CVE-2021-37191 is a medium-severity vulnerability affecting Siemens SINEMA Remote Connect Server versions prior to 3.0 SP2. The vulnerability is classified under CWE-799, which pertains to improper control of interaction frequency. Specifically, this flaw allows an unauthenticated attacker who is on the same network segment as the vulnerable system to perform brute-force attacks against usernames used by the SINEMA Remote Connect Server. The vulnerability arises because the software does not adequately limit or throttle repeated authentication attempts, enabling an attacker to enumerate valid usernames by systematically trying different inputs without being blocked or delayed. This can lead to information disclosure about valid user accounts, which could be leveraged in subsequent attacks such as password guessing or targeted exploitation. The CVSS v3.1 base score is 4.3, reflecting a medium severity level, with the vector indicating that the attack requires local network access (AV:A), low attack complexity (AC:L), no privileges or user interaction required (PR:N, UI:N), and impacts confidentiality only (C:L) without affecting integrity or availability. No known exploits have been reported in the wild, and Siemens has addressed the issue in version 3.0 SP2 of the product. SINEMA Remote Connect Server is used primarily for secure remote access to industrial control systems (ICS) and operational technology (OT) networks, making this vulnerability particularly relevant to critical infrastructure environments that rely on Siemens solutions for remote connectivity and management.

For European organizations, especially those operating in critical infrastructure sectors such as energy, manufacturing, transportation, and utilities, this vulnerability poses a risk of user enumeration within remote access gateways. Successful brute forcing of usernames can facilitate further attacks, including password guessing, credential stuffing, or social engineering campaigns targeting valid users. While the vulnerability itself does not directly allow system compromise or denial of service, the exposure of valid usernames weakens the security posture and increases the attack surface. Given the widespread use of Siemens industrial solutions across Europe, organizations that have not updated SINEMA Remote Connect Server to version 3.0 SP2 or later remain vulnerable. Attackers with local network access—such as malicious insiders, compromised devices within the same network segment, or attackers who have gained limited network foothold—could exploit this flaw to gather intelligence on user accounts. This can lead to lateral movement within OT environments, potentially disrupting industrial operations or leading to data breaches. The impact is heightened in environments where remote access is critical for operational continuity and where strong user authentication practices are not enforced. However, the lack of requirement for user interaction and the low complexity of the attack vector means that the vulnerability can be exploited relatively easily by attackers with network access, increasing the risk profile for affected organizations.

To mitigate this vulnerability, European organizations should prioritize upgrading SINEMA Remote Connect Server to version 3.0 SP2 or later, where the issue has been resolved. In addition to patching, organizations should implement network segmentation to restrict access to the SINEMA Remote Connect Server, ensuring that only trusted devices and users within secure network zones can reach the service. Deploying network-level controls such as access control lists (ACLs) and firewall rules to limit traffic to the remote connect server reduces exposure to potential attackers. Monitoring and logging authentication attempts on the SINEMA server can help detect brute-force activities early; organizations should configure alerts for repeated failed login attempts or unusual authentication patterns. Implementing multi-factor authentication (MFA) for remote access users adds a critical layer of defense, reducing the risk that username enumeration leads to account compromise. Additionally, organizations should enforce strong password policies and consider account lockout or throttling mechanisms at the network or application layer to further limit brute-force attempts. Regular security assessments and penetration testing focused on remote access infrastructure can help identify residual risks. Finally, educating OT and IT personnel about the risks of insider threats and the importance of network hygiene can reduce the likelihood of attackers gaining local network access required to exploit this vulnerability.