Does Salesforce REST API (not the Apex Rest API) inherently respect field level permissions while fetching data ? Tried to find this in the SF documentation but no luck. Any help is highly appriciated
Invoking a custom Apex REST Web service method always uses system context. Consequently, the current user's credentials are not used, and any user who has access to these methods can use their full power, regardless of permissions, field-level security, or sharing rules. Developers who expose methods using the Apex REST annotations should therefore take care that they are not inadvertently exposing any sensitive data.
Apex class methods that are exposed through the Apex REST API don't enforce object permissions and field-level security by default. We recommend that you make use of the appropriate object or field describe result methods to check the current user’s access level on the objects and fields that the Apex REST API method is accessing. See DescribeSObjectResult Class and DescribeFieldResult Class.
Also, sharing rules (record-level access) are enforced only when declaring a class with the with sharing keyword. This requirement applies to all Apex classes, including to classes that are exposed through Apex REST API. To enforce sharing rules for Apex REST API methods, declare the class that contains these methods with the with sharing keyword. See Using the with sharing or without sharing Keywords.
Reference from developer.salesforce.com
I tried this in my dev org, the Salesforce default rest api honors both sharing and field level permissions of the user(whose sessionId/access_token we are using) while fetching data.