They are licensed either under the Mozilla Public License v2.0 (MPLv2) or are in the public domain (CC0 license). The CC0 licensed plugins can be reused under any license and changes do not have to be published.

Here is an overview of the licenses for all plugins:

Plugin	License
ua_accesscontrol_default	CC0
ua_config_default	CC0
ua_config_json	CC0
ua_debug_dump_pkgs	CC0
ua_log_stdout	CC0
ua_log_syslog	CC0
ua_nodestore_hashmap	CC0
ua_nodestore_ziptree	CC0
crypto/mbedtls/securitypolicy_common	CC0
crypto/mbedtls/certificategroup	CC0
crypto/ua_certificategroup_none	CC0
crypto/openssl/certificategroup	CC0
crypto/ua_securitypolicy_none	CC0
crypto/ua_filestore_common	MPLv2
crypto/ua_securitypolicy_filestore	MPLv2
crypto/ua_certificategroup_filestore	MPLv2
crypto/mbedtls/create_certificate	MPLv2
crypto/mbedtls/securitypolicy_aes128sha256rsaoaep	MPLv2
crypto/mbedtls/securitypolicy_aes256sha256rsapss	MPLv2
crypto/mbedtls/securitypolicy_basic128rsa15	MPLv2
crypto/mbedtls/securitypolicy_basic256	MPLv2
crypto/mbedtls/securitypolicy_basic256sha256	MPLv2
crypto/mbedtls/securitypolicy_pubsub_aes128ctr	MPLv2
crypto/mbedtls/securitypolicy_pubsub_aes256ctr	MPLv2
crypto/openssl/create_certificate	MPLv2
crypto/openssl/securitypolicy_aes128sha256rsaoaep	MPLv2
crypto/openssl/securitypolicy_aes256sha256rsapss	MPLv2
crypto/openssl/securitypolicy_basic128rsa15	MPLv2
crypto/openssl/securitypolicy_basic256	MPLv2
crypto/openssl/securitypolicy_basic256sha256	MPLv2
crypto/openssl/securitypolicy_common	MPLv2
crypto/openssl/securitypolicy_eccnistp256	MPLv2
crypto/pkcs11/securitypolicy_pubsub_aes128ctr_tpm	MPLv2
crypto/pkcs11/securitypolicy_pubsub_aes256ctr_tpm	MPLv2
historydata/ua_history_data_backend_memory	MPLv2
historydata/ua_history_data_gathering_default	MPLv2
historydata/ua_history_database_default	MPLv2