Read all parameters from SYS_CONTEXT/USERENV

There is no way to determine the supported values for the userenv namespace. One way would be to create a table with the supported parameter names.

I guess it might be helpful for you. The query below dumps Oracle USERENV context as a table.

    select res.*
    from (
      select *
      from (
        select
          sys_context ('userenv','ACTION') ACTION,
          sys_context ('userenv','AUDITED_CURSORID') AUDITED_CURSORID,
          sys_context ('userenv','AUTHENTICATED_IDENTITY') AUTHENTICATED_IDENTITY,
          sys_context ('userenv','AUTHENTICATION_DATA') AUTHENTICATION_DATA,
          sys_context ('userenv','AUTHENTICATION_METHOD') AUTHENTICATION_METHOD,
          sys_context ('userenv','BG_JOB_ID') BG_JOB_ID,
          sys_context ('userenv','CLIENT_IDENTIFIER') CLIENT_IDENTIFIER,
          sys_context ('userenv','CLIENT_INFO') CLIENT_INFO,
          sys_context ('userenv','CURRENT_BIND') CURRENT_BIND,
          sys_context ('userenv','CURRENT_EDITION_ID') CURRENT_EDITION_ID,
          sys_context ('userenv','CURRENT_EDITION_NAME') CURRENT_EDITION_NAME,
          sys_context ('userenv','CURRENT_SCHEMA') CURRENT_SCHEMA,
          sys_context ('userenv','CURRENT_SCHEMAID') CURRENT_SCHEMAID,
          sys_context ('userenv','CURRENT_SQL') CURRENT_SQL,
          sys_context ('userenv','CURRENT_SQLn') CURRENT_SQLn,
          sys_context ('userenv','CURRENT_SQL_LENGTH') CURRENT_SQL_LENGTH,
          sys_context ('userenv','CURRENT_USER') CURRENT_USER,
          sys_context ('userenv','CURRENT_USERID') CURRENT_USERID,
          sys_context ('userenv','DATABASE_ROLE') DATABASE_ROLE,
          sys_context ('userenv','DB_DOMAIN') DB_DOMAIN,
          sys_context ('userenv','DB_NAME') DB_NAME,
          sys_context ('userenv','DB_UNIQUE_NAME') DB_UNIQUE_NAME,
          sys_context ('userenv','DBLINK_INFO') DBLINK_INFO,
          sys_context ('userenv','ENTRYID') ENTRYID,
          sys_context ('userenv','ENTERPRISE_IDENTITY') ENTERPRISE_IDENTITY,
          sys_context ('userenv','FG_JOB_ID') FG_JOB_ID,
          sys_context ('userenv','GLOBAL_CONTEXT_MEMORY') GLOBAL_CONTEXT_MEMORY,
          sys_context ('userenv','GLOBAL_UID') GLOBAL_UID,
          sys_context ('userenv','HOST') HOST,
          sys_context ('userenv','IDENTIFICATION_TYPE') IDENTIFICATION_TYPE,
          sys_context ('userenv','INSTANCE') INSTANCE,
          sys_context ('userenv','INSTANCE_NAME') INSTANCE_NAME,
          sys_context ('userenv','IP_ADDRESS') IP_ADDRESS,
          sys_context ('userenv','ISDBA') ISDBA,
          sys_context ('userenv','LANG') LANG,
          sys_context ('userenv','LANGUAGE') LANGUAGE,
          sys_context ('userenv','MODULE') MODULE,
          sys_context ('userenv','NETWORK_PROTOCOL') NETWORK_PROTOCOL,
          sys_context ('userenv','NLS_CALENDAR') NLS_CALENDAR,
          sys_context ('userenv','NLS_CURRENCY') NLS_CURRENCY,
          sys_context ('userenv','NLS_DATE_FORMAT') NLS_DATE_FORMAT,
          sys_context ('userenv','NLS_DATE_LANGUAGE') NLS_DATE_LANGUAGE,
          sys_context ('userenv','NLS_SORT') NLS_SORT,
          sys_context ('userenv','NLS_TERRITORY') NLS_TERRITORY,
          sys_context ('userenv','OS_USER') OS_USER,
          sys_context ('userenv','POLICY_INVOKER') POLICY_INVOKER,
          sys_context ('userenv','PROXY_ENTERPRISE_IDENTITY') PROXY_ENTERPRISE_IDENTITY,
          sys_context ('userenv','PROXY_USER') PROXY_USER,
          sys_context ('userenv','PROXY_USERID') PROXY_USERID,
          sys_context ('userenv','SERVER_HOST') SERVER_HOST,
          sys_context ('userenv','SERVICE_NAME') SERVICE_NAME,
          sys_context ('userenv','SESSION_EDITION_ID') SESSION_EDITION_ID,
          sys_context ('userenv','SESSION_EDITION_NAME') SESSION_EDITION_NAME,
          sys_context ('userenv','SESSION_USER') SESSION_USER,
          sys_context ('userenv','SESSION_USERID') SESSION_USERID,
          sys_context ('userenv','SESSIONID') SESSIONID,
          sys_context ('userenv','SID') SID,
          sys_context ('userenv','STATEMENTID') STATEMENTID,
          sys_context ('userenv','TERMINAL') TERMINAL
        from dual
        -- where sys_context ('userenv','SESSIONID') NOT in ('SYS', 'XDB')    -- <<<<< filter by user
      )
      unpivot include nulls (
        val for name in (action, audited_cursorid, authenticated_identity, authentication_data, authentication_method, bg_job_id, client_identifier, client_info, current_bind, current_edition_id, current_edition_name, current_schema, current_schemaid, current_sql, current_sqln, current_sql_length, current_user, current_userid, database_role, db_domain, db_name, db_unique_name, dblink_info, entryid, enterprise_identity, fg_job_id, global_context_memory, global_uid, host, identification_type, instance, instance_name, ip_address, isdba, lang, language, module, network_protocol, nls_calendar, nls_currency, nls_date_format, nls_date_language, nls_sort, nls_territory, os_user, policy_invoker, proxy_enterprise_identity, proxy_user, proxy_userid, server_host, service_name, session_edition_id, session_edition_name, session_user, session_userid, sessionid, sid, statementid, terminal)
      )
    ) res;