The main PayPal web site sets a cookie named “aksession” which contains a blob of base64-encoded ciphertext. This ciphertext is encrypted using a 64-bit block cipher in CBC mode and does not have any other integrity protection. Naturally, this means the aksession cookie is vulnerable to a padding oracle attack allowing full decryption and forgery…. The main PayPal web site sets a cookie named “aksession” which contains a blob of base64-encoded ciphertext. This ciphertext is encrypted using a 64-bit block cipher in CBC mode and does not have any other integrity protection. Naturally, this means the aksession cookie is vulnerable to a padding oracle attack allowing full decryption and forgery.

Read more http://packetstormsecurity.com/files/123079/paypalaksession-fail.txt