libcurl's URL API function [curl url get()](https://curl.se/libcurl/c/curl url get.html) offers punycode conversions, to and from IDN. Asking to convert a name that is exactly 256 bytes, libcurl ends up reading outside of a stack based buffer when built to use the macidn IDN backend. The conversion function then fills up the provided buffer exactly - but does not null terminate the string.

This flaw can lead to stack contents accidently getting returned as part of the converted string.