive approach can help prevent supply chain attacks.
  • Breach Detection: Our breach detection capabilities can help detect and respond to network intrusions resulting from exploited router vulnerabilities.
  • Dark Web Monitoring: We offer dark web monitoring service to identify if any confidential information from your organisation has been published there.
  • Underground Forum Intelligence: Our underground forum intelligence service will get you threat intel of potential threat actors planning on targetting your organisation.

Call to Action

Given the active exploitation of this critical Linksys router flaw, immediate action is essential. If you require assistance in assessing your network security or implementing remediation measures, we encourage you to explore PurpleOps’ services. Contact us today to learn more about how we can help secure your network infrastructure and protect against emerging threats.

FAQ

What is CVE-2025-34037?
CVE-2025-34037 is a critical security vulnerability affecting multiple Linksys E-Series routers, allowing unauthenticated attackers to inject shell commands.

Which Linksys router models are affected?
Confirmed vulnerable models include E4200, E3200, E3000, E2500, E2100L, E2000, E1550, E1500, E1200, E1000, and E900. Other models may also be affected.

What is TheMoon worm?
TheMoon worm is malware actively exploiting CVE-2025-34037 to infect Linksys routers and propagate across networks.

How can I protect my Linksys router?
Technical users should apply security patches, monitor network traffic, and implement network segmentation. Non-technical users should consult technical support and power cycle their routers.

What services does PurpleOps offer to help with router security?
PurpleOps offers cyber threat intelligence, supply-chain risk monitoring, and breach detection services to help organizations mitigate risks associated with router vulnerabilities.