# OAuth Validator Modules

PostgreSQL provides infrastructure for creating custom modules to perform server-side validation of OAuth bearer tokens. Because OAuth implementations vary so wildly, and bearer token validation is heavily dependent on the issuing party, the server cannot check the token itself; validator modules provide the integration layer between the server and the OAuth provider in use.

OAuth validator modules must at least consist of an initialization function (see oauth-validator-init) and the required callback for performing validation (see oauth-validator-callback-validate).

Since a misbehaving validator might let unauthorized users into the database, correct implementation is crucial for server safety. See oauth-validator-design for design considerations.

## Safely Designing a Validator Module

Read and understand the entirety of this section before implementing a validator module. A malfunctioning validator is potentially worse than no authentication at all, both because of the false sense of security it provides, and because it may contribute to attacks against other pieces of an OAuth ecosystem.

### Validator Responsibilities

Although different modules may take very different approaches to token validation, implementations generally need to perform three separate actions:

Validate the Token

- The validator must first ensure that the presented token is in fact a valid Bearer token for use in client authentication. The correct way to do this depends on the provider, but it generally involves either cryptographic operations to prove that the token was created by a trusted party (offline validation), or the presentation of the token to that trusted party so that it can perform validation for you (online validation).

  Online validation, usually implemented via OAuth Token Introspection, requires fewer steps of a validator module and allows central revocation of a token in the event that it is stolen or misissued. However, it does require the module to make at least one network call per authentication attempt (all of which must complete within the configured guc-authentication-timeout). Additionally, your provider may not provide introspection endpoints for use by external resource servers.

  Offline validation is much more involved, typically requiring a validator to maintain a list of trusted signing keys for a provider and then check the token's cryptographic signature along with its contents. Implementations must follow the provider's instructions to the letter, including any verification of issuer ("where is this token from?"), audience ("who is this token for?"), and validity period ("when can this token be used?"). Since there is no communication between the module and the provider, tokens cannot be centrally revoked using this method; offline validator implementations may wish to place restrictions on the maximum length of a token's validity period.

  If the token cannot be validated, the module should immediately fail. Further authentication/authorization is pointless if the bearer token wasn't issued by a trusted party.

Authorize the Client

- Next the validator must ensure that the end user has given the client permission to access the server on their behalf. This generally involves checking the scopes that have been assigned to the token, to make sure that they cover database access for the current HBA parameters.

  The purpose of this step is to prevent an OAuth client from obtaining a token under false pretenses. If the validator requires all tokens to carry scopes that cover database access, the provider should then loudly prompt the user to grant that access during the flow. This gives them the opportunity to reject the request if the client isn't supposed to be using their credentials to connect to databases.

  While it is possible to establish client authorization without explicit scopes by using out-of-band knowledge of the deployed architecture, doing so removes the user from the loop, which prevents them from catching deployment mistakes and allows any such mistakes to be exploited silently. Access to the database must be tightly restricted to only trusted clients

  That is, "trusted" in the sense that the OAuth client and the PostgreSQL server are controlled by the same entity. Notably, the Device Authorization client flow supported by libpq does not usually meet this bar, since it's designed for use by public/untrusted clients.

  if users are not prompted for additional scopes.

  Even if authorization fails, a module may choose to continue to pull authentication information from the token for use in auditing and debugging.

Authenticate the End User

- Finally, the validator should determine a user identifier for the token, either by asking the provider for this information or by extracting it from the token itself, and return that identifier to the server (which will then make a final authorization decision using the HBA configuration). This identifier will be available within the session via `system_user` and recorded in the server logs if guc-log-connections is enabled.

  Different providers may record a variety of different authentication information for an end user, typically referred to as claims. Providers usually document which of these claims are trustworthy enough to use for authorization decisions and which are not. (For instance, it would probably not be wise to use an end user's full name as the identifier for authentication, since many providers allow users to change their display names arbitrarily.) Ultimately, the choice of which claim (or combination of claims) to use comes down to the provider implementation and application requirements.

  Note that anonymous/pseudonymous login is possible as well, by enabling usermap delegation; see oauth-validator-design-usermap-delegation.

### General Coding Guidelines

Developers should keep the following in mind when implementing token validation:

Token Confidentiality

- Modules should not write tokens, or pieces of tokens, into the server log. This is true even if the module considers the token invalid; an attacker who confuses a client into communicating with the wrong provider should not be able to retrieve that (otherwise valid) token from the disk.

  Implementations that send tokens over the network (for example, to perform online token validation with a provider) must authenticate the peer and ensure that strong transport security is in use.

Logging

- Modules may use the same logging facilities as standard extensions; however, the rules for emitting log entries to the client are subtly different during the authentication phase of the connection. Generally speaking, modules should log verification problems at the COMMERROR level and return normally, instead of using ERROR/FATAL to unwind the stack, to avoid leaking information to unauthenticated clients.

Interruptibility

- Modules must remain interruptible by signals so that the server can correctly handle authentication timeouts and shutdown signals from pg_ctl. For example, blocking calls on sockets should generally be replaced with code that handles both socket events and interrupts without races (see `WaitLatchOrSocket()`, `WaitEventSetWait()`, et al), and long-running loops should periodically call `CHECK_FOR_INTERRUPTS()`. Failure to follow this guidance may result in unresponsive backend sessions.

Testing

- The breadth of testing an OAuth system is well beyond the scope of this documentation, but at minimum, negative testing should be considered mandatory. It's trivial to design a module that lets authorized users in; the whole point of the system is to keep unauthorized users out.

Documentation

- Validator implementations should document the contents and format of the authenticated ID that is reported to the server for each end user, since DBAs may need to use this information to construct pg_ident maps. (For instance, is it an email address? an organizational ID number? a UUID?) They should also document whether or not it is safe to use the module in delegate_ident_mapping=1 mode, and what additional configuration is required in order to do so.

### Authorizing Users (Usermap Delegation)

The standard deliverable of a validation module is the user identifier, which the server will then compare to any configured `pg_ident.conf` mappings and determine whether the end user is authorized to connect. However, OAuth is itself an authorization framework, and tokens may carry information about user privileges. For example, a token may be associated with the organizational groups that a user belongs to, or list the roles that a user may assume, and duplicating that knowledge into local usermaps for every server may not be desirable.

To bypass username mapping entirely, and have the validator module assume the additional responsibility of authorizing user connections, the HBA may be configured with auth-oauth-delegate-ident-mapping. The module may then use token scopes or an equivalent method to decide whether the user is allowed to connect under their desired role. The user identifier will still be recorded by the server, but it plays no part in determining whether to continue the connection.

Using this scheme, authentication itself is optional. As long as the module reports that the connection is authorized, login will continue even if there is no recorded user identifier at all. This makes it possible to implement anonymous or pseudonymous access to the database, where the third-party provider performs all necessary authentication but does not provide any user-identifying information to the server. (Some providers may create an anonymized ID number that can be recorded instead, for later auditing.)

Usermap delegation provides the most architectural flexibility, but it turns the validator module into a single point of failure for connection authorization. Use with caution.

## Initialization Functions

OAuth validator modules are dynamically loaded from the shared libraries listed in guc-oauth-validator-libraries. Modules are loaded on demand when requested from a login in progress. The normal library search path is used to locate the library. To provide the validator callbacks and to indicate that the library is an OAuth validator module a function named `_PG_oauth_validator_module_init` must be provided. The return value of the function must be a pointer to a struct of type OAuthValidatorCallbacks, which contains a magic number and pointers to the module's token validation functions. The returned pointer must be of server lifetime, which is typically achieved by defining it as a `static const` variable in global scope.

    typedef struct OAuthValidatorCallbacks
    {
        uint32        magic;            /* must be set to PG_OAUTH_VALIDATOR_MAGIC */

        ValidatorStartupCB startup_cb;
        ValidatorShutdownCB shutdown_cb;
        ValidatorValidateCB validate_cb;
    } OAuthValidatorCallbacks;

    typedef const OAuthValidatorCallbacks *(*OAuthValidatorModuleInit) (void);

Only the `validate_cb` callback is required, the others are optional.

## OAuth Validator Callbacks

OAuth validator modules implement their functionality by defining a set of callbacks. The server will call them as required to process the authentication request from the user.

### Startup Callback

The `startup_cb` callback is executed directly after loading the module. This callback can be used to set up local state and perform additional initialization if required. If the validator module has state it can use state->private_data to store it.

    typedef void (*ValidatorStartupCB) (ValidatorModuleState *state);

### Validate Callback

The `validate_cb` callback is executed during the OAuth exchange when a user attempts to authenticate using OAuth. Any state set in previous calls will be available in state->private_data.

    typedef bool (*ValidatorValidateCB) (const ValidatorModuleState *state,
                                         const char *token, const char *role,
                                         ValidatorModuleResult *result);

`token` will contain the bearer token to validate. PostgreSQL has ensured that the token is well-formed syntactically, but no other validation has been performed. `role` will contain the role the user has requested to log in as. The callback must set output parameters in the `result` struct, which is defined as below:

    typedef struct ValidatorModuleResult
    {
        bool        authorized;
        char       *authn_id;
    } ValidatorModuleResult;

The connection will only proceed if the module sets result->authorized to `true`. To authenticate the user, the authenticated user name (as determined using the token) shall be palloc'd and returned in the result->authn_id field. Alternatively, result->authn_id may be set to NULL if the token is valid but the associated user identity cannot be determined.

A validator may return `false` to signal an internal error, in which case any result parameters are ignored and the connection fails. Otherwise the validator should return `true` to indicate that it has processed the token and made an authorization decision.

The behavior after `validate_cb` returns depends on the specific HBA setup. Normally, the result->authn_id user name must exactly match the role that the user is logging in as. (This behavior may be modified with a usermap.) But when authenticating against an HBA rule with `delegate_ident_mapping` turned on, PostgreSQL will not perform any checks on the value of result->authn_id at all; in this case it is up to the validator to ensure that the token carries enough privileges for the user to log in under the indicated `role`.

### Shutdown Callback

The `shutdown_cb` callback is executed when the backend process associated with the connection exits. If the validator module has any allocated state, this callback should free it to avoid resource leaks.

    typedef void (*ValidatorShutdownCB) (ValidatorModuleState *state);