An integer underflow (CWE-191) exists in the DHCPv6 sub-option parser of Amazon FreeRTOS-Plus-TCP prior to versions 4.4.1 and 4.2.6. Exploitation by an adjacent network actor sending a crafted DHCPv6 packet can corrupt device IPv6 address assignment, DNS settings, and lease times, and cause a denial of service by permanently freezing the IP task, necessitating a hardware reset. The vulnerability affects devices with DHCPv6 enabled. The vendor advisory recommends upgrading to fixed versions 4.2.6 or 4.4.1 or later.

The vulnerability allows an adjacent network attacker to disrupt IPv6 network configuration on affected devices, leading to corrupted IP address assignment, DNS configuration, and lease times. It also enables denial of service by causing the IP task to freeze permanently, requiring a hardware reset to recover. There is no indication of confidentiality impact. The CVSS 3.1 base score is 8.1 (High), reflecting high impact on integrity and availability with low attack complexity and no required privileges or user interaction.

Users should upgrade Amazon FreeRTOS-Plus-TCP to version 4.2.6, 4.4.1, or newer as recommended by the AWS security advisory (https://aws.amazon.com/security/security-bulletins/2026-022-aws/). This is the official fix addressing the integer underflow vulnerability. No other mitigations are indicated or required once the upgrade is applied.