CVE-2025-36535: Embedded Web Server Auth Bypass Flaw

CVE-2025-36535 Overview

CVE-2025-36535 is a critical authentication bypass vulnerability affecting embedded web servers in industrial control system (ICS) devices. The embedded web server lacks authentication and access controls, allowing unrestricted remote access. This vulnerability could lead to configuration changes, operational disruption, or arbitrary code execution depending on the environment and exposed functionality.

Critical Impact

Remote attackers can access the device's web interface without any authentication, potentially taking full control of the industrial gateway device and disrupting critical operational technology (OT) environments.

Affected Products

• AutomationDirect EKI-1221-CE Modbus Gateway
• Devices with vulnerable embedded web server firmware
• Industrial protocol gateway devices exposed to network access

Discovery Timeline

• 2025-05-21 - CVE-2025-36535 published to NVD
• 2025-05-21 - Last updated in NVD database

Technical Details for CVE-2025-36535

Vulnerability Analysis

This vulnerability is classified under CWE-306 (Missing Authentication for Critical Function), which represents one of the most severe security weaknesses in embedded systems. The embedded web server on affected devices fails to implement any form of authentication or access control mechanism, meaning any network-reachable attacker can interact with the administrative interface without providing credentials.

In industrial control system environments, this type of vulnerability is particularly dangerous. The affected Modbus gateway devices serve as protocol translators between different industrial networks, making them critical infrastructure components. Unauthorized access to these devices could allow attackers to modify communication parameters, intercept or manipulate industrial protocol traffic, or potentially pivot to other connected systems.

The network-accessible nature of this vulnerability combined with no required privileges or user interaction makes exploitation trivial for any attacker who can reach the device on the network.

Root Cause

The root cause of CVE-2025-36535 is the complete absence of authentication mechanisms in the embedded web server implementation. The device firmware was designed or configured to serve the web management interface without requiring any form of credential verification. This design flaw allows any HTTP request to the device's web interface to be processed as if it came from an authorized administrator.

This type of vulnerability often stems from development practices that prioritize ease of initial setup over security, or from legacy firmware designs that predate modern security requirements for industrial devices.

Attack Vector

The attack vector is network-based and requires no authentication, privileges, or user interaction. An attacker can exploit this vulnerability by:

1. Identifying a vulnerable device on the network through port scanning or Shodan-style reconnaissance
2. Directly accessing the web management interface via HTTP/HTTPS
3. Making configuration changes, extracting sensitive information, or executing available administrative functions
4. Potentially uploading malicious firmware or configurations depending on exposed functionality

The exploitation is straightforward—simply navigating to the device's IP address in a web browser grants full administrative access. For more technical details, refer to the CISA ICS Advisory ICSA-25-140-09.

Detection Methods for CVE-2025-36535

Indicators of Compromise

• Unexpected configuration changes on Modbus gateway devices without corresponding authorized access logs
• Network traffic to the device's web management port (typically TCP/80 or TCP/443) from unauthorized IP addresses
• Unusual administrative actions or firmware modifications on affected devices
• Anomalous Modbus protocol behavior downstream from compromised gateways

Detection Strategies

• Implement network monitoring to detect unauthorized access attempts to device management interfaces
• Deploy ICS-aware intrusion detection systems (IDS) to identify reconnaissance and exploitation attempts targeting industrial devices
• Configure SentinelOne Singularity platform to monitor for network anomalies and unauthorized access patterns in OT environments
• Conduct regular configuration audits to detect unauthorized changes to gateway device settings

Monitoring Recommendations

• Monitor network traffic to/from affected devices for connections originating from untrusted network segments
• Enable logging on network security devices to capture all access attempts to industrial device management interfaces
• Implement alerting for any web traffic to Modbus gateway devices outside of scheduled maintenance windows
• Deploy network segmentation monitoring to detect lateral movement attempts from compromised OT devices

How to Mitigate CVE-2025-36535

Immediate Actions Required

• Isolate affected devices from untrusted networks immediately by implementing network segmentation
• Place vulnerable Modbus gateways behind firewalls with strict access control lists limiting management interface access
• Disable remote web management access if not operationally required
• Review CISA ICS Advisory ICSA-25-140-09 for vendor-specific remediation guidance

Patch Information

Organizations should consult the CISA ICS Advisory ICSA-25-140-09 for the latest patch information and vendor guidance. Contact AutomationDirect for firmware updates that address this authentication bypass vulnerability. The AutomationDirect Product Page may contain updated firmware or security bulletins.

Workarounds

• Implement network segmentation to restrict access to the device management interface to authorized management stations only
• Deploy a VPN or jump server architecture requiring authentication before accessing the device network segment
• Configure firewall rules to block all inbound connections to the web management port except from specific trusted IP addresses
• Consider placing affected devices on isolated network segments with no direct internet connectivity

# Example firewall rule to restrict management access (iptables)
# Allow management access only from trusted admin workstation
iptables -A INPUT -p tcp --dport 80 -s 192.168.100.10 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s 192.168.100.10 -j ACCEPT
# Block all other access to web management ports
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP