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implement the plus variants of scram, to bind the authentication exchange to the tls connection

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to get the security benefits (detecting mitm attempts), explicitly configure
clients to use a scram plus variant, e.g. scram-sha-256-plus. unfortunately,
not many clients support it yet.

imapserver scram plus support seems to work with the latest imtest (imap test
client) from cyrus-sasl. no success yet with mutt (with gsasl) though.
This commit is contained in:
Mechiel Lukkien
2023-12-23 23:07:21 +01:00
parent 4701857d7f
commit e7478ed6ac
23 changed files with 690 additions and 189 deletions
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47
smtpserver/server.go
View File

@ -869,7 +869,12 @@ func (c *conn) cmdHello(p *parser, ehlo bool) {
if c.submission {
// ../rfc/4954:123
if c.tls || !c.requireTLSForAuth {
c.bwritelinef("250-AUTH SCRAM-SHA-256 SCRAM-SHA-1 CRAM-MD5 PLAIN LOGIN")
// We always mention the SCRAM PLUS variants, even if TLS is not active: It is a
// hint to the client that a TLS connection can use TLS channel binding during
// authentication. The client should select the bare variant when TLS isn't
// present, and also not indicate the server supports the PLUS variant in that
// case, or it would trigger the mechanism downgrade detection.
c.bwritelinef("250-AUTH SCRAM-SHA-256-PLUS SCRAM-SHA-256 SCRAM-SHA-1-PLUS SCRAM-SHA-1 CRAM-MD5 PLAIN LOGIN")
} else {
c.bwritelinef("250-AUTH ")
}
@ -1190,22 +1195,35 @@ func (c *conn) cmdAuth(p *parser) {
// ../rfc/4954:276
c.writecodeline(smtp.C235AuthSuccess, smtp.SePol7Other0, "nice", nil)
case "SCRAM-SHA-1", "SCRAM-SHA-256":
case "SCRAM-SHA-256-PLUS", "SCRAM-SHA-256", "SCRAM-SHA-1-PLUS", "SCRAM-SHA-1":
// todo: improve handling of errors during scram. e.g. invalid parameters. should we abort the imap command, or continue until the end and respond with a scram-level error?
// todo: use single implementation between ../imapserver/server.go and ../smtpserver/server.go
authVariant = strings.ToLower(mech)
var h func() hash.Hash
if authVariant == "scram-sha-1" {
h = sha1.New
} else {
h = sha256.New
}
// Passwords cannot be retrieved or replayed from the trace.
authVariant = strings.ToLower(mech)
var h func() hash.Hash
switch authVariant {
case "scram-sha-1", "scram-sha-1-plus":
h = sha1.New
case "scram-sha-256", "scram-sha-256-plus":
h = sha256.New
default:
xsmtpServerErrorf(codes{smtp.C554TransactionFailed, smtp.SeSys3Other0}, "missing scram auth method case")
}
var cs *tls.ConnectionState
channelBindingRequired := strings.HasSuffix(authVariant, "-plus")
if channelBindingRequired && !c.tls {
// ../rfc/4954:630
xsmtpUserErrorf(smtp.C538EncReqForAuth, smtp.SePol7EncReqForAuth11, "scram plus mechanism requires tls connection")
}
if c.tls {
xcs := c.conn.(*tls.Conn).ConnectionState()
cs = &xcs
}
c0 := xreadInitial()
ss, err := scram.NewServer(h, c0)
ss, err := scram.NewServer(h, c0, cs, channelBindingRequired)
xcheckf(err, "starting scram")
c.log.Debug("scram auth", slog.String("authentication", ss.Authentication))
acc, _, err := store.OpenEmail(c.log, ss.Authentication)
@ -1229,10 +1247,13 @@ func (c *conn) cmdAuth(p *parser) {
acc.WithRLock(func() {
err := acc.DB.Read(context.TODO(), func(tx *bstore.Tx) error {
password, err := bstore.QueryTx[store.Password](tx).Get()
if authVariant == "scram-sha-1" {
switch authVariant {
case "scram-sha-1", "scram-sha-1-plus":
xscram = password.SCRAMSHA1
} else {
case "scram-sha-256", "scram-sha-256-plus":
xscram = password.SCRAMSHA256
default:
xsmtpServerErrorf(codes{smtp.C554TransactionFailed, smtp.SeSys3Other0}, "missing scram auth credentials case")
}
if err == bstore.ErrAbsent || err == nil && (len(xscram.Salt) == 0 || xscram.Iterations == 0 || len(xscram.SaltedPassword) == 0) {
c.log.Info("scram auth attempt without derived secrets set, save password again to store secrets", slog.String("address", ss.Authentication))

40
smtpserver/server_test.go
View File

@ -89,7 +89,7 @@ type testserver struct {
comm *store.Comm
cid int64
resolver dns.Resolver
auth []sasl.Client
auth func(mechanisms []string, cs *tls.ConnectionState) (sasl.Client, error)
user, pass string
submission bool
requiretls bool
@ -159,11 +159,11 @@ func (ts *testserver) run(fn func(helloErr error, client *smtpclient.Client)) {
close(serverdone)
}()
var auth []sasl.Client
if len(ts.auth) > 0 {
auth = ts.auth
} else if ts.user != "" {
auth = append(auth, sasl.NewClientPlain(ts.user, ts.pass))
auth := ts.auth
if auth == nil && ts.user != "" {
auth = func(mechanisms []string, cs *tls.ConnectionState) (sasl.Client, error) {
return sasl.NewClientPlain(ts.user, ts.pass), nil
}
}
ourHostname := mox.Conf.Static.HostnameDomain
@ -234,10 +234,12 @@ func TestSubmission(t *testing.T) {
}
mox.Conf.Dynamic.Domains["mox.example"] = dom
testAuth := func(authfn func(user, pass string) sasl.Client, user, pass string, expErr *smtpclient.Error) {
testAuth := func(authfn func(user, pass string, cs *tls.ConnectionState) sasl.Client, user, pass string, expErr *smtpclient.Error) {
t.Helper()
if authfn != nil {
ts.auth = []sasl.Client{authfn(user, pass)}
ts.auth = func(mechanisms []string, cs *tls.ConnectionState) (sasl.Client, error) {
return authfn(user, pass, cs), nil
}
} else {
ts.auth = nil
}
@ -258,12 +260,22 @@ func TestSubmission(t *testing.T) {
ts.submission = true
testAuth(nil, "", "", &smtpclient.Error{Permanent: true, Code: smtp.C530SecurityRequired, Secode: smtp.SePol7Other0})
authfns := []func(user, pass string) sasl.Client{
sasl.NewClientPlain,
sasl.NewClientLogin,
sasl.NewClientCRAMMD5,
sasl.NewClientSCRAMSHA1,
sasl.NewClientSCRAMSHA256,
authfns := []func(user, pass string, cs *tls.ConnectionState) sasl.Client{
func(user, pass string, cs *tls.ConnectionState) sasl.Client { return sasl.NewClientPlain(user, pass) },
func(user, pass string, cs *tls.ConnectionState) sasl.Client { return sasl.NewClientLogin(user, pass) },
func(user, pass string, cs *tls.ConnectionState) sasl.Client { return sasl.NewClientCRAMMD5(user, pass) },
func(user, pass string, cs *tls.ConnectionState) sasl.Client {
return sasl.NewClientSCRAMSHA1(user, pass, false)
},
func(user, pass string, cs *tls.ConnectionState) sasl.Client {
return sasl.NewClientSCRAMSHA256(user, pass, false)
},
func(user, pass string, cs *tls.ConnectionState) sasl.Client {
return sasl.NewClientSCRAMSHA1PLUS(user, pass, *cs)
},
func(user, pass string, cs *tls.ConnectionState) sasl.Client {
return sasl.NewClientSCRAMSHA256PLUS(user, pass, *cs)
},
}
for _, fn := range authfns {
testAuth(fn, "mjl@mox.example", "test", &smtpclient.Error{Secode: smtp.SePol7AuthBadCreds8}) // Bad (short) password.