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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
Download Patch File Download Diff File Expand all files Collapse all files

61
sendmail.go
View File

@ -3,6 +3,7 @@ package main
import (
"bufio"
"context"
"crypto/tls"
"errors"
"fmt"
"io"
@ -14,6 +15,8 @@ import (
"strings"
"time"
"golang.org/x/exp/slices"
"github.com/mjl-/sconf"
"github.com/mjl-/mox/dns"
@ -30,8 +33,8 @@ var submitconf struct {
TLS bool `sconf-doc:"Connect with TLS. Usually for connections to port 465."`
STARTTLS bool `sconf-doc:"After starting in plain text, use STARTTLS to enable TLS. For port 587 and 25."`
Username string `sconf-doc:"For SMTP authentication."`
Password string `sconf-doc:"For password-based SMTP authentication, e.g. SCRAM-SHA-256, SCRAM-SHA-1, CRAM-MD5, PLAIN."`
AuthMethod string `sconf-doc:"If set, only attempt this authentication mechanism. E.g. SCRAM-SHA-256. If not set, any mutually supported algorithm can be used, in order of most to least secure."`
Password string `sconf-doc:"For password-based SMTP authentication, e.g. SCRAM-SHA-256-PLUS, CRAM-MD5, PLAIN."`
AuthMethod string `sconf-doc:"If set, only attempt this authentication mechanism. E.g. SCRAM-SHA-256-PLUS, SCRAM-SHA-256, SCRAM-SHA-1-PLUS, SCRAM-SHA-1, CRAM-MD5, PLAIN. If not set, any mutually supported algorithm can be used, in order listed, from most to least secure. It is recommended to specify the strongest authentication mechanism known to be implemented by the server, to prevent mechanism downgrade attacks."`
From string `sconf-doc:"Address for MAIL FROM in SMTP and From-header in message."`
DefaultDestination string `sconf:"optional" sconf-doc:"Used when specified address does not contain an @ and may be a local user (eg root)."`
RequireTLS RequireTLSOption `sconf:"optional" sconf-doc:"If yes, submission server must implement SMTP REQUIRETLS extension, and connection to submission server must use verified TLS. If no, a TLS-Required header with value no is added to the message, allowing fallback to unverified TLS or plain text delivery despite recpient domain policies. By default, the submission server will follow the policies of the recipient domain (MTA-STS and/or DANE), and apply unverified opportunistic TLS with STARTTLS."`
@ -252,23 +255,45 @@ binary should be setgid that group:
conn, err := d.Dial("tcp", addr)
xsavecheckf(err, "dial submit server")
var auth []sasl.Client
switch submitconf.AuthMethod {
case "SCRAM-SHA-256":
auth = []sasl.Client{sasl.NewClientSCRAMSHA256(submitconf.Username, submitconf.Password)}
case "SCRAM-SHA-1":
auth = []sasl.Client{sasl.NewClientSCRAMSHA1(submitconf.Username, submitconf.Password)}
case "CRAM-MD5":
auth = []sasl.Client{sasl.NewClientCRAMMD5(submitconf.Username, submitconf.Password)}
case "PLAIN":
auth = []sasl.Client{sasl.NewClientPlain(submitconf.Username, submitconf.Password)}
default:
auth = []sasl.Client{
sasl.NewClientSCRAMSHA256(submitconf.Username, submitconf.Password),
sasl.NewClientSCRAMSHA1(submitconf.Username, submitconf.Password),
sasl.NewClientCRAMMD5(submitconf.Username, submitconf.Password),
sasl.NewClientPlain(submitconf.Username, submitconf.Password),
auth := func(mechanisms []string, cs *tls.ConnectionState) (sasl.Client, error) {
// Check explicitly configured mechanisms.
switch submitconf.AuthMethod {
case "SCRAM-SHA-256-PLUS":
if cs == nil {
return nil, fmt.Errorf("scram plus authentication mechanism requires tls")
}
return sasl.NewClientSCRAMSHA256PLUS(submitconf.Username, submitconf.Password, *cs), nil
case "SCRAM-SHA-256":
return sasl.NewClientSCRAMSHA256(submitconf.Username, submitconf.Password, false), nil
case "SCRAM-SHA-1-PLUS":
if cs == nil {
return nil, fmt.Errorf("scram plus authentication mechanism requires tls")
}
return sasl.NewClientSCRAMSHA1PLUS(submitconf.Username, submitconf.Password, *cs), nil
case "SCRAM-SHA-1":
return sasl.NewClientSCRAMSHA1(submitconf.Username, submitconf.Password, false), nil
case "CRAM-MD5":
return sasl.NewClientCRAMMD5(submitconf.Username, submitconf.Password), nil
case "PLAIN":
return sasl.NewClientPlain(submitconf.Username, submitconf.Password), nil
}
// Try the defaults, from more to less secure.
if cs != nil && slices.Contains(mechanisms, "SCRAM-SHA-256-PLUS") {
return sasl.NewClientSCRAMSHA256PLUS(submitconf.Username, submitconf.Password, *cs), nil
} else if slices.Contains(mechanisms, "SCRAM-SHA-256") {
return sasl.NewClientSCRAMSHA256(submitconf.Username, submitconf.Password, true), nil
} else if cs != nil && slices.Contains(mechanisms, "SCRAM-SHA-1-PLUS") {
return sasl.NewClientSCRAMSHA1PLUS(submitconf.Username, submitconf.Password, *cs), nil
} else if slices.Contains(mechanisms, "SCRAM-SHA-1") {
return sasl.NewClientSCRAMSHA1(submitconf.Username, submitconf.Password, true), nil
} else if slices.Contains(mechanisms, "CRAM-MD5") {
return sasl.NewClientCRAMMD5(submitconf.Username, submitconf.Password), nil
} else if slices.Contains(mechanisms, "PLAIN") {
return sasl.NewClientPlain(submitconf.Username, submitconf.Password), nil
}
// No mutually supported mechanism.
return nil, nil
}
ctx, cancel := context.WithTimeout(context.Background(), 3*time.Minute)