Question

I’ve heard that exposing database IDs (in URLs, for example) is a security risk, but I’m having trouble understanding why.

Any opinions or links on why it’s a risk, or why it isn’t?

EDIT: of course the access is scoped, e.g. if you can’t see resource foo?id=123 you’ll get an error page. Otherwise the URL itself should be secret.

EDIT: if the URL is secret, it will probably contain a generated token that has a limited lifetime, e.g. valid for 1 hour and can only be used once.

EDIT (months later): my current preferred practice for this is to use UUIDS for IDs and expose them. If I’m using sequential numbers (usually for performance on some DBs) as IDs I like generating a UUID token for each entry as an alternate key, and expose that.

Answer 1

There are risks associated with exposing database identifiers. On the other hand, it would be extremely burdensome to design a web application without exposing them at all. Thus, it’s important to understand the risks and take care to address them.

The first danger is what OWASP called "insecure direct object references." If someone discovers the id of an entity, and your application lacks sufficient authorization controls to prevent it, they can do things that you didn’t intend.

Here are some good rules to follow:

1. Use role-based security to control access to an operation. How this is done depends on the platform and framework you’ve chosen, but many support a declarative security model that will automatically redirect browsers to an authentication step when an action requires some authority.
2. Use programmatic security to control access to an object. This is harder to do at a framework level. More often, it is something you have to write into your code and is therefore more error prone. This check goes beyond role-based checking by ensuring not only that the user has authority for the operation, but also has necessary rights on the specific object being modified. In a role-based system, it’s easy to check that only managers can give raises, but beyond that, you need to make sure that the employee belongs to the particular manager’s department.

There are schemes to hide the real identifier from an end user (e.g., map between the real identifier and a temporary, user-specific identifier on the server), but I would argue that this is a form of security by obscurity. I want to focus on keeping real cryptographic secrets, not trying to conceal application data. In a web context, it also runs counter to widely used REST design, where identifiers commonly show up in URLs to address a resource, which is subject to access control.

Another challenge is prediction or discovery of the identifiers. The easiest way for an attacker to discover an unauthorized object is to guess it from a numbering sequence. The following guidelines can help mitigate that:

3. Expose only unpredictable identifiers. For the sake of performance, you might use sequence numbers in foreign key relationships inside the database, but any entity you want to reference from the web application should also have an unpredictable surrogate identifier. This is the only one that should ever be exposed to the client. Using random UUIDs for these is a practical solution for assigning these surrogate keys, even though they aren’t cryptographically secure.

4. One place where cryptographically unpredictable identifiers is a necessity, however, is in session IDs or other authentication tokens, where the ID itself authenticates a request. These should be generated by a cryptographic RNG.