The option is usually European, exercisable only at one date in the future at a specific strike price defined as a coupon on the credit default swap.
Therefore, buying a payer option is not a good protection against an actual default, only against a rise in the credit spread.
They may also feature quite high implied volatilities, as shown by Damiano Brigo (2005).
This is expressed at times by stating that the options offer "front-end protection".
Proper inclusion of front end protection complicates index options valuation, see for example Claus M. Pedersen (2003), or Brigo and Morini (2008).