[exim] SQL Injection

Hi,

I've noticed that SQL Injection is possible when using a SQL Query for
SMTP AUTH.
By having
server_condition = ${if crypteq {$3}{${lookup pgsql {SELECT password FROM users WHERE username='$2'}}}{yes}{no}}
in the authenticator it was possible for me to execute a bad SQL query
by sending this username:
test'; INSERT INTO valid_email_addresses VALUES ('adrian', 'evil@???'); SELECT '

Is there a way to prevent this except by disabling write access for
ths database user (which is certainly not a way to circumvent sql
injection problems). Maybe there exists a function to escape the
arguments before they are inserted in the query?

--
Adrian