Created
April 14, 2020 05:53
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Auth0 PKCE flow for a CLI built in golang
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| package auth | |
| import ( | |
| "encoding/json" | |
| "fmt" | |
| "io" | |
| "io/ioutil" | |
| "net" | |
| "net/http" | |
| "net/url" | |
| "os" | |
| "strings" | |
| cv "github.com/nirasan/go-oauth-pkce-code-verifier" | |
| "github.com/skratchdot/open-golang/open" | |
| "github.com/spf13/viper" | |
| ) | |
| // AuthorizeUser implements the PKCE OAuth2 flow. | |
| func AuthorizeUser(clientID string, authDomain string, redirectURL string) { | |
| // initialize the code verifier | |
| var CodeVerifier, _ = cv.CreateCodeVerifier() | |
| // Create code_challenge with S256 method | |
| codeChallenge := CodeVerifier.CodeChallengeS256() | |
| // construct the authorization URL (with Auth0 as the authorization provider) | |
| authorizationURL := fmt.Sprintf( | |
| "https://%s/authorize?audience=https://api.snapmaster.io"+ | |
| "&scope=openid"+ | |
| "&response_type=code&client_id=%s"+ | |
| "&code_challenge=%s"+ | |
| "&code_challenge_method=S256&redirect_uri=%s", | |
| authDomain, clientID, codeChallenge, redirectURL) | |
| // start a web server to listen on a callback URL | |
| server := &http.Server{Addr: redirectURL} | |
| // define a handler that will get the authorization code, call the token endpoint, and close the HTTP server | |
| http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) { | |
| // get the authorization code | |
| code := r.URL.Query().Get("code") | |
| if code == "" { | |
| fmt.Println("snap: Url Param 'code' is missing") | |
| io.WriteString(w, "Error: could not find 'code' URL parameter\n") | |
| // close the HTTP server and return | |
| cleanup(server) | |
| return | |
| } | |
| // trade the authorization code and the code verifier for an access token | |
| codeVerifier := CodeVerifier.String() | |
| token, err := getAccessToken(clientID, codeVerifier, code, redirectURL) | |
| if err != nil { | |
| fmt.Println("snap: could not get access token") | |
| io.WriteString(w, "Error: could not retrieve access token\n") | |
| // close the HTTP server and return | |
| cleanup(server) | |
| return | |
| } | |
| viper.Set("AccessToken", token) | |
| err = viper.WriteConfig() | |
| //_, err = config.WriteConfigFile("auth.json", token) | |
| if err != nil { | |
| fmt.Println("snap: could not write config file") | |
| io.WriteString(w, "Error: could not store access token\n") | |
| // close the HTTP server and return | |
| cleanup(server) | |
| return | |
| } | |
| // return an indication of success to the caller | |
| io.WriteString(w, ` | |
| <html> | |
| <body> | |
| <h1>Login successful!</h1> | |
| <h2>You can close this window and return to the snap CLI.</h2> | |
| </body> | |
| </html>`) | |
| fmt.Println("Successfully logged into snapmaster API.") | |
| // close the HTTP server | |
| cleanup(server) | |
| }) | |
| // parse the redirect URL for the port number | |
| u, err := url.Parse(redirectURL) | |
| if err != nil { | |
| fmt.Printf("snap: bad redirect URL: %s\n", err) | |
| os.Exit(1) | |
| } | |
| // set up a listener on the redirect port | |
| port := fmt.Sprintf(":%s", u.Port()) | |
| l, err := net.Listen("tcp", port) | |
| if err != nil { | |
| fmt.Printf("snap: can't listen to port %s: %s\n", port, err) | |
| os.Exit(1) | |
| } | |
| // open a browser window to the authorizationURL | |
| err = open.Start(authorizationURL) | |
| if err != nil { | |
| fmt.Printf("snap: can't open browser to URL %s: %s\n", authorizationURL, err) | |
| os.Exit(1) | |
| } | |
| // start the blocking web server loop | |
| // this will exit when the handler gets fired and calls server.Close() | |
| server.Serve(l) | |
| } | |
| // getAccessToken trades the authorization code retrieved from the first OAuth2 leg for an access token | |
| func getAccessToken(clientID string, codeVerifier string, authorizationCode string, callbackURL string) (string, error) { | |
| // set the url and form-encoded data for the POST to the access token endpoint | |
| url := "https://snapmaster-dev.auth0.com/oauth/token" | |
| data := fmt.Sprintf( | |
| "grant_type=authorization_code&client_id=%s"+ | |
| "&code_verifier=%s"+ | |
| "&code=%s"+ | |
| "&redirect_uri=%s", | |
| clientID, codeVerifier, authorizationCode, callbackURL) | |
| payload := strings.NewReader(data) | |
| // create the request and execute it | |
| req, _ := http.NewRequest("POST", url, payload) | |
| req.Header.Add("content-type", "application/x-www-form-urlencoded") | |
| res, err := http.DefaultClient.Do(req) | |
| if err != nil { | |
| fmt.Printf("snap: HTTP error: %s", err) | |
| return "", err | |
| } | |
| // process the response | |
| defer res.Body.Close() | |
| var responseData map[string]interface{} | |
| body, _ := ioutil.ReadAll(res.Body) | |
| // unmarshal the json into a string map | |
| err = json.Unmarshal(body, &responseData) | |
| if err != nil { | |
| fmt.Printf("snap: JSON error: %s", err) | |
| return "", err | |
| } | |
| // retrieve the access token out of the map, and return to caller | |
| accessToken := responseData["access_token"].(string) | |
| return accessToken, nil | |
| } | |
| // cleanup closes the HTTP server | |
| func cleanup(server *http.Server) { | |
| // we run this as a goroutine so that this function falls through and | |
| // the socket to the browser gets flushed/closed before the server goes away | |
| go server.Close() | |
| } |
FYI: github.com/nirasan/go-oauth-pkce-code-verifier uses math/rand to generate a non-cryptographic random string for the verifier which means it has a timing vulnerability.
I've opened a PR to address it: nirasan/go-oauth-pkce-code-verifier#1
Until the PR is merged, I would suggest not using that package for PKCE implementations.
The pull request from @jimlambrt was merged on May 9, 2022.
I believe this should be safe[r] to use now...
Just to update, go 1.21 now has pkce support with
verifier = oauth2.GenerateVerifier()
authCodeURL := oauthConfig.AuthCodeURL(state, oauth2.AccessTypeOffline, oauth2.S256ChallengeOption(verifier))
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Thanks @cadethacker! Glad it was helpful. Also, good find on the code that finds an open port - that's a great practical addition to the gist.