CVE-2025-20666 is a high-severity vulnerability identified in multiple MediaTek modem chipsets, including models MT2735, MT6833, MT6853, MT6873, MT6880, MT6890, MT8666, MT8673, MT8771, MT8791, and others. The vulnerability is classified under CWE-617 (Reachable Assertion), indicating that an assertion in the modem firmware can be triggered improperly, leading to an uncaught exception and subsequent system crash. This flaw exists in the modem's NR15 version and can be exploited remotely without requiring any user interaction or prior authentication. The attack vector involves a user equipment (UE) device connecting to a rogue base station controlled by an attacker. Upon connection, the attacker can send specially crafted signals or messages that trigger the assertion failure, causing the modem to crash and resulting in a denial of service (DoS) condition. The CVSS v3.1 base score is 7.5, reflecting the vulnerability's high impact on availability with no impact on confidentiality or integrity. The vulnerability does not grant additional execution privileges but disrupts normal modem operation, potentially causing service outages or device reboots. No known exploits are currently reported in the wild, and MediaTek has assigned a patch ID (MOLY00650610) to address the issue. The vulnerability affects a broad range of MediaTek modem chipsets widely used in mobile devices, IoT equipment, and embedded systems, making it a significant concern for network reliability and device stability.

For European organizations, the primary impact of CVE-2025-20666 is the potential for remote denial of service on devices using affected MediaTek modems. This can disrupt critical communications, especially in sectors relying on mobile connectivity such as telecommunications providers, emergency services, transportation, and industrial IoT deployments. The vulnerability could be exploited by attackers deploying rogue base stations in proximity to target devices, causing widespread service interruptions without requiring device user interaction. This poses risks to operational continuity, particularly for enterprises with remote or mobile workforces and for organizations deploying 5G or LTE-based IoT infrastructure. Additionally, denial of service in communication devices may indirectly affect data availability and business processes dependent on network connectivity. While no direct data breach or privilege escalation is involved, the disruption of service could lead to financial losses, reputational damage, and safety concerns in critical infrastructure sectors.

To mitigate CVE-2025-20666, European organizations should: 1) Ensure all devices using affected MediaTek modems are updated promptly with the official patch (MOLY00650610) provided by MediaTek or device manufacturers. 2) Implement network monitoring to detect and alert on the presence of rogue base stations or suspicious radio signals indicative of attack attempts. 3) Employ mobile device management (MDM) solutions to enforce firmware updates and maintain device security hygiene. 4) For critical infrastructure, consider deploying network-level protections such as base station authentication and anomaly detection systems to prevent unauthorized base station connections. 5) Educate IT and security teams about the threat vector to recognize symptoms of modem crashes and service disruptions. 6) Collaborate with telecom providers to ensure network-level mitigations and rapid incident response capabilities. 7) For IoT deployments, isolate vulnerable devices in segmented network zones to limit impact scope. These steps go beyond generic patching by emphasizing detection, network defense, and operational preparedness.