Secure connection between client and server

问题:

I'm developing a server component that will serve requests for a embedded client, which is also under my control.

Right now everything is beta and the security works like this:

1. client sends username / password over https.

2. server returns access token.

3. client makes further requests over http with the access token in a custom header.

This is fine for a demo, but it has some problems that need to be fixed before releasing it:

• Anyone can copy a login request, re-send it and get an access token back. As some users replied this is not an issue since it goes over https. My mistake.

• Anyone can listen and get an access key just by inspecting the request headers.

I can think of a symmetric key encryption, with a timestamp so I can reject duplicate requests, but I was wondering if there are some well known good practices for this scenario (that seems a pretty common).

Thanks a lot for the insight.

PS: I'm using Java for the server and the client is coded in C++, just in case.

回答1:

I don't get the first part, If the login request is https, how can anyone just copy it?

Regarding the second part, t This is a pretty standard session hijacking scenario. See this question. Of course you don't have the built-in browser options here, but the basic idea is the same - either send the token only over a secure connection when it matters, or in some way associate the token with the sending device.

In a browser, basically all you have is IP address (which isn't very good), but in your case you may be able to express something specific about your device that you validate against the request to ensure the same token isn't being used from somewhere else.

Edit: You could just be lucky here and be able to rule out the IP address changing behind proxies, and actually use it for this purpose.

But at the end of the day, it is much more secure to use https from a well-known and reviewed library rather than trying to roll your own here. I realize that https is an overhead, but rolling your own has big risks around missing obvious things that an attacker can exploit.

回答2:

First question, just to get it out there: if you're concerned enough about nefarious client-impersonator accesses, why not carry out the entire conversation over HTTPS? Is the minimal performance hit significant enough for this application that it's not worth the added layer of security?

Second, how can someone replay the login request? If I'm not mistaken, that's taking place over HTTPS; if the connection is set up correctly, HTTPS prevents replay attacks using one-time nonces (see here).

回答3:

One of the common recommendations is - use https

https man in the middle attack aside using https for the entire session should be reliable enough. You do not even need to worry about access tokens - https takes care of this for you.

Using http for further requests seems to introduce some vulnerabilities. Now anybody with a network sniffer can intercept your traffic steal the token and spoof your requests. you can build protection to prevent it - token encryption, use once tokens, etc. but in doing so you will be re-creating https.

Going back to the https man in the middle attack - it is based on somebody's ability to insert himself between your server and your client and funnel your requests through their code. It is all doable i.e. in case the attacker has access to the physical network. The problem such attacker will face is that he will not be able to give you a proper digital certificat - he does not have the private key you used to sign it. When https is accessed through a browser, the browser gives you a warning but still can let you through to the page.

In your case it is your client who will communicate with the server. And you can make sure that all proper validations of the certificate are in place. If you do that you should be fine

Edit

Seconding Yishai - yes some overhead is involved, primarily CPU, but if this additional overhead pushes your server over board, you have bigger problems with your app