Active Directory authentication for SaaS product

If you work on .NET 3.5 or newer, you can use the System.DirectoryServices.AccountManagement namespace and easily verify your credentials:

// create a "principal context" - e.g. your domain (could be machine, too)
using(PrincipalContext pc = new PrincipalContext(ContextType.Domain, "YOURDOMAIN"))
{
    // validate the credentials
    bool isValid = pc.ValidateCredentials("myuser", "mypassword");
}

It's simple, it's reliable, it's 100% C# managed code on your end - what more can you ask for? :-)

Read all about it here:

• Managing Directory Security Principals in the .NET Framework 3.5
• MSDN docs on System.DirectoryServices.AccountManagement

Update:

As outlined in this other SO question (and its answers), there is an issue with this call possibly returning True for old passwords of a user. Just be aware of this behavior and don't be too surprised if this happens :-) (thanks to @MikeGledhill for pointing this out!)

How to handle authentication in a RESTful Client-Server architecture is a matter of debate.

Commonly, it can be achieved, in the SOA over HTTP world via:

• HTTP basic auth over HTTPS;
• Cookies and session management;
• Token in HTTP headers (e.g. OAuth 2.0 + JWT);
• Query Authentication with additional signature parameters.

You'll have to adapt, or even better mix those techniques, to match your software architecture at best.

Each authentication scheme has its own PROs and CONs, depending on the purpose of your security policy and software architecture.

HTTP basic auth over HTTPS

This first solution, based on the standard HTTPS protocol, is used by most web services.

GET /spec.html HTTP/1.1
Host: www.example.org
Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

It's easy to implement, available by default on all browsers, but has some known drawbacks, like the awful authentication window displayed on the Browser, which will persist (there is no LogOut-like feature here), some server-side additional CPU consumption, and the fact that the user-name and password are transmitted (over HTTPS) into the Server (it should be more secure to let the password stay only on the client side, during keyboard entry, and be stored as secure hash on the Server).

We may use Digest Authentication, but it requires also HTTPS, since it is vulnerable to MiM or Replay attacks, and is specific to HTTP.

Session via Cookies

To be honest, a session managed on the Server is not truly Stateless.

One possibility could be to maintain all data within the cookie content. And, by design, the cookie is handled on the Server side (Client, in fact, does even not try to interpret this cookie data: it just hands it back to the server on each successive request). But this cookie data is application state data, so the client should manage it, not the server, in a pure Stateless world.

GET /spec.html HTTP/1.1
Host: www.example.org
Cookie: theme=light; sessionToken=abc123

The cookie technique itself is HTTP-linked, so it's not truly RESTful, which should be protocol-independent, IMHO. It is vulnerable to MiM or Replay attacks.

Granted via Token (OAuth2)

An alternative is to put a token within the HTTP headers so that the request is authenticated. This is what OAuth 2.0 does, for instance. See the RFC 6749:

 GET /resource/1 HTTP/1.1
 Host: example.com
 Authorization: Bearer mF_9.B5f-4.1JqM

In short, this is very similar to a cookie and suffers to the same issues: not stateless, relying on HTTP transmission details, and subject to a lot of security weaknesses - including MiM and Replay - so is to be used only over HTTPS. Typically, a JWT is used as a token.

Query Authentication

Query Authentication consists in signing each RESTful request via some additional parameters on the URI. See this reference article.

It was defined as such in this article:

All REST queries must be authenticated by signing the query parameters sorted in lower-case, alphabetical order using the private credential as the signing token. Signing should occur before URL encoding the query string.

This technique is perhaps the more compatible with a Stateless architecture, and can also be implemented with a light session management (using in-memory sessions instead of DB persistence).

For instance, here is a generic URI sample from the link above:

GET /object?apiKey=Qwerty2010

should be transmitted as such:

GET /object?timestamp=1261496500&apiKey=Qwerty2010&signature=abcdef0123456789

The string being signed is /object?apikey=Qwerty2010&timestamp=1261496500 and the signature is the SHA256 hash of that string using the private component of the API key.

Server-side data caching can be always available. For instance, in our framework, we cache the responses at the SQL level, not at the URI level. So adding this extra parameter doesn't break the cache mechanism.

See this article for some details about RESTful authentication in our client-server ORM/SOA/MVC framework, based on JSON and REST. Since we allow communication not only over HTTP/1.1, but also named pipes or GDI messages (locally), we tried to implement a truly RESTful authentication pattern, and not rely on HTTP specificity (like header or cookies).

Later Note: adding a signature in the URI can be seen as bad practice (since for instance it will appear in the http server logs) so it has to be mitigated, e.g. by a proper TTL to avoid replays. But if your http logs are compromised, you will certainly have bigger security problems.

In practice, the upcoming MAC Tokens Authentication for OAuth 2.0 may be a huge improvement in respect to the "Granted by Token" current scheme. But this is still a work in progress and is tied to HTTP transmission.

Conclusion

It's worth concluding that REST is not only HTTP-based, even if, in practice, it's also mostly implemented over HTTP. REST can use other communication layers. So a RESTful authentication is not just a synonym of HTTP authentication, whatever Google answers. It should even not use the HTTP mechanism at all but shall be abstracted from the communication layer. And if you use HTTP communication, thanks to the Let's Encrypt initiative there is no reason not to use proper HTTPS, which is required in addition to any authentication scheme.