A DHCP server, or anyone on the LAN who answers a DISCOVER first, can make the client read about a kilobyte past the end of the received message. The option walk keeps a pointer and an offset in step, and the only bound check uses the offset: A TLV option occupies size + 2 bytes. "data" is advanced by size + 2 in total, "i" by size + 1, so the offset falls one byte behind the real read position for every option the walk skips. After enough skipped options the check "i < length - 1" still holds while "data" is already past the end of the message, and the subsequent read of the type and length bytes comes from whatever follows. A single OFFER carrying a long run of skippable options is enough: A well formed OFFER through the same path is handled normally, the client records the offer and moves to REQUESTING, so the difference is the option layout rather than the harness. The read runs in the DHCP client thread while the client is still unconfigured, so it happens on every boot in reach of a hostile DHCP responder. The values read are used to configure the interface, which is how the disclosed bytes become observable. Advance "i" by size + 2, or derive the bound from "data" rather than keeping a second counter.