We analyze a countermeasure against differential power and electromagnetic attacks that was recently introduced under the name of split mask. We show a general weakness of the split mask countermeasure that makes standard DPA attacks with a full key recovery applicable to masked AES and DES implementations. Complexity of the attacks is the same as for unmasked implementations. We implement the most efficient attack on an 8-bit AVR microcontroller. We also show that the strengthened variant of the countermeasure is susceptible to a second order DPA attack independently of the number of used mask tables.