Avoid using cookie data for a security-related decision.
Perform thorough input validation (i.e.: server side validation) on the cookie data if you're going to use it for a security related decision.
Add integrity checks to detect tampering.
Protect critical cookies from replay attacks, since cross-site scripting or other attacks may allow attackers to steal a strongly-encrypted cookie that also passes integrity checks. This mitigation applies to cookies that should only be valid during a single transaction or session. By enforcing timeouts, you may limit the scope of an attack. As part of your integrity check, use an unpredictable, server-side value that is not exposed to the client.