GNU libidn2 before 2.2.0 fails to perform the roundtrip checks specified in RFC3490 Section 4.2 when converting A-labels to U-labels. This makes it possible in some circumstances for one domain to impersonate another. By creating a malicious domain that matches a target domain except for the inclusion of certain punycoded Unicode characters (that would be discarded when converted first to a Unicode label and then back to an ASCII label), arbitrary domains can be impersonated.
https://usn.ubuntu.com/4168-1/
https://security.gentoo.org/glsa/202003-63
https://gitlab.com/libidn/libidn2/merge_requests/71
https://gitlab.com/libidn/libidn2/commit/614117ef6e4c60e1950d742e3edf0a0ef8d389de
https://gitlab.com/libidn/libidn2/commit/241e8f486134793cb0f4a5b0e5817a97883401f5
http://lists.opensuse.org/opensuse-security-announce/2019-12/msg00009.html
http://lists.opensuse.org/opensuse-security-announce/2019-12/msg00008.html