CVE-2026-27624: Coturn: IPv4-mapped IPv6 (::ffff:0:0/96) bypasses denied-peer-ip ACL

Coturn is a free open source implementation of TURN and STUN Server. Coturn is commonly configured to block loopback and internal ranges using "denied-peer-ip" and/or default loopback restrictions. CVE-2020-26262 addressed bypasses involving "0.0.0.0", "[::1]" and "[::]", but IPv4-mapped IPv6 is not covered. When sending a "CreatePermission" or "ChannelBind" request with the "XOR-PEER-ADDRESS" value of "::ffff:127.0.0.1", a successful response is received, even though "127.0.0.0/8" is blocked via "denied-peer-ip". The root cause is that, prior to the updated fix implemented in version 4.9.0, three functions in "src/client/nsturnioaddr.c" do not check "IN6ISADDRV4MAPPED". "ioaaddrisloopback()" checks "127.x.x.x" (AFINET) and "::1" (AFINET6), but not "::ffff:127.0.0.1." "ioaaddriszero()" checks "0.0.0.0" and "::", but not "::ffff:0.0.0.0." "addrlesseq()" used by "ioaaddrinrange()" for "denied-peer-ip" matching: when the range is AFINET and the peer is AFINET6, the comparison returns 0 without extracting the embedded IPv4. Version 4.9.0 contains an updated fix to address the bypass of the fix for CVE-2020-26262.