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prevent unicode-confusion in password by applying PRECIS, and username/email address by applying unicode NFC normalization

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an é (e with accent) can also be written as e+\u0301. the first form is NFC,
the second NFD. when logging in, we transform usernames (email addresses) to
NFC. so both forms will be accepted. if a client is using NFD, they can log
in too.

for passwords, we apply the PRECIS "opaquestring", which (despite the name)
transforms the value too: unicode spaces are replaced with ascii spaces. the
string is also normalized to NFC. PRECIS may reject confusing passwords when
you set a password.
This commit is contained in:
Mechiel Lukkien
2024-03-08 23:29:15 +01:00
parent 8e6fe7459b
commit c57aeac7f0
99 changed files with 59625 additions and 114 deletions
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27
smtpserver/server.go
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@ -28,6 +28,7 @@ import (
"time"
"golang.org/x/exp/maps"
"golang.org/x/text/unicode/norm"
"github.com/prometheus/client_golang/prometheus"
"github.com/prometheus/client_golang/prometheus/promauto"
@ -962,8 +963,6 @@ func (c *conn) cmdAuth(p *parser) {
xsmtpUserErrorf(smtp.C503BadCmdSeq, smtp.SeProto5BadCmdOrSeq1, "authentication not allowed during mail transaction")
}
// todo future: we may want to normalize usernames and passwords, see stringprep in ../rfc/4013:38 and possibly newer mechanisms (though they are opt-in and that may not have happened yet).
// If authentication fails due to missing derived secrets, we don't hold it against
// the connection. There is no way to indicate server support for an authentication
// mechanism, but that a mechanism won't work for an account.
@ -1071,8 +1070,8 @@ func (c *conn) cmdAuth(p *parser) {
if len(plain) != 3 {
xsmtpUserErrorf(smtp.C501BadParamSyntax, smtp.SeProto5BadParams4, "auth data should have 3 nul-separated tokens, got %d", len(plain))
}
authz := string(plain[0])
authc := string(plain[1])
authz := norm.NFC.String(string(plain[0]))
authc := norm.NFC.String(string(plain[1]))
password := string(plain[2])
if authz != "" && authz != authc {
@ -1115,6 +1114,7 @@ func (c *conn) cmdAuth(p *parser) {
// I-D says maximum length must be 64 bytes. We allow more, for long user names
// (domains).
username := string(xreadInitial())
username = norm.NFC.String(username)
// Again, client should ignore the challenge, we send the same as the example in
// the I-D.
@ -1156,7 +1156,7 @@ func (c *conn) cmdAuth(p *parser) {
if len(t) != 2 || len(t[1]) != 2*md5.Size {
xsmtpUserErrorf(smtp.C501BadParamSyntax, smtp.SeProto5BadParams4, "malformed cram-md5 response")
}
addr := t[0]
addr := norm.NFC.String(t[0])
c.log.Debug("cram-md5 auth", slog.String("address", addr))
acc, _, err := store.OpenEmail(c.log, addr)
if err != nil {
@ -1245,13 +1245,14 @@ func (c *conn) cmdAuth(p *parser) {
c0 := xreadInitial()
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)
authc := norm.NFC.String(ss.Authentication)
c.log.Debug("scram auth", slog.String("authentication", authc))
acc, _, err := store.OpenEmail(c.log, authc)
if err != nil {
// todo: we could continue scram with a generated salt, deterministically generated
// from the username. that way we don't have to store anything but attackers cannot
// learn if an account exists. same for absent scram saltedpassword below.
c.log.Info("failed authentication attempt", slog.String("username", ss.Authentication), slog.Any("remote", c.remoteIP))
c.log.Info("failed authentication attempt", slog.String("username", authc), slog.Any("remote", c.remoteIP))
xsmtpUserErrorf(smtp.C454TempAuthFail, smtp.SeSys3Other0, "scram not possible")
}
defer func() {
@ -1268,7 +1269,7 @@ func (c *conn) cmdAuth(p *parser) {
err := acc.DB.Read(context.TODO(), func(tx *bstore.Tx) error {
password, err := bstore.QueryTx[store.Password](tx).Get()
if err == bstore.ErrAbsent {
c.log.Info("failed authentication attempt", slog.String("username", ss.Authentication), slog.Any("remote", c.remoteIP))
c.log.Info("failed authentication attempt", slog.String("username", authc), slog.Any("remote", c.remoteIP))
xsmtpUserErrorf(smtp.C535AuthBadCreds, smtp.SePol7AuthBadCreds8, "bad user/pass")
}
xcheckf(err, "fetching credentials")
@ -1282,8 +1283,8 @@ func (c *conn) cmdAuth(p *parser) {
}
if len(xscram.Salt) == 0 || xscram.Iterations == 0 || len(xscram.SaltedPassword) == 0 {
missingDerivedSecrets = true
c.log.Info("scram auth attempt without derived secrets set, save password again to store secrets", slog.String("address", ss.Authentication))
c.log.Info("failed authentication attempt", slog.String("username", ss.Authentication), slog.Any("remote", c.remoteIP))
c.log.Info("scram auth attempt without derived secrets set, save password again to store secrets", slog.String("address", authc))
c.log.Info("failed authentication attempt", slog.String("username", authc), slog.Any("remote", c.remoteIP))
xsmtpUserErrorf(smtp.C454TempAuthFail, smtp.SeSys3Other0, "scram not possible")
}
return nil
@ -1302,7 +1303,7 @@ func (c *conn) cmdAuth(p *parser) {
c.readline() // Should be "*" for cancellation.
if errors.Is(err, scram.ErrInvalidProof) {
authResult = "badcreds"
c.log.Info("failed authentication attempt", slog.String("username", ss.Authentication), slog.Any("remote", c.remoteIP))
c.log.Info("failed authentication attempt", slog.String("username", authc), slog.Any("remote", c.remoteIP))
xsmtpUserErrorf(smtp.C535AuthBadCreds, smtp.SePol7AuthBadCreds8, "bad credentials")
}
xcheckf(err, "server final")
@ -1317,7 +1318,7 @@ func (c *conn) cmdAuth(p *parser) {
c.setSlow(false)
c.account = acc
acc = nil // Cancel cleanup.
c.username = ss.Authentication
c.username = authc
// ../rfc/4954:276
c.writecodeline(smtp.C235AuthSuccess, smtp.SePol7Other0, "nice", nil)