This chapter discusses possible security vulnerabilities Keycloak could have, how Keycloak mitigates those vulnerabilities, and what steps you need to do to configure Keycloak to mitigate some vulnerabilities. A good list of potential vulnerabilities and what security implementations should do to mitigate them can be found in the OAuth 2.0 Thread Model document put out by the IETF. Many of those vulnerabilities are discussed here.

If you do not use SSL/HTTPS for all communication between the Keycloak auth server and the clients it secures you will be very vulnerable to man in the middle attacks. OAuth 2.0/OpenID Connect uses access tokens for security. Without SSL/HTTPS, attackers can sniff your network and obtain an access token. Once they have an access token they can do any operation that the token has been given permission for. Keycloak has three modes for SSL/HTTPS. SSL can be hard to set up, so out of the box, Keycloak allows non-HTTPS communication over private IP addresses like localhost, 192.168.x.x, and other private IP addresses. In production, you should make sure SSL is enabled and required across the board. On the adapter/client side, Keycloak allows you to turn off the SSL trust manager. The trust manager ensures identity the client is talking to. It checks the DNS domain name against the server's certificate. In production you should make sure that each of your client adapters is configured to use a truststore. Otherwise you are vulnerable to DNS man in the middle attacks.

Cross-site request forgery (CSRF) is a web-based attack whereby HTTP requests are transmitted from a user that the web site trusts or has authenticated (e.g., via HTTP redirects or HTML forms). Any site that uses cookie based authentication is vulnerable for these types of attacks. These attacks are mitigated by matching a state cookie against a posted form or query parameter. OAuth 2.0 login specification requires that a state cookie be used and matched against a transmitted state parameter. Keycloak fully implements this part of the specification so all logins are protected. The Keycloak adminstration console is a pure Javascript/HTML5 application that makes REST calls to the backend Keycloak admin API. These calls all require bearer token authentication and are made via Javascript Ajax calls. CSRF does not apply here. The admin REST API can also be configured to validate CORS origins as well. The only part of Keycloak that really falls into CSRF is the user account management pages. To mitigate this Keycloak sets a state cookie and also embeds the value of this state cookie within hidden form fields or query parameters in action links. This query or form parameter is checked against the state cookie to verify that the call was made by the user.

With clickjacking, a malicious site loads the target site in a transparent iFrame overlaid on top of a set of dummy buttons that are carefully constructed to be placed directly under important buttons on the target site. When a user clicks a visible button, they are actually clicking a button (such as an "Authorize" button) on the hidden page. An attacker can steal a user's authentication credentials and access their resources. By default, every response by Keycloak sets some specific browser headers that can prevent this from happening specifically X-FRAME_OPTIONS and Content-Security-Policy. You should take a look at both of these headers. In the admin console you can specify the values these headers will have. By default, Keycloak only sets up a same-origin policy for iframes.

It would be very hard for an attacker to compromise Keycloak access codes. Keycloak generates a cryptographically strong random value for its access codes so it would be very hard to guess an access token. An access code can only be turned into an access token once so it can't be replayed. In the admin console you can specify how long an access token is valid for. This value should be really short. Like a seconds. Just long enough for the client to make the request to turn the code into an token.

There's a few things you can do to mitigate access tokens and refresh tokens from being stolen. Most importantly is to enforce SSL/HTTPS communication between Keycloak and its clients and applications. Short lifespans (minutes) for access tokens allows Keycloak to check the validity of a refresh token. Making sure refresh tokens always stay private to the client and are never transmitted ever is very important as well. If an access token or refresh token is compromised, the first thing you should do is go to the admin console and push a not-before revocation policy to all applications. This will enforce that any tokens issued prior to that date are now invalid. You can also disable specific applications, clients, and users if you feel that any one of those entities is completely compromised.

An attacker could use the end-user authorization endpoint and the redirect URI parameter to abuse the authorization server as an open redirector. An open redirector is an endpoint using a parameter to automatically redirect a user agent to the location specified by the parameter value without any validation. An attacker could utilize a user's trust in an authorization server to launch a phishing attack. Keycloak requires that all registered applications and clients register at least one redirection uri pattern. Any time a client asks Keycloak to perform a redirect (on login or logout for example), Keycloak will check the redirect uri vs. the list of valid registered uri patterns. It is important that clients and applications register as specific a URI pattern as possible to mitigate open redirector attacks.

A brute force attack happens when an attacker is trying to guess a user's password. Keycloak has some limited brute force detection capabilities. If turned on, a user account will be temporarily disabled if a threshold of login failures is reached. The downside of this is that this makes Keycloak vulnerable to denial of service attacks. Eventually we will expand this functionality to take client IP address into account when deciding whether to block a user. Another thing you can do to prevent password guessing is to point a tool like Fail2Ban to the Keycloak server's log file. Keycloak logs every login failure and client IP address that had the failure. In the admin console, per realm, you can set up a password policy to enforce that users pick hard to guess passwords. Finally, the best way to mitigate against brute force attacks is to require user to set up a one-time-password (OTP).

Keycloak does not store passwords in raw text. It stores a hash of them. Because of performance reasons, Keycloak only hashes passwords once. While a human could probably never crack a hashed password, it is very possible that a computer could. The security community suggests around 20,000 (yes thousand!) hashing iterations to be done to each password. This number grows every year due to increasing computing power (It was 1000 12 years ago). The problem with this is that password hashing is a huge performance hit as each login would require the entered password to be hashed that many times and compared to the stored hash. So, its up to the admin to configure the password hash iterations. This can be done in the admin console password policy configuration. Again, the default value is 1 as we thought it might be more important for Keycloak to scale out of the box. There's a lot of other measures admins can do to protect their password databases.

At this point in time, there is no knowledge of any SQL injection vulnerabilities in Keycloak

Using the Scope menu in the admin console for oauth clients or applications, you can control exactly which role mappings will be included within the token sent back to the client or application. This allows you to limit the scope of permissions given to the application or client which is great if the client isn't very trusted and is known to not being very careful with its tokens.