radvd is a router advertisement daemon for IPv6. Prior to version 2.21, the `radvdump` utility shipped with radvd contains a stack buffer overflow in the Route Information option parser. When processing a crafted ICMPv6 Router Advertisement, `print_ff()` copies up to 2032 bytes from attacker-controlled packet data into a 16-byte `struct in6_addr` on the stack, overflowing by up to 2016 bytes. Note that the main `radvd` daemon is not affected by the vulnerability. Version 2.21 patches the issue.