feat: modify auth token url based on instance param

If an instance parameter is provided in the redirect, use it to modify
the URL from where the oauth token is obtained.

The instance provided is the Snyk region domain, minus the api. host
prefix.

pkg/auth/oauth2authenticator.go

@@ -14,6 +14,8 @@ import (
 	"math/big"
 	"net"
 	"net/http"
+	"net/url"
+	"regexp"
 	"sync"
 	"time"
 
@@ -287,6 +289,7 @@ func (o *oAuth2Authenticator) authenticateWithAuthorizationCode() error {
 	var responseCode string
 	var responseState string
 	var responseError string
+	var responseInstance string
 	verifier, err := createVerifier(128)
 	if err != nil {
 		return err
@@ -336,6 +339,7 @@ func (o *oAuth2Authenticator) authenticateWithAuthorizationCode() error {
 			appUrl := o.config.GetString(configuration.WEB_APP_URL)
 			responseCode = html.EscapeString(r.URL.Query().Get("code"))
 			responseState = html.EscapeString(r.URL.Query().Get("state"))
+			responseInstance = html.EscapeString(r.URL.Query().Get("instance"))
 			w.Header().Add("Location", appUrl+"/authenticated?type=oauth")
 			w.WriteHeader(http.StatusMovedPermanently)
 		}
@@ -388,6 +392,23 @@ func (o *oAuth2Authenticator) authenticateWithAuthorizationCode() error {
 		return fmt.Errorf("incorrect response state: %s != %s", responseState, state)
 	}
 
+	if responseInstance != "" {
+		authHost := redirectAuthHost(responseInstance)
+		if err != nil {
+			return fmt.Errorf("invalid instance: %q", responseInstance)
+		}
+		if !isValidAuthHost(authHost) {
+			return fmt.Errorf("invalid instance: %q", responseInstance)
+		}
+
+		authURL, err := url.Parse(o.oauthConfig.Endpoint.AuthURL)
+		if err != nil {
+			return fmt.Errorf("failed to parse auth url: %w", err)
+		}
+		authURL.Host = authHost
+		o.oauthConfig.Endpoint.AuthURL = authURL.String()
+	}
+
 	// Use the custom HTTP client when requesting a token.
 	if o.httpClient != nil {
 		ctx = context.WithValue(ctx, oauth2.HTTPClient, o.httpClient)
@@ -402,6 +423,16 @@ func (o *oAuth2Authenticator) authenticateWithAuthorizationCode() error {
 	return err
 }
 
+func redirectAuthHost(instance string) string {
+	return fmt.Sprintf("api.%s", instance)
+}
+
+var redirectAuthHostRE = regexp.MustCompile(`^api\.(.+)\.snyk\.io$`)
+
+func isValidAuthHost(authHost string) bool {
+	return redirectAuthHostRE.MatchString(authHost)
+}
+
 func (o *oAuth2Authenticator) AddAuthenticationHeader(request *http.Request) error {
 	if request == nil {
 		return fmt.Errorf("request must not be nil")