imap,smtp: add XOAUTH2 support

Add XOAUTH2 authentication support for IMAP and SMTP. Although XOAUTH2
is now deprecated in favor of OAuthBearer, it is the only way to connect
to Office365 since Basic Auth is now completely removed.

Since XOAUTH2 is very similar to OAuthBearer and uses the same
configuration parameters, this is basically a copy-paste of the existing
OAuthBearer code.

However, XOAUTH2 support was removed from go-sasl library, so this
change reimports the code that was removed from go-sasl and offers it
a new home in lib/xoauth2.go. Hopefully it shouldn't be too hard to
maintain, being less than 50 SLOC.

Link: https://github.com/emersion/go-sasl/commit/7bfe0ed36a21
Implements: https://todo.sr.ht/~rjarry/aerc/78
Signed-off-by: Julian Pidancet <julian.pidancet@oracle.com>
Tested-by: Inwit <inwit@sindominio.net>
Acked-by: Tim Culverhouse <tim@timculverhouse.com>

CHANGELOG.md

@@ -18,6 +18,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Override `:open` handler on a per-MIME-type basis in `aerc.conf`.
 - Specify opener as the first `:open` param instead of always using default
   handler (i.e. `:open gimp` to open attachment in GIMP).
+- Restored XOAUTH2 support for IMAP and SMTP.
 
 ### Changed
 

commands/compose/send.go

@@ -302,6 +302,28 @@ func newSaslClient(auth string, uri *url.URL) (sasl.Client, error) {
 			Username: uri.User.Username(),
 			Token:    password,
 		})
+	case "xoauth2":
+		q := uri.Query()
+		oauth2 := &oauth2.Config{}
+		if q.Get("token_endpoint") != "" {
+			oauth2.ClientID = q.Get("client_id")
+			oauth2.ClientSecret = q.Get("client_secret")
+			oauth2.Scopes = []string{q.Get("scope")}
+			oauth2.Endpoint.TokenURL = q.Get("token_endpoint")
+		}
+		password, _ := uri.User.Password()
+		bearer := lib.Xoauth2{
+			OAuth2:  oauth2,
+			Enabled: true,
+		}
+		if bearer.OAuth2.Endpoint.TokenURL != "" {
+			token, err := bearer.ExchangeRefreshToken(password)
+			if err != nil {
+				return nil, err
+			}
+			password = token.AccessToken
+		}
+		saslClient = lib.NewXoauth2Client(uri.User.Username(), password)
 	default:
 		return nil, fmt.Errorf("Unsupported auth mechanism %s", auth)
 	}

doc/aerc-imap.5.scd

@@ -19,7 +19,7 @@ In accounts.conf (see *aerc-config*(5)), the following IMAP-specific options are
 available:
 
 *source*
-	imap[s][+insecure|+oauthbearer]://username[:password]@hostname[:port]?[:oauth2_params]
+	imap[s][+insecure|+oauthbearer|+xoauth2]://username[:password]@hostname[:port]?[:oauth2_params]
 
 	Remember that all fields must be URL encoded. The "@" symbol, when URL
 	encoded, is *%40*.
@@ -53,6 +53,10 @@ available:
 		Example:
 		imaps+oauthbearer://...?token_endpoint=https://...&client_id=
 
+	*imaps+xoauth2://*
+		IMAP with TLS/SSL using XOAUTH2 Authentication. Parameters are
+		the same as OAUTHBEARER.
+
 *source-cred-cmd*
 	Specifies the command to run to get the password for the IMAP
 	account. This command will be run using `sh -c [command]`. If a

doc/aerc-smtp.5.scd

@@ -16,7 +16,7 @@ In accounts.conf (see *aerc-config*(5)), the following SMTP-specific options are
 available:
 
 *outgoing*
-	smtp[s][+plain|+login|+none|+oauthbearer]://username[:password]@hostname[:port]?[:oauth2_params]
+	smtp[s][+plain|+login|+none|+oauthbearer|+xoauth2]://username[:password]@hostname[:port]?[:oauth2_params]
 
 	Remember that all fields must be URL encoded. The "@" symbol, when URL
 	encoded, is *%40*.
@@ -47,6 +47,10 @@ available:
 		SMTP with TLS/SSL using OAUTHBEARER Authentication.  See documentation in
 		*aerc-imap*(5) for usage.
 
+	*+xoauth2*:
+		SMTP with TLS/SSL using XOAUTH2 Authentication.  See documentation in
+		*aerc-imap*(5) for usage.
+
 *outgoing-cred-cmd*
 	Specifies the command to run to get the password for the SMTP
 	account. This command will be run using `sh -c [command]`. If a

lib/xoauth2.go

@@ -0,0 +1,88 @@
+//
+// This code is derived from the go-sasl library.
+//
+// Copyright (c) 2016 emersion
+// Copyright (c) 2022, Oracle and/or its affiliates.
+//
+// SPDX-License-Identifier: MIT
+
+package lib
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/emersion/go-imap/client"
+	"github.com/emersion/go-sasl"
+	"golang.org/x/oauth2"
+)
+
+// An XOAUTH2 error.
+type Xoauth2Error struct {
+	Status  string `json:"status"`
+	Schemes string `json:"schemes"`
+	Scope   string `json:"scope"`
+}
+
+// Implements error.
+func (err *Xoauth2Error) Error() string {
+	return fmt.Sprintf("XOAUTH2 authentication error (%v)", err.Status)
+}
+
+type xoauth2Client struct {
+	Username string
+	Token    string
+}
+
+func (a *xoauth2Client) Start() (mech string, ir []byte, err error) {
+	mech = "XOAUTH2"
+	ir = []byte("user=" + a.Username + "\x01auth=Bearer " + a.Token + "\x01\x01")
+	return
+}
+
+func (a *xoauth2Client) Next(challenge []byte) ([]byte, error) {
+	// Server sent an error response
+	xoauth2Err := &Xoauth2Error{}
+	if err := json.Unmarshal(challenge, xoauth2Err); err != nil {
+		return nil, err
+	} else {
+		return nil, xoauth2Err
+	}
+}
+
+// An implementation of the XOAUTH2 authentication mechanism, as
+// described in https://developers.google.com/gmail/xoauth2_protocol.
+func NewXoauth2Client(username, token string) sasl.Client {
+	return &xoauth2Client{username, token}
+}
+
+type Xoauth2 struct {
+	OAuth2  *oauth2.Config
+	Enabled bool
+}
+
+func (c *Xoauth2) ExchangeRefreshToken(refreshToken string) (*oauth2.Token, error) {
+	token := new(oauth2.Token)
+	token.RefreshToken = refreshToken
+	token.TokenType = "Bearer"
+	return c.OAuth2.TokenSource(context.TODO(), token).Token()
+}
+
+func (c *Xoauth2) Authenticate(username string, password string, client *client.Client) error {
+	if ok, err := client.SupportAuth("XOAUTH2"); err != nil || !ok {
+		return fmt.Errorf("Xoauth2 not supported %w", err)
+	}
+
+	if c.OAuth2.Endpoint.TokenURL != "" {
+		token, err := c.ExchangeRefreshToken(password)
+		if err != nil {
+			return err
+		}
+		password = token.AccessToken
+	}
+
+	saslClient := NewXoauth2Client(username, password)
+
+	return client.Authenticate(saslClient)
+}

worker/imap/configure.go

@@ -38,6 +38,21 @@ func (w *IMAPWorker) handleConfigure(msg *types.Configure) error {
 		w.config.oauthBearer.OAuth2 = oauth2
 	}
 
+	if strings.HasSuffix(w.config.scheme, "+xoauth2") {
+		w.config.scheme = strings.TrimSuffix(w.config.scheme, "+xoauth2")
+		w.config.xoauth2.Enabled = true
+		q := u.Query()
+
+		oauth2 := &oauth2.Config{}
+		if q.Get("token_endpoint") != "" {
+			oauth2.ClientID = q.Get("client_id")
+			oauth2.ClientSecret = q.Get("client_secret")
+			oauth2.Scopes = []string{q.Get("scope")}
+			oauth2.Endpoint.TokenURL = q.Get("token_endpoint")
+		}
+		w.config.xoauth2.OAuth2 = oauth2
+	}
+
 	w.config.addr = u.Host
 	if !strings.ContainsRune(w.config.addr, ':') {
 		w.config.addr += ":" + w.config.scheme

worker/imap/connect.go

@@ -80,6 +80,11 @@ func (w *IMAPWorker) connect() (*client.Client, error) {
 				username, password, c); err != nil {
 				return nil, err
 			}
+		} else if w.config.xoauth2.Enabled {
+			if err := w.config.xoauth2.Authenticate(
+				username, password, c); err != nil {
+				return nil, err
+			}
 		} else if err := c.Login(username, password); err != nil {
 			return nil, err
 		}

worker/imap/worker.go

@@ -43,6 +43,7 @@ type imapConfig struct {
 	user              *url.Userinfo
 	folders           []string
 	oauthBearer       lib.OAuthBearer
+	xoauth2           lib.Xoauth2
 	idle_timeout      time.Duration
 	idle_debounce     time.Duration
 	reconnect_maxwait time.Duration