RIOT is an open-source microcontroller operating system, designed to match the requirements of Internet of Things (IoT) devices and other embedded devices. A vulnerability was discovered in the IPv6 fragmentation reassembly implementation of RIOT OS v2025.07. When copying the contents of the first fragment (offset=0) into the reassembly buffer, no size check is performed. It is possible to force the creation of a small reassembly buffer by first sending a shorter fragment (also with offset=0). Overflowing the reassembly buffer corrupts the state of other packet buffers which an attacker might be able to used to achieve further memory corruption (potentially resulting in remote code execution). To trigger the vulnerability, the `gnrc_ipv6_ext_frag` module must be included and the attacker must be able to send arbitrary IPv6 packets to the victim. Version 2025.10 fixes the issue.