missing_extension
missing_extension (109) is a TLS 1.3 alert sent when a mandatory extension was not included in a handshake message. TLS 1.3 requires specific extensions in ClientHello (like supported_versions and key_share). If these are absent, the receiver sends missing_extension.
Alert Code
109
Level
Fatal
Standard
RFC 8446
Description
TLS 1.3 introduced strict requirements for which extensions must be present in each handshake message. RFC 8446 §9.2 lists mandatory extensions for ClientHello: supported_versions, supported_groups, and signature_algorithms when certain features are used. A TLS 1.3 server receiving a ClientHello without the supported_versions extension sends missing_extension. This primarily occurs when a TLS 1.3 server receives a ClientHello from a library that claims to support TLS 1.3 but does not correctly implement its mandatory extensions.
Common Causes
- –ClientHello missing the supported_versions extension (required for TLS 1.3)
- –ClientHello missing key_share extension when needed
- –TLS 1.3 implementation sending incomplete handshake messages
- –Partial TLS 1.3 implementation that omits required extensions