TLS Alert Codes

unexpected_message

unexpected_message (10) is sent when a TLS record is received that is inappropriate for the current state of the connection – a message that should not have been sent at this point in the handshake or session.

bad_record_mac

bad_record_mac (20) is sent when a TLS record fails its MAC or AEAD authentication check.

record_overflow

record_overflow (22) is sent when a TLS record exceeds the maximum allowed length.

handshake_failure

handshake_failure (40) is sent when the sender cannot negotiate an acceptable set of security parameters for the TLS handshake.

bad_certificate

bad_certificate (42) is sent when a certificate is corrupt, has an invalid signature, or cannot be parsed.

unsupported_certificate

unsupported_certificate (43) is sent when the certificate type is not supported by the receiver.

certificate_revoked

certificate_revoked (44) is sent when a certificate has been revoked by the issuing Certificate Authority.

certificate_expired

certificate_expired (45) is sent when a certificate's validity period has passed – its notAfter date is in the past, or its notBefore date is in the future.

certificate_unknown

certificate_unknown (46) is a catch-all alert for certificate problems not covered by more specific codes.

illegal_parameter

illegal_parameter (47) is sent when a field in the TLS handshake is out of range or inconsistent with other fields.

unknown_ca

unknown_ca (48) is sent when the certificate chain was valid but the root CA is not recognized or trusted.

access_denied

access_denied (49) is sent when a valid certificate was received but access was denied by the server's authorization policy.

decode_error

decode_error (50) is sent when a TLS message cannot be decoded because a field is out of the specified range or the length of the message is incorrect.

decrypt_error

decrypt_error (51) is sent when a cryptographic operation in the handshake fails – specifically when verifying a signature or a Finished message MAC.

protocol_version

protocol_version (70) is sent when the TLS version proposed by the client is not supported by the server.

insufficient_security

insufficient_security (71) is sent when the server requires security parameters stronger than those offered by the client.

internal_error

internal_error (80) is sent when an internal error unrelated to the peer or the correctness of the protocol made it impossible to continue the TLS handshake or session.

inappropriate_fallback

inappropriate_fallback (86) is sent by a server when a client performed a TLS version downgrade using the TLS_FALLBACK_SCSV signaling cipher suite but the downgraded version is lower than the server's highest supported version.

missing_extension

missing_extension (109) is a TLS 1.

unsupported_extension

unsupported_extension (110) is sent when a TLS extension appears in a handshake message where it is not permitted.

bad_certificate_status_response

bad_certificate_status_response (113) is sent when an invalid OCSP response was received via the status_request extension (OCSP stapling).

unknown_psk_identity

unknown_psk_identity (115) is sent in TLS 1.

certificate_required

certificate_required (116) is a TLS 1.

no_application_protocol

no_application_protocol (120) is sent when the client offered ALPN (Application-Layer Protocol Negotiation) extension but none of the offered protocols are supported by the server.

ech_required

ech_required (121) is sent when the server requires Encrypted Client Hello (ECH) but the client did not use it.

close_notify

close_notify (0) signals that the sender will not send any more messages on this connection.

user_canceled

user_canceled (90) is sent to indicate that the handshake is being canceled for reasons unrelated to a protocol failure.

unrecognized_name

unrecognized_name (112) is sent when the server does not recognize the hostname sent in the SNI (Server Name Indication) extension.