RFC 2617 does an admirable job of summarizing some of the security risks inherent in HTTP authentication schemes. This section describes some of these risks.
To provide a foolproof system against header tampering, you need either end-to-end encryption or a digital signature of the headers—preferably a combination of both! Digest authentication is focused on providing a tamper-proof authentication scheme, but it does not necessarily extend that protection to the data. The only headers that have some level of protection are WWW-Authenticate and Authorization.
A replay attack, in the current context, is when someone uses a set of snooped authentication credentials from a given transaction for another transaction. While this problem is an issue with GET requests, it is vital that a foolproof method for avoiding replay attacks be available for POST and PUT requests. The ability to successfully replay previously used credentials while transporting form data could cause security nightmares.
Thus, in order for a server to accept "replayed" credentials, the nonce values must be repeated. One of the ways to mitigate this problem is to have the server generate a nonce containing a digest of the client's IP address, a time-stamp, the resource ETag, and a private server key (as recommended earlier). In such a scenario, the combination of an IP address and a short timeout value may provide a huge hurdle for the attacker.
However, this solution ...