Test case

recreate table SQL_SOURCE(
    SQL_SOURCE varchar(32000)
);
commit;

insert into SQL_SOURCE values(
'execute block as
declare variable SQL type of column SQL_SOURCE.SQL_SOURCE;
begin
    select first(1) SQL_SOURCE from SQL_SOURCE into :SQL;
    execute statement :SQL
    -- YOUR DB
    on external ''127.0.0.1:S:\X.fdb''
    as user ''SYSDBA'' password ''masterkey'';
end'
);
commit;

execute block as
declare variable SQL type of column SQL_SOURCE.SQL_SOURCE;
begin
    select first(1) SQL_SOURCE from SQL_SOURCE into :SQL;
    execute statement :SQL;
end


SS and SC hungs, CS creates a lot of processes.

Yes, test case looks crazy, but recursion must be limited somehow.