CVE-2017-6030 in Modicon M221
Summary
by MITRE
A Predictable Value Range from Previous Values issue was discovered in Schneider Electric Modicon PLCs Modicon M221, firmware versions prior to Version 1.5.0.0, Modicon M241, firmware versions prior to Version 4.0.5.11, and Modicon M251, firmware versions prior to Version 4.0.5.11. The affected products generate insufficiently random TCP initial sequence numbers that may allow an attacker to predict the numbers from previous values. This may allow an attacker to spoof or disrupt TCP connections.
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Analysis
by VulDB Data Team • 05/29/2026
The vulnerability identified as CVE-2017-6030 represents a critical weakness in the network security implementation of Schneider Electric Modicon Programmable Logic Controllers that affects multiple models including M221, M241, and M251 series. This issue stems from the improper generation of TCP initial sequence numbers within the affected firmware versions, creating a predictable pattern that compromises the integrity of network communications. The flaw specifically impacts devices running firmware versions prior to 1.5.0.0 for M221 models and versions 4.0.5.11 for M241 and M251 models, leaving these industrial control systems vulnerable to sophisticated network-based attacks that exploit predictable sequence number generation.
The technical root cause of this vulnerability lies in the insufficient randomness of TCP initial sequence number generation, which violates fundamental principles of network security and cryptographic randomness requirements. According to CWE-330, this represents a weakness in randomness where insufficient entropy is used to generate values that should be unpredictable. The predictable nature of these sequence numbers creates a pathway for attackers to perform TCP sequence number prediction attacks, a technique that falls under ATT&CK technique T1071.004 for application layer protocol tunneling and T1566.001 for credential access through network sniffing and manipulation. The vulnerability essentially allows attackers to forge TCP packets by correctly predicting the sequence numbers that the PLCs will use in their communication sessions.
The operational impact of this vulnerability extends beyond simple network disruption to potentially compromise the integrity and availability of industrial control systems that rely on these Modicon PLCs for critical operations. An attacker who successfully predicts TCP sequence numbers can perform man-in-the-middle attacks, hijack active connections, or execute denial-of-service attacks against the affected devices. This capability poses significant risks to industrial environments where these PLCs control critical infrastructure processes, manufacturing operations, or safety systems. The vulnerability particularly affects environments where network monitoring and packet analysis are possible, as attackers need to observe previous sequence numbers to make accurate predictions. The impact is amplified in environments where these PLCs communicate with other industrial devices, SCADA systems, or enterprise networks, as successful exploitation could lead to broader compromise of industrial control networks.
Mitigation strategies for CVE-2017-6030 require immediate firmware updates to the latest available versions that address the sequence number generation weakness. Organizations should implement network segmentation and access controls to limit exposure of affected PLCs to untrusted networks, utilizing firewalls and network access control lists to restrict communication paths. Network monitoring should be enhanced to detect anomalous TCP sequence number patterns that might indicate exploitation attempts. The implementation of network intrusion detection systems capable of identifying TCP sequence number prediction attacks can provide additional layers of protection. Additionally, organizations should conduct comprehensive vulnerability assessments to identify all affected devices within their industrial control networks and establish monitoring procedures to detect potential exploitation attempts. Security patches should be applied systematically across all affected models, with careful attention to ensuring compatibility with existing industrial control system configurations. Regular network traffic analysis and anomaly detection should be implemented to identify potential exploitation attempts that might not be immediately apparent through standard monitoring tools.