CVE-2026-20403 is a vulnerability classified as CWE-787 (out-of-bounds write) found in the modem component of MediaTek chipsets. The flaw arises due to a missing bounds check in the modem firmware, which can be triggered remotely when a user equipment (UE) device connects to a maliciously controlled rogue base station. This out-of-bounds write can cause a system crash, resulting in a denial of service condition on the affected device. Exploitation does not require any privileges or user interaction, making it relatively easy to trigger once the device is connected to the attacker's base station. The affected chipsets span a broad range of MediaTek models, including MT2735 through MT8893 series, covering many modern mobile devices. The vulnerability primarily impacts device availability, as it leads to system crashes without affecting confidentiality or integrity. MediaTek has released patches identified as MOLY01689254 and MOLY01689259 for different platform versions (N15, NR16, NR17, NR17R). No public exploits have been reported yet, but the potential for remote DoS attacks exists, especially in environments where rogue base stations can be deployed. The CVSS v3.1 score of 6.5 reflects a medium severity rating, with attack vector as adjacent network, low attack complexity, no privileges required, no user interaction, and impact limited to availability.

The primary impact of CVE-2026-20403 is the potential for remote denial of service on devices using affected MediaTek chipsets. This can disrupt mobile device availability, causing crashes and requiring device reboots, which may lead to service interruptions for end users. For organizations, this can translate into operational disruptions, especially for enterprises relying on mobile devices for critical communications or IoT deployments using MediaTek hardware. Attackers controlling rogue base stations could target specific users or groups, potentially causing localized outages or degrading service quality. While confidentiality and integrity are not directly impacted, the loss of availability can affect business continuity, emergency communications, and user productivity. The ease of exploitation without user interaction or privileges increases the risk in environments where attackers can deploy or simulate rogue base stations, such as public spaces or unsecured networks. The broad range of affected chipsets means many consumer and enterprise devices globally could be vulnerable until patched.

1. Deploy the official patches released by MediaTek (MOLY01689254 for N15 and NR16 platforms, MOLY01689259 for NR17 and NR17R platforms) as soon as possible to remediate the vulnerability. 2. Implement network monitoring to detect and block rogue base stations or suspicious cellular network activity, using specialized detection tools or services. 3. Encourage users to avoid connecting to untrusted or unknown cellular networks, especially in high-risk or public areas. 4. For enterprise environments, consider mobile device management (MDM) solutions that can enforce security policies and monitor device connectivity. 5. Collaborate with mobile network operators to identify and mitigate rogue base station threats in critical regions. 6. Maintain up-to-date firmware and software on all devices using MediaTek chipsets to reduce exposure to known vulnerabilities. 7. Educate users about the risks of connecting to unauthorized cellular networks and the importance of applying security updates promptly.