Development:External services security
Descriptions of security framework for web services, also used for RSS feeds, embedded application and similar parts that can not use normal HTTP cookies.
Overview
Current solutions:
- user keys for gradebook import and export - see require_user_key_login() and db table user_private_key
- open RSS feeds - no security at all
- chat_sid tokens - generated separately for each user in each chat
- calendar export - hash from user name, password and salt
- hacky cookie emulation in visual gradebook plugin
Design
Different uses
The external API may be used from different places:
- directly from PHP - no authentication, current user session is used ($USER, $SESSION)
- simple web service layer - ws is responsible for faking of $USER and $SESSION object, no persistent session is maintained, webservice auth plugin is used for login/password authentication
- when embedding external applications - external application receives unique token which is used instead of normal browser session cookie, the session is linked to the current user session in browser, the token is automatically invalidated after logout
- RSS feeds, iCals, etc. - token login, no permanent session
- full WS layer with emulated session - performance is the main issue here, the faking of $USER and $SESSION is very expensive, we will have to use some token which mimics the browser cookies.
API layers
Three layers:
- external server interface (SOAP, REST, RSS, etc.) - deals with tokens, emulates user session, parameter processing
- public PHP API - functions usable directly from PHP, list generated from inline PHP docs, need to verify all parameters and access control, may access $USER, should not manipulate $SESSION directly, must not read $_POST or $_GET
- low level internal API - as fast as possible, basic param validation, no access control, must not touch $USER, $SESSION, $_GET or $_POST, must not use has_capability() or require_login()!
Context restrictions
Context restriction of token validity should be effective against security problems in external applications interacting with Moodle. Some external applications do not have any access to http cookies, solution is to create temporary tokens. Context restrictions would allow us to grant external access to individual activities, courses ,etc..
Implementation
PHP API
This simplest case of using the external API. The calling call has to make sure the current $USER and $SESSION is valid. Optionally the caller may set up the context restriction manually. This should be the easiest way to start with Moodle modifications.
Simplified web services
This type of web service is using simple username+password authentication (in future could be username+certificate). The communication integrity/privacy can be protected using https. Each external application is using own user account. These user accounts can not be used for normal login into moodle vie web interface.
This type of WS is intended primarily for integration with student information systems or other enterprise software.
webservice auth plugin
It is recommended to create separate user for each external application. The benefit of this auth type is that it can not be used for normal log-in from the web interface. User preferences may be later used for storage of extra data such as public certificates.
WS entry point
Separate php file /webservice/xxx/simpleserver.php
external_services_users table
Specifies which user may use each simplified web service. All services available through this interface should have the restrictedusers flag enabled.
Field | Type | Default | Description |
---|---|---|---|
id | int(10) | auto-incrementing | |
externalserviceid | int(10) | foreign key, reference external_services.id | |
userid | int(10) | foreign key, reference user.id | |
iprestriction | char(255) | restrict access to some ips or subnets only | |
validuntil | int(10) | invalidate after | |
timecreated | int(10) | time when this record added |
Steps needed to configure simple web services access
- enable simple web services - global switch affecting all simple web services (admin UI)
- create new service in some local plug-ip (add code into /local/yourplugin/db/services.php)
- add new user, set auth plugin to webservice (normal add user UI)
- enable web service layer plugin (admin UI)
- add user into list of users that are allowed to use specific external services (admin UI)
- set up user permissions needed in function implementations
- set up permission to use specific web services (optional, services do not need to specify required capability)
General web services
More flexible than the WS with simplified authentication and setup. The performance may be significantly improved by emulation of sessions. The authentication is based on security tokens (user, context and purpose specific) that are generated in normal Moodle interface and then used in other external applications that are using our web services. This allows us to give only partial access to some parts of WS api, externall applications and services do not need to be fully trusted.
The main drawback is that the complex configuration does not encourage admins to keep everything secured.
Application embedding
Capabilities
It would not be practical to list all approved users, instead we can use capabilities for access restrictions. The required capability is given in the service definition table.
RSS and other feeds
external_tokens
Stores tokens for cookieless access, script runs without real session, $USER and $SESSION is emulated. Use is relatively expensive because each scripts has to initialize accessdata in acceslib.php again. Existing data from user_private_key table are migrated here.
Field | Type | Default | Description |
---|---|---|---|
id | int(10) | auto-incrementing | |
userid | int(10) | foreign key, references user.id | |
token | varchar(128) | private access key value | |
restrictioncontextid | int(10) | security restriction, key usable only in this context, references context.id | |
externalserviceid | int(10) | foreign key, references external_services.id | |
itemid | int(10) | Service specific item id | |
iprestriction | varchar(255) | null | IP address restriction, list of allowed addresses |
validuntil | int(10) | null | timestampt - valid until date |
timecreated | int(10) | time when key created | |
lastaccess | int(10) | time when key last used for access |
Examples: gradebook exports, private RSS feeds