API3:2019 Excessive Data Exposure

In the previous post we covered API2:2019 Broken User Authentication, which was the second post in this series. If you want to start from the beginning, go to the first post, API1:2019 Broken Object Level Authorization.

#RSAC 2022

Excessive data exposure is something that web applications can face, not just APIs. That said, because web-based APIs are basically services on the web, they can be abused even more easily to exfiltrate sensitive data than a regular web app. It’s easy for an attacker to find APIs (just connect to any web app or mobile app using a web proxy and see the API calls for yourself!), to call them, and then look at the responses to see if anything being sent looks potentially sensitive. For instance, if the data in a field passed back is named “password”, “sin” or “secret”, you’re most likely onto something.

Using a web proxy to watch the API calls go back and forth is sometimes called “sniffing”, but no matter what you call it, it’s easy to do! Anyone with the tiniest amount of web-app hacking training can do this on day one. This means this threat is prevalent (happens all the time) and very dangerous (because unsophisticated attackers can easily execute it).

Some APIs are *supposed* to return sensitive data. This vulnerability is when sensitive data is exposed to someone it should not be (for instance, someone who is not a valid user, seeing another user’s sensitive data, or for whom that specific data should not be shown due to their role within the system). Since whether data is sensitive in nature is not obvious to automated testing tools, it can be a bit more difficult to identify than other types of vulnerabilities.

* Note: occasionally the vulnerability rears its head via poorly-generated and/or overly-populated responses. For instance, the API delivers the entire table worth of data, which includes sensitive information, but then the client-side front-end sifts through it and only reveals the non-sensitive/appropriate data to the end user. Unfortunately, if the API call is not encrypted in transit, this means a malicious actor could see all of the data if they were sniffing the API at the time.

How do we avoid this?

Let’s look at some great advice from the project team (I may have added a bit onto their list):

  • Never rely on the client side to filter sensitive data. By this we mean, only return the data you need to return! Don’t send a ton of stuff you do not need to, then let the GUI/front end decide what to show the user. Make these important decisions on the server.
  • Classifying then label all your data. If you know immediately when you look at something that it is sensitive, it’s automatic to treat it in a certain way. Educate your developers and other areas of IT on how to classify data, and to ask the security team if they aren’t sure.
  • In the design of your API, add user stories and/or threat models around this potential vulnerability. Making protecting sensitive data part of your design.
  • Review the responses from the API to make sure they contain only legitimate data, data that the specific user (or users with that role inside your system) are allowed to access.
  • Back-end engineers should always ask themselves “who is the consumer of the data?” before exposing a new API endpoint. Or better yet, perform threat modelling on your data flows, THEN design.
  • Avoid using generic methods such as to_json() and to_string(). Instead, cherry-pick specific properties you really want to return. You do not need to return everything. In fact, it’s better for your cloud bills to return only what you need, even if it requires a bit more programming.
  • Classify sensitive and personally identifiable information (PII) that your application stores and works with, reviewing all API calls returning such information to see if these responses pose a security issue.
  • Implement a schema-based response validation mechanism as an extra layer of security. As part of this mechanism define and enforce data returned by all API methods, including errors.
  • Perform strict linting on your API definition file, to ensure you have input validation built-in, by default, for every variable.

In the next blog post we will be talking about API4:2019 Lack of Resources & Rate Limiting!

API2:2019 Broken User Authentication

In the previous post we covered API1:2019 Broken Object Level Authorization, which was the first post in this series. Feel free to start there and work your way back to here if you want! Or just keep on reading.

You can read the official document from the OWASP Project team here.

Tanya Janca presenting "Shifting Security Everywhere" at OWASP Global AppSec, Dublin, Ireland, Feb 2023
Tanya Janca presenting “Shifting Security Everywhere” at OWASP Global AppSec, Dublin, Ireland, Feb 2023

Broken User Authentication is a vulnerability that can affect any web app or API that has user accounts, which is a very large percentage of applications currently on the internet. The app or API asks the user “Who are you?” (authentication) and if the user is able to trick the app into ‘recognizing them’ when they are not a valid user, that is broken authentication.

When someone exploits this vulnerability, they have access to a system they should not, and quite likely, sensitive data of that user, plus the ability to change account settings of whatever account they have broken into. This is a total nightmare for the user whose account has been compromised, not only the security team.

The most common ways this vulnerability is exploited would be via credential stuffing (trying thousands of stolen credentials to see if any of them work), and brute force attacks (allowing end users to try over and over again to guess a password and not locking them out when they deserve it).

Other types of attacks of this nature include:

  • Allowing users to make very weak passwords, such as “Winter2023”, which is likely the English-speaking-world’s most popular password this very moment. </sigh>
  • Sending auth-tokens in insecure ways, such as in an unencrypted/insecurely configured cookie, or the URL parameters
  • Not checking if your auth tokens are valid, every-single-call/page/action/time
  • Using auth tokens incorrectly/with poor config/very old versions they shouldn’t use anymore
  • Following all the right programming steps, but then sending it using very insecure encryption, such that it the data can be decrypted
  • Insecure storage of passwords (passwords should be salted and hashed, possibly peppered, but never plain text or encrypted)

How can I ensure this never happens?

  • Follow auth best practices and *ideally* use an API gateway. API Gateways can handle a lot of this for you, and that makes life WAY BETTER. Writing your own is very complex, time consuming, and potentially risky.
  • Test. Test this manually, and with tools. Test this thoroughly and if you make changes test it again.
  • Threat model this or make a security user story about this. It’s a user story/potential threat to almost every app and API on the net. Take it very seriously.
  • This OWASP Resource from the Cheat Sheets Project is extremely helpful: OWASP Authentication Cheatsheet

Advice from the OWASP Project team (it is also excellent!)

  • Make sure you know all the possible flows to authenticate to the API (mobile/ web/deep links that implement one-click authentication/etc.)
  • Ask your engineers what flows you missed.
  • Read about your authentication mechanisms. Make sure you understand what and how they are used. OAuth is not authentication, and neither is API keys.
  • Don’t reinvent the wheel in authentication, token generation, password storage. Use the standards.
  • Credential recovery/forget password endpoints should be treated as login endpoints in terms of brute force, rate limiting, and lockout protections.
  • Use the OWASP Authentication Cheatsheet. It’s very good!
  • Where possible, implement multi-factor authentication.
  • Implement anti brute force mechanisms to mitigate credential stuffing, dictionary attack, and brute force attacks on your authentication endpoints. This mechanism should be stricter than the regular rate limiting mechanism on your API.
  • Implement account lockout / captcha mechanism to prevent brute force against specific users. Implement weak-password checks.
  • API keys should not be used for user authentication, but for client app/ project authentication.

In the next blog post we will be talking about API3:2019 Excessive Data Exposure!