CVE-2024-41798: SENTRON 7KM PAC3200 Auth Bypass Flaw

CVE-2024-41798 Overview

A critical authentication bypass vulnerability has been identified in Siemens SENTRON 7KM PAC3200 power monitoring devices. The affected devices implement a weak 4-digit PIN mechanism to protect administrative access via the Modbus TCP interface. This insufficient authentication mechanism allows attackers with network access to the Modbus TCP interface to easily bypass protection through brute-force attacks or by sniffing the Modbus clear text communication.

Critical Impact

This vulnerability allows unauthenticated remote attackers to gain full administrative control over industrial power monitoring equipment, potentially enabling manipulation of power measurements, unauthorized configuration changes, or disruption of critical infrastructure monitoring systems.

Affected Products

• SENTRON 7KM PAC3200 (All versions)

Discovery Timeline

• 2024-10-08 - CVE CVE-2024-41798 published to NVD
• 2024-10-10 - Last updated in NVD database

Technical Details for CVE-2024-41798

Vulnerability Analysis

This vulnerability stems from improper authentication (CWE-287) in the Siemens SENTRON 7KM PAC3200 power monitoring device. The device relies solely on a 4-digit PIN for administrative access protection via the Modbus TCP interface, which provides only 10,000 possible combinations (0000-9999). This severely limited keyspace makes brute-force attacks trivially achievable within minutes.

Compounding the authentication weakness, the Modbus TCP protocol transmits data in clear text without encryption. This design flaw means that even if the PIN is complex within its 4-digit constraints, an attacker with network visibility can simply intercept the PIN during legitimate administrative sessions.

The combination of weak authentication and unencrypted communication creates multiple attack paths for adversaries with access to the same network segment as the affected device.

Root Cause

The root cause of this vulnerability is twofold. First, the implementation of an insufficiently complex authentication mechanism—a 4-digit PIN provides inadequate protection against automated brute-force attacks. Second, the use of the Modbus TCP protocol without transport layer encryption allows credential interception. These design decisions prioritize backward compatibility and simplicity over security, which is particularly concerning for devices deployed in industrial control system environments.

Attack Vector

The vulnerability is exploitable over the network by any attacker with access to the Modbus TCP interface. Exploitation can occur through two primary methods:

Brute-Force Attack: An attacker can systematically attempt all 10,000 possible 4-digit PIN combinations. Without rate limiting or account lockout mechanisms, this attack can complete in minutes or even seconds depending on network conditions and the target device's response time.

Passive Credential Sniffing: Since Modbus TCP transmits data without encryption, an attacker positioned on the network path between a legitimate administrator and the device can capture the PIN during a valid authentication session using standard network sniffing tools.

Both attack methods require network-level access to the Modbus TCP interface (typically port 502), but do not require any prior authentication or privileges on the target device.

Detection Methods for CVE-2024-41798

Indicators of Compromise

• Unusual volume of Modbus TCP connection attempts to port 502 on SENTRON 7KM PAC3200 devices
• Sequential or rapid authentication attempts indicating brute-force activity
• Unexpected administrative configuration changes on affected devices
• Unauthorized access logs or session activity outside normal maintenance windows

Detection Strategies

• Deploy network intrusion detection systems (IDS) with rules to detect brute-force patterns on Modbus TCP communications
• Monitor for excessive failed authentication attempts against the device's administrative interface
• Implement network traffic analysis to identify clear-text Modbus communications containing authentication data
• Use SIEM correlation to detect authentication anomalies from unusual source IP addresses

Monitoring Recommendations

• Enable logging on network firewalls and switches to capture all traffic to and from SENTRON 7KM PAC3200 devices
• Establish baseline administrative access patterns and alert on deviations
• Deploy OT/ICS-aware security monitoring tools capable of parsing Modbus TCP protocol traffic
• Conduct regular audits of device configurations to detect unauthorized changes

How to Mitigate CVE-2024-41798

Immediate Actions Required

• Isolate SENTRON 7KM PAC3200 devices on a segregated network segment inaccessible from general corporate networks
• Implement strict firewall rules limiting Modbus TCP access (port 502) to authorized administrative workstations only
• Deploy VPN or encrypted tunneling for any remote administrative access requirements
• Review access logs for any indicators of unauthorized access or brute-force attempts

Patch Information

Siemens has published a security advisory (SSA-850560) addressing this vulnerability. Organizations should consult the Siemens Security Advisory for the latest guidance on available updates and recommended mitigations. Given that all versions are affected, network-level compensating controls are critical.

Workarounds

• Implement network segmentation to restrict access to the Modbus TCP interface to authorized systems only
• Deploy application-layer firewalls or protocol-aware gateways that can filter and monitor Modbus TCP traffic
• Use a VPN or encrypted tunnel for all administrative communications to the device
• Consider disabling remote Modbus TCP administrative access entirely where operationally feasible
• Implement monitoring and alerting for any authentication attempts against the device

# Example firewall rule to restrict Modbus TCP access
# Allow only authorized administrator workstation (192.168.10.50) to access SENTRON device
iptables -A INPUT -p tcp --dport 502 -s 192.168.10.50 -j ACCEPT
iptables -A INPUT -p tcp --dport 502 -j DROP