Identity and Access Management for REST APIs - REST Security Cheat Sheet


Why

REST APIs are ubiquitous, yet there is a lack of solid insights into securing REST APIs. The OWASP cheat sheet is notably out of date, focusing on issues such as session management that reflect a past where REST services were merely a data layer for simple web applications. Contemporary REST APIs serve a multitude of consumers and cover a wide variety of profiles. The game has changed and we need to rethink the security measures we take to protect the APIs, consumers, and end users.

What

  • Which current authN and authZ standards make sense in the REST ecosystems? In which scenarios?
  • Are there any tried-and-tested implementations of those standards?
  • Are there any common vulnerabilities?
  • Are there any implementation/integration guidelines?

Candidate technologies to try out

  • OAuth 2.0
  • OpenID Connect
  • Oz
  • UMA
  • JWT/JOSE
  • HoK

Outcomes

  • Use simpler terms: user, api, application - good enough for most scenarios and more comprehensible - guideline should outlive the specific schemes
  • Glossary of terms to clarify our use of terms like credentials, api keys, …
  • Need to list the different common scenarios (use cases), providing guidelines for each, e.g.:
  • Application consuming an api without a separate identity provider - api issues credentials/secret/mutual ssl securely - supplied on requests in an authorisation header over secure channel, e.g. HTTPS - application must be able to keep the credentials secret, e.g. not in public repositories, encrypted in configuration, … api could use internal identity (authorisation) provider - application not aware of an IDP - one-legged
  • Application calling multiple APIs with shared identity provider (application is aware of the IDP) - two-legged c.g. OAuth client credential grant - microservices scenario
  • User (resource owner) and application (client) and api - OAuth three-legged case - adds authorisation claims
  • As above but api calls api - no way to prevent api propagating rights illegitimately - JWT or signing bearer tokens is one way to mitigate this
  • Fine-grained resource control

Synopsis & takeaways

REST IAM

Discussion

  • Current section doesn’t make sense, makes lot of upfront assumptions
  • Prefer time-limited tokens to sending credentials with every request
  • Tokens should be short-lived and scoped
  • Long-running processes still need to be identified and authorised
  • Exchanging short-lived tokens for longer-lived tokens with narrower scope - narrow scope is a good principle anyways - principle of least privilege
  • Complexity of implementation is a consideration and trade-off to consider - more likely to get it wrong if complex
  • Need to at least address the simple case and offer good advice
  • Signed bearer tokens are an interesting idea

Scenario 1

Scenario1

Scenario 2

Scenario2

Scenario 3

Scenario3

Scenario 4

Scenario4

Scenario5

Scenario5

(sources for all these images are (https://drive.google.com/drive/u/0/folders/0B8Z12l6_ShfuSkQxaUc2SFFjQzg) )

Who

The target audience for this Working Session is:

  • Back-end REST services developers
  • SPA developers
  • Mobile developers
  • Authors of the REST Security Cheat Sheet

References


Working materials

Here are the points that I think need to be addressed in the cheat sheet. Give me feedback ASAP and I will start making changes on the OWASP site:

  • drop references to session management. At most mention that REST services should be stateless
  • Security-sensitive REST services should consume a signed JWT access token for authN and authZ
  • Current implementations only support JWT bearer tokens. Hence REST APIs that require authN/Z should only be available via an HTTPS URL. HSTS flag should be set
  • API key can be useful in an attempt to identify the client. In many cases, it is a bit of a stretch to expect authentication from an API key as publicly available clients cannot reliably keep the key secret
  • An access token is issued to a specific client to signal that the resource owner grants scoped access to the resource to that client for a given time period. The scope specifies the permissible operations, e.g. can the API consumer read the email address or also change it? Hence the REST service should at least check whether
    • the token is issued by a trusted issuer, sometimes called a Security Token Service or authorization server
    • the token is currently valid
    • the resource owner granted access to one of more resources on this resource server
    • the resource owner granted access to the API consumer invoking the services
    • the requested operation is in scope.
  • CSRF protection cannot be achieved in a multi-client REST service with a CSRF token. Origin header should be mandated and checked with configured list of allowed origins.
  • Direct object references are fine - this is how you point to a resource. It is just that access to that resource needs to be protected with an access token.
  • Input validation section is hard to understand. The bottom line is that any data included in the call needs to be output encoded before being used in processing the request.
  • Drop references to parsing XML - I assume no-one seriously wants to do that.
  • AFAIK, JWT cannot be used to guarantee message integrity.
  • A couple of important topics are missing and should be added:
    • CORS
    • HoK

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