Exposing database IDs - security risk?

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.

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.

While not a data security risk this is absolutely a business intelligence security risk as it exposes both data size and velocity. I've seen businesses get harmed by this and have written about this anti-pattern in depth. Unless you're just building an experiment and not a business I'd highly suggest keeping your private ids out of public eye. https://medium.com/lightrail/prevent-business-intelligence-leaks-by-using-uuids-instead-of-database-ids-on-urls-and-in-apis-17f15669fd2e

It depends on what the IDs stand for.

Consider a site that for competitive reason don't want to make public how many members they have but by using sequential IDs reveals it anyway in the URL: http://some.domain.name/user?id=3933

On the other hand, if they used the login name of the user instead: http://some.domain.name/user?id=some they haven't disclosed anything the user didn't already know.

The general thought goes along these lines: "Disclose as little information about the inner workings of your app to anyone."

Exposing the database ID counts as disclosing some information.

Reasons for this is that hackers can use any information about your apps inner workings to attack you, or a user can change the URL to get into a database he/she isn't suppose to see?

We use GUIDs for database ids. Leaking them is a lot less dangerous.

If you are using integer IDs in your db, you may make it easy for users to see data they shouldn't by changing qs variables.

E.g. a user could easily change the id parameter in this qs and see/modify data they shouldn't http://someurl?id=1

When you send database id's to your client you are forced to check security in both cases. If you keep the id's in your web session you can choose if you want/need to do it, meaning potentially less processing.

You are constantly trying to delegate things to your access control ;) This may be the case in your application but I have never seen such a consistent back-end system in my entire career. Most of them have security models that were designed for non-web usage and some have had additional roles added posthumously, and some of these have been bolted on outside of the core security model (because the role was added in a different operational context, say before the web).

So we use synthetic session local id's because it hides as much as we can get away with.

There is also the issue of non-integer key fields, which may be the case for enumerated values and similar. You can try to sanitize that data, but chances are you'll end up like little bobby drop tables.

From the perspective of code design, a database ID should be considered a private implementation detail of the persistence technology to keep track of a row. If possible, you should be designing your application with absolutely no reference to this ID in any way. Instead, you should be thinking about how entities are identified in general. Is a person identified with their social security number? Is a person identified with their email? If so, your account model should only ever have a reference to those attributes. If there is no real way to identify a user with such a field, then you should be generating a UUID before hitting the DB.

Doing so has a lot of advantages as it would allow you to divorce your domain models from persistence technologies. That would mean that you can substitute database technologies without worrying about primary key compatibility. Leaking your primary key to your data model is not necessarily a security issue if you write the appropriate authorization code but its indicative of less than optimal code design.