Please El Reg these stories make ma rage at breakfast, what's this?
The bug will come as another argument against Systemd as the Linux management tool continues to fight for the hearts and minds of admins and developers alike.
Less against systemd (which should get attacked on the design & implementation level) or against IPv6 than against the use of buffer-overflowable languages in 2018 in code that processes input from the Internet (it's not the middle ages anymore) or at least very hard linting of the same.
But in the end, what did it was a violation of the Don't Repeat Yourself principle and lack of sufficently high-level datastructures. Pointer into buffer, and the remaining buffer length are two discrete variables that need to be updated simultaneously to keep the invariant and this happens in several places. This is just a catastrophe waiting to happen. You forget to update it once, you are out! Use structs and functions updating the structs correctly.
And use assertions in the code, this stuff all seems disturbingly assertion-free.
Excellent explanation by Felix Wilhelm:
The function receives a pointer to the option buffer buf, it's remaining size buflen and the IA to be added to the buffer. While the check at (A) tries to ensure that the buffer has enough space left to store the IA option, it does not take the additional 4 bytes from the DHCP6Option header into account (B). Due to this the memcpy at (C) can go out-of-bound and *buflen can underflow [i.e. you suddenly have a gazillion byte buffer, Ed.] in (D) giving an attacker a very powerful and largely controlled OOB heap write starting at (E).