Contiki-NG is an open-source, cross-platform operating system for Next-Generation IoT devices. The low-power IPv6 network stack of Contiki-NG has a buffer module (os/net/ipv6/uipbuf.c) that processes IPv6 extension headers in incoming data packets. As part of this processing, the function uipbuf_get_next_header casts a pointer to a uip_ext_hdr structure into the packet buffer at different offsets where extension headers are expected to be found, and then reads from this structure. Because of a lack of bounds checking, the casting can be done so that the structure extends beyond the packet's end. Hence, with a carefully crafted packet, it is possible to cause the Contiki-NG system to read data outside the packet buffer. A patch that fixes the vulnerability is included in Contiki-NG 4.8.