app.mail.host = 10.1.1.1
app.mail.debug = true
app.db.username = spate
app.db.password = 1#4FnL&@7!

sed 's/app.db.password="[^"]+/app.db.password="XXXXXXXX"/g' foo.txt

sed 's/app.db.password[ t]*=.*/app.db.password="xxxxx"/