A vulnerability has been discovered in iOS 5.1 and earlier related to validating in-app purchase receipts by connecting to the App Store server directly from an iOS device. An attacker can alter the DNS table to redirect these requests to a server controlled by the attacker. Using a certificate authority controlled by the attacker and installed on the device by the user, the attacker can issue a SSL certificate that fraudulently identifies the attacker's server as an App Store server. When this fraudulent server is asked to validate an invalid receipt, it responds as if the receipt were valid.
iOS 6 will address this vulnerability. If your app follows the best practices described below then it is not affected by this attack.
Apple also provides steps developers can take to circumvent the vulnerability now.
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