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.

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-12290
• https://gitlab.com/libidn/libidn2/commit/241e8f486134793cb0f4a5b0e5817a97883401f5
• https://gitlab.com/libidn/libidn2/commit/614117ef6e4c60e1950d742e3edf0a0ef8d389de
• https://gitlab.com/libidn/libidn2/merge_requests/71
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/3UFT76Y7OSGPZV3EBEHD6ISVUM3DLARM
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/KXDKYWFV6N2HHVSE67FFDM7G3FEL2ZNE
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ONG3GJRRJO35COPGVJXXSZLU4J5Y42AT
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/RSI4TI2JTQWQ3YEUX5X36GTVGKO4QKZ5
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/U6ZXL2RDNQRAHCMKWPOMJFKYJ344X4HL
• https://security.gentoo.org/glsa/202003-63
• https://usn.ubuntu.com/4168-1
• http://lists.opensuse.org/opensuse-security-announce/2019-12/msg00008.html
• http://lists.opensuse.org/opensuse-security-announce/2019-12/msg00009.html