unsupported_extension
unsupported_extension (110) is sent when a TLS extension appears in a handshake message where it is not permitted. TLS 1.3 strictly defines which extensions are allowed in each message type. If a ServerHello contains an extension that is not legal in ServerHello, the client sends unsupported_extension.
Alert Code
110
Level
Fatal
Standard
RFC 8446
Description
TLS 1.3 moved extensions from ServerHello into EncryptedExtensions for most purposes, leaving only a few extensions legal in ServerHello (supported_versions, key_share, pre_shared_key). If a server includes an extension in ServerHello that belongs in EncryptedExtensions, the client sends unsupported_extension. Also sent when a server includes an extension in a Certificate message that is not allowed there. This enforces the TLS 1.3 security principle that fewer handshake fields are visible to network observers.
Common Causes
- –Server included an extension in ServerHello not permitted there in TLS 1.3
- –Extension appears in a Certificate message where it is not allowed
- –TLS 1.3 server incorrectly placing EncryptedExtensions content in ServerHello