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.
From Advisory: AFFECTED VERSIONS The vulnerable code can only be reached when curl is built to use macidn, the native IDN conversion library bundled with Apple's operating systems: macOS, iOS, ipadOS etc. Builds using other IDN backends are not vulnerable. Affected version: curl 8.8.0 Not affected versions: curl < 8.8.0 and >= 8.9.0 Introduced-in: https://github.com/curl/curl/commit/add22feeef07858307be57