Is ActiveRecord's "order" method vulnerable to SQL injection?

Yes, ActiveRecord's “order” method is vulnerable to SQL injection.

No, it is not safe to use interpolated strings when calling .order.

The above answers to my question have been confirmed by Aaron Patterson, who pointed me to http://rails-sqli.org/#order . From that page:

Taking advantage of SQL injection in ORDER BY clauses is tricky, but a CASE statement can be used to test other fields, switching the sort column for true or false. While it can take many queries, an attacker can determine the value of the field.

Therefore it's important to manually check anything going to order is safe; perhaps by using methods similar to @dmcnally's suggestions.

Thanks all.

Short answer is you need to sanitize your inputs.

If the strings you are planning to interpolate come from an untrusted source (e.g. web browser) then you need to first map them to trusted values. You could do this via a hash:

# Mappings from known values to SQL
order_mappings = {
  'first_name_asc'  => 'first_name ASC',
  'first_name_desc' => 'first_name DESC',
  'last_name_asc'   => 'last_name ASC',
  'last_name_desc'  => 'last_name DESC',
}

# Ordering options passed in as an array from some source:
order_options = ['last_name_asc', 'first_name_asc']

# Map them to the correct SQL:
order = order_options.map{|o| order_mappings[o] }.compact.join(', ')
Client.order(order)