CVE-2018-7791 in Modicon M221
Summary
by MITRE
A Permissions, Privileges, and Access Control vulnerability exists in Schneider Electric's Modicon M221 product (all references, all versions prior to firmware V1.6.2.0). The vulnerability allows unauthorized users to overwrite the original password with their password. If an attacker exploits this vulnerability and overwrite the password, the attacker can upload the original program from the PLC.
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Analysis
by VulDB Data Team • 05/29/2026
The vulnerability identified as CVE-2018-7791 represents a critical permissions and access control weakness within Schneider Electric's Modicon M221 programmable logic controller product line. This flaw exists in all versions prior to firmware revision V1.6.2.0 and fundamentally compromises the device's security architecture by allowing unauthorized individuals to manipulate the system's authentication mechanism. The vulnerability resides in the password management system where legitimate users can be displaced by unauthorized parties through simple password overwriting operations, creating a direct pathway for privilege escalation and system compromise.
The technical implementation of this vulnerability stems from inadequate access control mechanisms within the Modicon M221's authentication framework. When an unauthorized user successfully overwrites the original password, they effectively gain administrative privileges over the device, bypassing the intended security boundaries that should protect against unauthorized access. This flaw operates under the broader category of CWE-284, which addresses improper access control vulnerabilities, specifically manifesting as insufficient authorization checks during password modification processes. The vulnerability essentially creates a backdoor scenario where legitimate system administrators can be locked out while malicious actors gain control of the PLC's operational capabilities.
From an operational standpoint, the impact of CVE-2018-7791 extends far beyond simple authentication bypass. Once an attacker has overwritten the password and gained access, they can upload and execute arbitrary programs on the PLC, potentially leading to complete system compromise. This capability allows for the deployment of malicious code that could disrupt industrial processes, manipulate production outputs, or create persistent access points for further attacks. The vulnerability directly aligns with ATT&CK technique T1078 which covers valid accounts and privilege escalation, as attackers can leverage compromised credentials to gain deeper system access and maintain persistence within industrial control systems.
The exploitation of this vulnerability demonstrates a fundamental flaw in the device's security model where password modification operations lack proper authentication checks to verify the identity of the user attempting to make changes. This weakness enables what security researchers would categorize as a privilege escalation attack, where an attacker moves from a lower-privileged state to one with full administrative control over the PLC. The vulnerability also presents significant risk to industrial control system environments where these devices operate, as they often control critical infrastructure components and production processes where unauthorized modifications could result in physical damage, safety hazards, or operational disruptions.
Organizations utilizing Modicon M221 devices should immediately implement firmware updates to version V1.6.2.0 or later to address this vulnerability. Additionally, network segmentation and access controls should be strengthened to limit physical and network access to these devices. Regular security assessments of industrial control systems should include verification of authentication mechanisms and privilege management configurations. The vulnerability highlights the importance of maintaining current firmware versions and implementing robust security practices specifically tailored for industrial environments where the consequences of system compromise can extend beyond traditional cybersecurity impacts into physical safety and operational integrity domains.