The OAuth2 Filter

The OAuth2 filter type performs OAuth2 authorization against an identity provider implementing OIDC Discovery. The filter is both:

An OAuth Client, which fetches resources from the Resource Server on the user's behalf.
Half of a Resource Server, validating the Access Token before allowing the request through to the upstream service, which implements the other half of the Resource Server.

This is different from most OAuth implementations where the Authorization Server and the Resource Server are in the same security domain. With the Ambassador Edge Stack, the Client and the Resource Server are in the same security domain, and there is an independent Authorization Server.

The Ambassador Authentication Flow

This is what the authentication process looks like at a high level when using Ambassador Edge Stack with an external identity provider. The use case is an end-user accessing a secured app service.

Ambassador Authentication OAuth/OIDC

Some basic authentication terms

For those unfamiliar with authentication, here is a basic set of definitions.

OpenID: is an open standard and decentralized authentication protocol. OpenID allows users to be authenticated by co-operating sites, referred to as "relying parties" (RP) using a third-party authentication service. End-users can create accounts by selecting an OpenID identity provider (such as Auth0, Okta, etc), and then use those accounts to sign onto any website that accepts OpenID authentication.
Open Authorization (OAuth): an open standard for token-based authentication and authorization on the Internet. OAuth provides to clients a "secure delegated access" to server or application resources on behalf of an owner, which means that although you won't manage a user's authentication credentials, you can specify what they can access within your application once they have been successfully authenticated. The current latest version of this standard is OAuth 2.0.
Identity Provider (IdP): an entity that creates, maintains, and manages identity information for user accounts (also referred to "principals") while providing authentication services to external applications (referred to as "relying parties") within a distributed network, such as the web.
OpenID Connect (OIDC): is an authentication layer that is built on top of OAuth 2.0, which allows applications to verify the identity of an end-user based on the authentication performed by an IdP, using a well-specified RESTful HTTP API with JSON as a data format. Typically an OIDC implementation will allow you to obtain basic profile information for a user that successfully authenticates, which in turn can be used for implementing additional security measures like Role-based Access Control (RBAC).
JSON Web Token (JWT): is a JSON-based open standard for creating access tokens, such as those generated from an OAuth authentication. JWTs are compact, web-safe (or URL-safe), and are often used in the context of implementing single sign-on (SSO) within federated applications and organizations. Additional profile information, claims, or role-based information can be added to a JWT, and the token can be passed from the edge of an application right through the application's service call stack.

If you look back at the authentication process diagram, the function of the entities involved should now be much clearer.

Using an Identity Hub

Using an identity hub or broker allows you to support many IdPs without having to code individual integrations with them. For example, Auth0 and Keycloak both offer support for using Google and GitHub as an IdP.

An identity hub sits between your application and the IdP that authenticates your users, which not only adds a level of abstraction so that your application (and Ambassador Edge Stack) is isolated from any changes to each provider's implementation, but it also allows your users to chose which provider they use to authenticate (and you can set a default, or restrict these options).

The Auth0 docs provide a guide for adding social IdP "connections" to your Auth0 account, and the Keycloak docs provide a guide for adding social identity "brokers".

`OAuth2` Global Arguments

---
apiVersion: getambassador.io/v2
kind: Filter
metadata:
  name: "example-oauth2-filter"
  namespace: "example-namespace"
spec:
  OAuth2:
    authorizationURL:       "url"      # required
    ############################################################################
    # OAuth Client settings                                                    #
    ############################################################################
    expirationSafetyMargin: "duration" # optional; default is "0"
    # Which settings exist depends on the grantType; supported grantTypes
    # are "AuthorizationCode", "Password", and "ClientCredentials".
    grantType:              "enum"     # optional; default is "AuthorizationCode"
    ## OAuth Client settings: grantType=="AuthorizationCode" ###################
    clientURL:              "string"   # deprecated; use 'protectedOrigins' instead
    protectedOrigins:                  # required; must have at least 1 item
    - origin: "url"                      # required
      internalOrigin: "url"              # optional; default is to just use the 'origin' field
      includeSubdomains: bool            # optional; default is false
    useSessionCookies:                 # optional; default is { value: false }
      value: bool                        # optional: default is true
      ifRequestHeader:                   # optional; default to apply "useSessionCookies.value" to all requests
        name: "string"                     # required
        negate: bool                       # optional; default is false
        # It is invalid to specify both "value" and "valueRegex".
        value: "string"                    # optional; default is any non-empty string
        valueRegex: "regex"                # optional; default is any non-empty string
    extraAuthorizationParameters:      # optional; default is {}
      "string": "string"
    ## OAuth Client settings: grantType=="AuthorizationCode" or "Password" #####
    clientID:               "string"   # required
    # The client secret can be specified by including the raw secret as a
    # string in "secret", or by referencing Kubernetes secret with
    # "secretName" (and "secretNamespace").  It is invalid to specify both
    # "secret" and "secretName".
    secret:                 "string"   # required (unless secretName is set)
    secretName:             "string"   # required (unless secret is set)
    secretNamespace:        "string"   # optional; default is the same namespace as the Filter
    ## OAuth Client settings (grantType=="ClientCredentials") ##################
    #
    # (there are no additional client settings for
    # grantType=="ClientCredentials")
    ############################################################################
    # OAuth Resource Server settings                                           #
    ############################################################################
    allowMalformedAccessToken: bool     # optional; default is false
    accessTokenValidation:     "enum"   # optional; default is "auto"
    accessTokenJWTFilter:               # optional; default is null
      name: "string"                     # required
      namespace: "string"                # optional; default is the same namespace as the Filter
      arguments: JWT-Filter-Arguments    # optional
    ############################################################################
    # HTTP client settings for talking with the identity provider              #
    ############################################################################
    insecureTLS:           bool       # optional; default is false
    renegotiateTLS:        "enum"     # optional; default is "never"
    maxStale:              "duration" # optional; default is "0"

General settings

authorizationURL: Identifies where to look for the /.well-known/openid-configuration descriptor to figure out how to talk to the OAuth2 provider

OAuth Client settings

These settings configure the OAuth Client part of the filter.

grantType: Which type of OAuth 2.0 authorization grant to request from the identity provider. Currently supported are:
- "AuthorizationCode": Authenticate by redirecting to a login page served by the identity provider.
- "ClientCredentials": Authenticate by requiring X-Ambassador-Client-ID and X-Ambassador-Client-Secret HTTP headers on incoming requests, and using them to authenticate to the identity provider. Support for the ClientCredentials is currently preliminary, and only goes through limited testing.
- "Password": Authenticate by requiring X-Ambassador-Username and X-Ambassador-Password on all incoming requests, and use them to authenticate with the identity provider using the OAuth2 Resource Owner Password Credentials grant type.
expirationSafetyMargin: Check that access tokens not expire for at least this much longer; otherwise consider them to be already expired. This provides a safety margin of time for your application to send it to an upstream Resource Server that grants insufficient leeway to account for clock skew and network/application latency.

Depending on which grantType is used, different settings exist.

Settings that are only valid when grantType: "AuthorizationCode" or grantType: "Password":

clientID: The Client ID you get from your identity provider.
The client secret you get from your identity provider can be specified 2 different ways:
- As a string, in the secret field.
- As a Kubernetes generic Secret, named by secretName/secretNamespace. The Kubernetes secret must of the generic type, with the value stored under the keyoauth2-client-secret. If secretNamespace is not given, it defaults to the namespace of the Filter resource.
- Note: It is invalid to set both secret and secretName.

Settings that are only valid when grantType: "AuthorizationCode":

protectedOrigins: (You determine these, and must register them with your identity provider) Identifies hostnames that can appropriately set cookies for the application. Only the scheme (https://) and authority (example.com:1234) parts are used; the path part of the URL is ignored.
You will need to register each origin in protectedOrigins as an authorized callback endpoint with your identity provider. The URL will look like {{ORIGIN}}/.ambassador/oauth2/redirection-endpoint.
If you provide more than one protectedOrigin, all share the same authentication system, so that logging into one origin logs you into all origins; to have multiple domains that have separate logins, use separate Filters.
- internalOrigin: This sub-field of protectedOrigins[i] allows you to tell Ambassador that there is another gateway in front of Ambassador that rewrites the Host header, so that on the internal network between that gateway and Ambassador, the origin appears to be internalOrigin instead of origin. As a special-case the scheme and/or authority of the internalOrigin may be *, which matches any scheme or any domain respectively. The * is most useful in configurations with exactly one protected origin; in such a configuration, Ambassador doesn't need to know what the origin looks like on the internal network, just that a gateway in front of Ambassador is rewriting it. It is invalid to use * with includeSubdomains: true.
  For example, if you have a gateway in front of Ambassador handling traffic for myservice.example.com, terminating TLS and routing that traffic to Ambassador with the name ambassador.internal, you might write:
  Terminal
```
```yaml
  protectedOrigins:
  - origin: https://myservice.example.com
    internalOrigin: http://ambassador.internal
```
```
  or, to avoid being fragile to renaming ambassador.internal to something else, since there are not multiple origins that the Filter must distinguish between, you could instead write:
  Terminal
```
```yaml
  protectedOrigins:
  - origin: https://myservice.example.com
    internalOrigin: "*://*"
```
```

clientURL is deprecated, and is equivalent to setting

protectedOrigins:
- origin: clientURL-value
  internalOrigin: "*://*"

extraAuthorizationParameters: Extra (non-standard or extension) OAuth authorization parameters to use. It is not valid to specify a parameter used by OAuth itself ("response_type", "client_id", "redirect_uri", "scope", or "state").
By default, any cookies set by the Ambassador Edge Stack will be set to expire when the session expires naturally. The useSessionCookies setting may be used to cause session cookies to be used instead.
- Normally cookies are set to be deleted at a specific time; session cookies are deleted whenever the user closes their web browser. This may mean that the cookies are deleted sooner than normal if the user closes their web browser; conversely, it may mean that cookies persist for longer than normal if the use does not close their browser.
- The cookies being deleted sooner may or may not affect user-perceived behavior, depending on the behavior of the identity provider.
- Any cookies persisting longer will not affect behavior of the system; the Ambassador Edge Stack validates whether the session is expired when considering the cookie.
If useSessionCookies is non-null, then:
- By default it will have the cookies for all requests be session cookies or not according to the useSessionCookies.value sub-argument.
- Setting the useSessionCookies.ifRequestHeader sub-argument tells it to use useSessionCookies.value for requests that match the condition, and !useSessionCookies.value for requests don't match.
  When determining if a request matches, it looks at the HTTP header field named by useSessionCookies.ifRequestHeader.name (case-insensitive), and checks if it is either set to (if useSessionCookies.ifRequestHeader.negate: false) or not set to (if useSessionCookies.ifRequestHeader.negate: true)...
  - a non-empty string (if neither useSessionCookies.ifRequestHeader.value nor useSessionCookies.ifRequestHeader.valueRegex are set)
  - the exact string value (case-sensitive) (if useSessionCookies.ifRequestHeader.value is set)
  - a string that matches the regular expression useSessionCookies.ifRequestHeader.valueRegex (if valueRegex is set). This uses RE2 syntax (always, not obeying regex_type in the ambassador Module) but does not support the \C escape sequence.
  - (it is invalid to have both value and valueRegex set)

OAuth Resource Server settings

allowMalformedAccessToken: Allow any access token, even if they are not RFC 6750-compliant.
accessTokenValidation: How to verify the liveness and scope of Access Tokens issued by the identity provider. Valid values are either "auto", "jwt", or "userinfo". Empty or unset is equivalent to "auto".
- "jwt": Validates the Access Token as a JWT.
  - By default: It accepts the RS256, RS384, or RS512 signature algorithms, and validates the signature against the JWKS from OIDC Discovery. It then validates the exp, iat, nbf, iss (with the Issuer from OIDC Discovery), and scope claims: if present, none of the scopes are required to be present. This relies on the identity provider using non-encrypted signed JWTs as Access Tokens, and configuring the signing appropriately
  - This behavior can be modified by delegating to JWT Filter with accessTokenJWTFilter. The arguments are the same as the arguments when referring to a JWT Filter from a FilterPolicy.
- "userinfo": Validates the access token by polling the OIDC UserInfo Endpoint. This means that the Ambassador Edge Stack must initiate an HTTP request to the identity provider for each authorized request to a protected resource. This performs poorly, but functions properly with a wider range of identity providers. It is not valid to set accessTokenJWTFilter if accessTokenValidation: userinfo.
- "auto" attempts to do "jwt" validation if accessTokenJWTFilter is set or if the Access Token parses as a JWT and the signature is valid, and otherwise falls back to "userinfo" validation.

HTTP client

These HTTP client settings are used for talking to the identity provider:

maxStale: How long to keep stale cached OIDC replies for. This sets the max-stale Cache-Control directive on requests, and also ignores the no-store and no-cache Cache-Control directives on responses. This is useful for maintaining good performance when working with identity providers with misconfigured Cache-Control.
insecureTLS disables TLS verification when speaking to an identity provider with an https:// authorizationURL. This is discouraged in favor of either using plain http:// or installing a self-signed certificate.
renegotiateTLS allows a remote server to request TLS renegotiation. Accepted values are "never", "onceAsClient", and "freelyAsClient".

"duration" strings are parsed as a sequence of decimal numbers, each with optional fraction and a unit suffix, such as "300ms", "-1.5h" or "2h45m". Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h". See Go time.ParseDuration.

`OAuth2` Path-Specific Arguments

---
apiVersion: getambassador.io/v2
kind: FilterPolicy
metadata:
  name: "example-filter-policy"
  namespace: "example-namespace"
spec:
  rules:
  - host: "*"
    path: "*"
    filters:
    - name: "example-oauth2-filter"
      arguments:
        scopes:                     # optional; default is ["openid"] for `grantType=="AuthorizationCode"`; [] for `grantType=="ClientCredentials"` and `grantType=="Password"`
        - "scope1"
        - "scope2"
        insteadOfRedirect:          # optional for "AuthorizationCode"; default is to do a redirect to the identity provider
          ifRequestHeader:            # optional; default is to return httpStatusCode for all requests that would redirect-to-identity-provider
            name: "string"              # required
            negate: bool                # optional; default is false
            # It is invalid to specify both "value" and "valueRegex".
            value: "string"             # optional; default is any non-empty string
            valueRegex: "regex"  # optional; default is any non-empty string
          # option 1:
          httpStatusCode: integer     # optional; default is 403 (unless `filters` is set)
          # option 2:
          filters:                    # optional; default is to use `httpStatusCode` instead
          - name: "string"              # required
            namespace: "string"         # optional; default is the same namespace as the FilterPolicy
            ifRequestHeader:            # optional; default to apply this filter to all requests matching the host & path
              name: "string"              # required
              negate: bool                # optional; default is false
              # It is invalid to specify both "value" and "valueRegex".
              value: "string"             # optional; default is any non-empty string
              valueRegex: "regex"         # optional; default is any non-empty string
            onDeny: "enum"              # optional; default is "break"
            onAllow: "enum"             # optional; default is "continue"
            arguments: DEPENDS          # optional

scopes: A list of OAuth scope values to include in the scope of the authorization request. If one of the scope values for a path is not granted, then access to that resource is forbidden; if the scopes argument lists foo, but the authorization response from the provider does not include foo in the scope, then it will be taken to mean that the authorization server forbade access to this path, as the authenticated user does not have the foo resource scope.
If grantType: "AuthorizationCode", then the openid scope value is always included in the requested scope, even if it is not listed in the FilterPolicy argument.
If grantType: "ClientCredentials" or grantType: "Password", then the default scope is empty. If your identity provider does not have a default scope, then you will need to configure one here.
As a special case, if the offline_access scope value is requested, but not included in the response then access is not forbidden. With many identity providers, requesting the offline_access scope is necessary to receive a Refresh Token.
The ordering of scope values does not matter, and is ignored.
insteadOfRedirect: An action to perform instead of redirecting the User-Agent to the identity provider, when using grantType: "AuthorizationCode". By default, if the User-Agent does not have a currently-authenticated session, then the Ambassador Edge Stack will redirect the User-Agent to the identity provider. Setting insteadOfRedirect allows you to modify this behavior. insteadOfRedirect does nothing when grantType: "ClientCredentials", because the Ambassador Edge Stack will never redirect the User-Agent to the identity provider for the client credentials grant type.
- If insteadOfRedirect is non-null, then by default it will apply to all requests that would cause the redirect; setting the ifRequestHeader sub-argument causes it to only apply to requests that have the HTTP header field name (case-insensitive) either set to (if negate: false) or not set to (if negate: true)
  - a non-empty string if neither value nor valueRegex are set
  - the exact string value (case-sensitive) (if value is set)
  - a string that matches the regular expression valueRegex (if valueRegex is set). This uses RE2 syntax (always, not obeying regex_type in the ambassador Module) but does not support the \C escape sequence.
- By default, it serves an authorization-denied error page; by default HTTP 403 ("Forbidden"), but this can be configured by the httpStatusCode sub-argument.
- Instead of serving that simple error page, it can instead be configured to call out to a list of other Filters, by setting the filters list. The syntax and semantics of this list are the same as .spec.rules[].filters in a FilterPolicy. Be aware that if one of these filters modify the request rather than returning a response, then the request will be allowed through to the backend service, even though the OAuth2 Filter denied it.
- It is invalid to specify both httpStatusCode and filters.

XSRF protection

The ambassador_xsrf.NAME.NAMESPACE cookie is an opaque string that should be used as an XSRF token. Applications wishing to leverage the Ambassador Edge Stack in their XSRF attack protection should take two extra steps:

When generating an HTML form, the server should read the cookie, and include a <input type="hidden" name="_xsrf" value="COOKIE_VALUE" /> element in the form.
When handling submitted form data should verify that the form value and the cookie value match. If they do not match, it should refuse to handle the request, and return an HTTP 4XX response.

Applications using request submission formats other than HTML forms should perform analogous steps of ensuring that the value is present in the request duplicated in the cookie and also in either the request body or secure header field. A secure header field is one that is not Cookie, is not "simple", and is not explicitly allowed by the CORS policy.

Note: Prior versions of the Ambassador Edge Stack did not have an ambassador_xsrf.NAME.NAMESPACE cookie, and instead required you to use the ambassador_session.NAME.NAMESPACE cookie. The ambassador_session.NAME.NAMESPACE cookie should no longer be used for XSRF-protection purposes

RP-initiated logout

When a logout occurs, it is often not enough to delete the Ambassador Edge Stack's session cookie or session data; after this happens, and the web browser is redirected to the Identity Provider to re-log-in, the Identity Provider may remember the previous login, and immediately re-authorize the user; it would be like the logout never even happened.

To solve this, the Ambassador Edge Stack can use OpenID Connect Session Management to perform an "RP-Initiated Logout", where the Ambassador Edge Stack (the OpenID Connect "Relying Party" or "RP") communicates directly with Identity Providers that support OpenID Connect Session Management, to properly log out the user. Unfortunately, many Identity Providers do not support OpenID Connect Session Management.

This is done by having your application direct the web browser POST and navigate to /.ambassador/oauth2/logout. There are 2 form-encoded values that you need to include:

realm: The name.namespace of the Filter that you want to log out of. This may be submitted as part of the POST body, or may be set as a URL query parameter.
_xsrf: The value of the ambassador_xsrf.{{realm}} cookie (where {{realm}} is as described above). This must be set in the POST body, the URL query part will not be checked.

For example:

<form method="POST" action="/.ambassador/oauth2/logout" target="_blank">
  <input type="hidden" name="realm" value="myfilter.mynamespace" />
  <input type="hidden" name="_xsrf" value="{{ .Cookie.Value }}" />
  <input type="submit" value="Log out" />
</form>

<form method="POST" action="/.ambassador/oauth2/logout?realm=myfilter.mynamespace" target="_blank">
  <input type="hidden" name="_xsrf" value="{{ .Cookie.Value }}" />
  <input type="submit" value="Log out" />
</form>

or from JavaScript

function getCookie(name) {
    var prefix = name + "=";
    var cookies = document.cookie.split(';');
    for (var i = 0; i < cookies.length; i++) {
        var cookie = cookies[i].trimStart();
        if (cookie.indexOf(prefix) == 0) {
            return cookie.slice(prefix.length);
        }
    }
    return "";
}
function logout(realm) {
    var form = document.createElement('form');
    form.method = 'post';
    form.action = '/.ambassador/oauth2/logout?realm='+realm;
    //form.target = '_blank'; // uncomment to open the identity provider's page in a new tab
    var xsrfInput = document.createElement('input');
    xsrfInput.type = 'hidden';
    xsrfInput.name = '_xsrf';
    xsrfInput.value = getCookie("ambassador_xsrf."+realm);
    form.appendChild(xsrfInput);
    document.body.appendChild(form);
    form.submit();
}

Redis

The Ambassador Edge Stack relies on Redis to store short-lived authentication credentials and rate limiting information. If the Redis data store is lost, users will need to log back in and all existing rate-limits would be reset.

Questions?

We’re here to help. If you have questions, join our Slack or contact us.

Version:

Search the Docs

The OAuth2 Filter

The Ambassador Authentication Flow

Some basic authentication terms

Using an Identity Hub

`OAuth2` Global Arguments

General settings

OAuth Client settings

OAuth Resource Server settings

HTTP client

`OAuth2` Path-Specific Arguments

XSRF protection

RP-initiated logout

Redis

Further reading

Questions?

Version:

Search the Docs

The OAuth2 Filter

The Ambassador Authentication Flow

Some basic authentication terms

Using an Identity Hub

OAuth2 Global Arguments

General settings

OAuth Client settings

OAuth Resource Server settings

HTTP client

OAuth2 Path-Specific Arguments

XSRF protection

RP-initiated logout

Redis

Further reading

Questions?

`OAuth2` Global Arguments

`OAuth2` Path-Specific Arguments