CVE-2026-20405 is a vulnerability classified under CWE-617 (Reachable Assertion) found in the modem firmware of a broad range of MediaTek chipsets, including MT2735 through MT8893 series. The root cause is a missing bounds check in the modem software, which leads to a reachable assertion failure. When a user equipment (UE) device equipped with one of these affected chipsets connects to a rogue base station controlled by an attacker, the attacker can remotely trigger a system crash, causing a denial of service (DoS). This attack vector does not require any user interaction or elevated privileges, making it easier to exploit in scenarios where the attacker can simulate or control a base station environment. The affected modem versions include NR15, NR16, NR17, and NR17R, which are commonly integrated into mobile devices and IoT equipment. The vulnerability impacts the availability of the device by causing crashes, but does not affect confidentiality or integrity. The CVSS v3.1 score is 6.5, indicating a medium severity level, with an attack vector of adjacent network (AV:A), low attack complexity (AC:L), no privileges required (PR:N), and no user interaction (UI:N). No known exploits have been reported in the wild as of the publication date. The vendor has assigned a patch ID (MOLY01688495) and issue ID (MSV-4818), but no public patch links are currently available. This vulnerability highlights the risks posed by rogue base stations and the importance of robust modem firmware validation and bounds checking.

For European organizations, the primary impact of CVE-2026-20405 is the potential for remote denial of service on devices using affected MediaTek modems. This can disrupt mobile communications, impacting business operations, especially in sectors reliant on mobile connectivity such as telecommunications providers, emergency services, transportation, and critical infrastructure. The vulnerability could be exploited in targeted attacks where adversaries deploy rogue base stations to cause service outages or degrade network reliability. Although it does not compromise data confidentiality or integrity, the availability impact can lead to operational downtime and loss of productivity. Enterprises deploying IoT devices or mobile endpoints with these chipsets may experience device instability or failure, complicating incident response and recovery. The lack of user interaction and no privilege requirements lower the barrier for exploitation in environments where attackers can position rogue base stations, such as public spaces or near corporate premises. The absence of known exploits in the wild reduces immediate risk but does not eliminate the threat, especially as rogue base station technology becomes more accessible.

1. Apply vendor-provided patches immediately once available to address the missing bounds check in the modem firmware. 2. Implement network monitoring solutions capable of detecting and alerting on rogue base station activity, including unusual base station identifiers or signal characteristics. 3. Employ mobile device management (MDM) policies to restrict connections to trusted networks and enforce firmware updates on devices using affected chipsets. 4. Use endpoint security solutions that can monitor modem behavior and detect abnormal crashes or resets indicative of exploitation attempts. 5. Educate users and administrators about the risks of rogue base stations and encourage reporting of suspicious network behavior. 6. Collaborate with telecom providers to enhance detection and mitigation of unauthorized base stations within organizational vicinities. 7. For critical infrastructure, consider deploying redundant communication paths or failover mechanisms to maintain availability in case of targeted DoS attacks. 8. Regularly audit and inventory devices to identify those using vulnerable MediaTek chipsets and prioritize them for remediation.