HttpWebRequest losing cookies

I have a client application that is communicating with an ASP.NET web service using cookie-based authentication. (the clients call a login method which sets a persistent cookie that is then re-used across requests within the logon session).

This has been working fine for some time, but I have started getting error reports from a few machines used by a new client that seem to indicate that the cookie has not been successfully roundtripped. (Login requests are all successful, but all subsequent requests fail with a 302-redirect to the logon resource)

I am using a CookieContainer which I manually attach to each HttpWebRequest I am using to ensure that the cookies are preserved across every requ开发者_如何学编程est.

Is it possible that there is some "security" software on these machines that is intercepting/blocking the cookie transmission? (I am using SSL). If so, is there anything that can be done to tell what is getting in the way?

It's highly unlike that security software can even see inside your packet if you're using SSL. SSL data should be encrypted even before they get into packet form; generally they are encrypted even before using the send() to the socket. Unless you have some awesome security software that has broken SSL encryption and can look inside the packet, this shouldn't be possible.

Are the same machines failing every time? Or are some machines failing randomly at times, and others failing at other times? If it's the latter, maybe there's something going on on the server, not the clients.