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7.2

Coturn: IPv4-mapped IPv6 bypasses blocked IP restrictions

CVE-2026-27624
Summary

Coturn, a popular TURN and STUN server, has a security flaw that allows attackers to bypass IP restrictions. This means that even if you've blocked certain IP addresses, an attacker can still send traffic to your server by using a specific type of IPv6 address. To protect yourself, update Coturn to version 4.9.0 or later, or configure your server to block IPv4-mapped IPv6 addresses explicitly.

Original title
Coturn: IPv4-mapped IPv6 (::ffff:0:0/96) bypasses denied-peer-ip ACL
Original description
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/ns_turn_ioaddr.c" do not check "IN6_IS_ADDR_V4MAPPED". "ioa_addr_is_loopback()" checks "127.x.x.x" (AF_INET) and "::1" (AF_INET6), but not "::ffff:127.0.0.1." "ioa_addr_is_zero()" checks "0.0.0.0" and "::", but not "::ffff:0.0.0.0." "addr_less_eq()" used by "ioa_addr_in_range()" for "denied-peer-ip" matching: when the range is AF_INET and the peer is AF_INET6, 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.
osv CVSS3.1 7.2
Vulnerability type
CWE-284 Improper Access Control
CWE-441
Published: 25 Feb 2026 · Updated: 13 Mar 2026 · First seen: 9 Mar 2026