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go-chasquid-smtp/internal/dkim/verify_test.go
T
Alberto Bertogli 10aa1a55d0 dkim: Validate domain in s= and d= tags
We currently use the s= and d= tags as-is without any validation.

They are used to construct the DNS name to look up the key, which is
generally fine, but also the d= tag is used as-is in the
Authentication-Results header.

The latter can be used to inject malicious content, for example:

  d=example.com dkim=pass header.d=example.net

can result in the header including:

  header.d=example.com dkim=pass header.d=example.net

regardless of the validation status.

To fix that problem, this patch now validates the domain (using the same
normalization we use in other areas, like RCPT TO and MAIL FROM) on both
tags.

If any of the tags have invalid domains, we consider the signature
invalid as per RFC 6376 section 3.5.
2026-10-12 19:53:47 +01:00

593 lines
18 KiB
Go

package dkim
import (
"context"
"crypto"
"encoding/base64"
"errors"
"fmt"
"net"
"strings"
"testing"
"github.com/google/go-cmp/cmp"
)
func toCRLF(s string) string {
return strings.ReplaceAll(s, "\n", "\r\n")
}
func makeLookupTXT(results map[string][]string) lookupTXTFunc {
return func(ctx context.Context, domain string) ([]string, error) {
return results[domain], nil
}
}
func TestVerifyRF6376CExample(t *testing.T) {
ctx := t.Context()
ctx = WithTraceFunc(ctx, t.Logf)
// Use the public key from the example in RFC 6376 appendix C.
// https://datatracker.ietf.org/doc/html/rfc6376#appendix-C
ctx = WithLookupTXTFunc(ctx, makeLookupTXT(map[string][]string{
"brisbane._domainkey.example.com": {
"v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ" +
"KBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYt" +
"IxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v" +
"/RtdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhi" +
"tdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB",
},
}))
// Note that the examples in the RFC text have multiple issues:
// - The double space in "game. Are" should be a single
// space. Otherwise, the body hash does not match.
// https://www.rfc-editor.org/errata/eid3192
// - The header indentation is incorrect. This causes
// signature validation failure (because the example uses simple
// canonicalization, which leaves the indentation untouched).
// https://www.rfc-editor.org/errata/eid4926
message := toCRLF(
`DKIM-Signature: v=1; a=rsa-sha256; s=brisbane; d=example.com;
c=simple/simple; q=dns/txt; i=joe@football.example.com;
h=Received : From : To : Subject : Date : Message-ID;
bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
b=AuUoFEfDxTDkHlLXSZEpZj79LICEps6eda7W3deTVFOk4yAUoqOB
4nujc7YopdG5dWLSdNg6xNAZpOPr+kHxt1IrE+NahM6L/LbvaHut
KVdkLLkpVaVVQPzeRDI009SO2Il5Lu7rDNH6mZckBdrIx0orEtZV
4bmp/YzhwvcubU4=;
Received: from client1.football.example.com [192.0.2.1]
by submitserver.example.com with SUBMISSION;
Fri, 11 Jul 2003 21:01:54 -0700 (PDT)
From: Joe SixPack <joe@football.example.com>
To: Suzie Q <suzie@shopping.example.net>
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.
`)
res, err := VerifyMessage(ctx, message)
if res.Valid != 1 || err != nil {
t.Errorf("VerifyMessage: wanted 1 valid / nil; got %v / %v", res, err)
}
// Extend the message, check it does not pass validation.
res, err = VerifyMessage(ctx, message+"Extra line.\r\n")
if res.Valid != 0 || err != nil {
t.Errorf("VerifyMessage: wanted 0 valid / nil; got %v / %v", res, err)
}
// Alter a header, check it does not pass validation.
res, err = VerifyMessage(ctx,
strings.Replace(message, "Subject", "X-Subject", 1))
if res.Valid != 0 || err != nil {
t.Errorf("VerifyMessage: wanted 0 valid / nil; got %v / %v", res, err)
}
}
func TestVerifyRFC8463Example(t *testing.T) {
ctx := t.Context()
ctx = WithTraceFunc(ctx, t.Logf)
// Use the public keys from the example in RFC 8463 appendix A.2.
// https://datatracker.ietf.org/doc/html/rfc6376#appendix-C
ctx = WithLookupTXTFunc(ctx, makeLookupTXT(map[string][]string{
"brisbane._domainkey.football.example.com": {
"v=DKIM1; k=ed25519; " +
"p=11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo="},
"test._domainkey.football.example.com": {
"v=DKIM1; k=rsa; " +
"p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDkHlOQoBTzWR" +
"iGs5V6NpP3idY6Wk08a5qhdR6wy5bdOKb2jLQiY/J16JYi0Qvx/b" +
"yYzCNb3W91y3FutACDfzwQ/BC/e/8uBsCR+yz1Lxj+PL6lHvqMKr" +
"M3rG4hstT5QjvHO9PzoxZyVYLzBfO2EeC3Ip3G+2kryOTIKT+l/K" +
"4w3QIDAQAB"},
}))
message := toCRLF(
`DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed;
d=football.example.com; i=@football.example.com;
q=dns/txt; s=brisbane; t=1528637909; h=from : to :
subject : date : message-id : from : subject : date;
bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
b=/gCrinpcQOoIfuHNQIbq4pgh9kyIK3AQUdt9OdqQehSwhEIug4D11Bus
Fa3bT3FY5OsU7ZbnKELq+eXdp1Q1Dw==
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=football.example.com; i=@football.example.com;
q=dns/txt; s=test; t=1528637909; h=from : to : subject :
date : message-id : from : subject : date;
bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
b=F45dVWDfMbQDGHJFlXUNB2HKfbCeLRyhDXgFpEL8GwpsRe0IeIixNTe3
DhCVlUrSjV4BwcVcOF6+FF3Zo9Rpo1tFOeS9mPYQTnGdaSGsgeefOsk2Jz
dA+L10TeYt9BgDfQNZtKdN1WO//KgIqXP7OdEFE4LjFYNcUxZQ4FADY+8=
From: Joe SixPack <joe@football.example.com>
To: Suzie Q <suzie@shopping.example.net>
Subject: Is dinner ready?
Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT)
Message-ID: <20030712040037.46341.5F8J@football.example.com>
Hi.
We lost the game. Are you hungry yet?
Joe.
`)
expected := &VerifyResult{
Found: 2,
Valid: 2,
Results: []*OneResult{
{
SignatureHeader: toCRLF(
` v=1; a=ed25519-sha256; c=relaxed/relaxed;
d=football.example.com; i=@football.example.com;
q=dns/txt; s=brisbane; t=1528637909; h=from : to :
subject : date : message-id : from : subject : date;
bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
b=/gCrinpcQOoIfuHNQIbq4pgh9kyIK3AQUdt9OdqQehSwhEIug4D11Bus
Fa3bT3FY5OsU7ZbnKELq+eXdp1Q1Dw==`),
Domain: "football.example.com",
Selector: "brisbane",
B: "/gCrinpcQOoIfuHNQIbq4pgh9kyIK3AQUdt9OdqQehSwhEIug4D11" +
"BusFa3bT3FY5OsU7ZbnKELq+eXdp1Q1Dw==",
State: SUCCESS,
Error: nil,
},
{
SignatureHeader: toCRLF(
` v=1; a=rsa-sha256; c=relaxed/relaxed;
d=football.example.com; i=@football.example.com;
q=dns/txt; s=test; t=1528637909; h=from : to : subject :
date : message-id : from : subject : date;
bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=;
b=F45dVWDfMbQDGHJFlXUNB2HKfbCeLRyhDXgFpEL8GwpsRe0IeIixNTe3
DhCVlUrSjV4BwcVcOF6+FF3Zo9Rpo1tFOeS9mPYQTnGdaSGsgeefOsk2Jz
dA+L10TeYt9BgDfQNZtKdN1WO//KgIqXP7OdEFE4LjFYNcUxZQ4FADY+8=`),
Domain: "football.example.com",
Selector: "test",
B: "F45dVWDfMbQDGHJFlXUNB2HKfbCeLRyhDXgFpEL8GwpsRe0IeIixNTe" +
"3DhCVlUrSjV4BwcVcOF6+FF3Zo9Rpo1tFOeS9mPYQTnGdaSGsgeefO" +
"sk2JzdA+L10TeYt9BgDfQNZtKdN1WO//KgIqXP7OdEFE4LjFYNcUxZ" +
"Q4FADY+8=",
State: SUCCESS,
Error: nil,
},
},
}
res, err := VerifyMessage(ctx, message)
if err != nil {
t.Fatalf("VerifyMessage returned error: %v", err)
}
if diff := cmp.Diff(expected, res); diff != "" {
t.Errorf("VerifyMessage diff (-want +got):\n%s", diff)
}
// Extend the message, check it does not pass validation.
res, err = VerifyMessage(ctx, message+"Extra line.\r\n")
if res.Found != 2 || res.Valid != 0 || err != nil {
t.Errorf("VerifyMessage: wanted 2 found, 0 valid / nil; got %v / %v",
res, err)
}
// Alter a header, check it does not pass validation.
res, err = VerifyMessage(ctx,
strings.Replace(message, "Subject", "X-Subject", 1))
if res.Found != 2 || res.Valid != 0 || err != nil {
t.Errorf("VerifyMessage: wanted 2 found, 0 valid / nil; got %v / %v",
res, err)
}
}
func TestVerifyBodyLength(t *testing.T) {
ctx := t.Context()
ctx = WithTraceFunc(ctx, t.Logf)
// Any valid key works, since none of the signatures here are valid. We
// use the one from the example in RFC 8463 appendix A.2.
ctx = WithLookupTXTFunc(ctx, makeLookupTXT(map[string][]string{
"brisbane._domainkey.example.com": {
"v=DKIM1; k=ed25519; " +
"p=11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo="},
}))
// Use a valid body hash (of the "signed" part of the body), so we can
// tell the l= cases apart: when l= covers exactly the signed part, the
// body hash matches and we fail later on (because b= is bogus).
body := "Hi.\r\n"
cases := []struct {
l string
signed string
want error
}{
{"0", "", ErrVerificationFailed},
{"0", body, ErrBodyHashMismatch},
{"4", body, ErrBodyHashMismatch},
{"5", body, ErrVerificationFailed},
{"6", body, errInvalidTag},
{"18446744073709551615", body, errInvalidTag},
}
for _, c := range cases {
bh := base64.StdEncoding.EncodeToString(
hashWith(crypto.SHA256, []byte(c.signed)))
message := toCRLF(
"DKIM-Signature: v=1; a=ed25519-sha256; c=simple/simple;\n"+
" d=example.com; s=brisbane; h=from; l="+c.l+";\n"+
" bh="+bh+"; b=AAAA\n"+
"From: Joe SixPack <joe@example.com>\n"+
"\n") + body
res, err := VerifyMessage(ctx, message)
if err != nil {
t.Fatalf("l=%s signed=%q: VerifyMessage returned error: %v",
c.l, c.signed, err)
}
if res.Found != 1 || res.Valid != 0 {
t.Fatalf("l=%s signed=%q: wanted 1 found, 0 valid; got %v",
c.l, c.signed, res)
}
r := res.Results[0]
if r.State != PERMFAIL || !errors.Is(r.Error, c.want) {
t.Errorf("l=%s signed=%q: wanted PERMFAIL / %v; got %v / %v",
c.l, c.signed, c.want, r.State, r.Error)
}
}
}
func TestVerifyInvalidDomain(t *testing.T) {
ctx := t.Context()
ctx = WithTraceFunc(ctx, t.Logf)
ctx = WithLookupTXTFunc(ctx, makeLookupTXT(map[string][]string{}))
// A d= tag that is not a valid domain must be rejected, and must not
// make it to the Authentication-Results header, where it could inject
// content.
message := toCRLF(
"DKIM-Signature: v=1; a=ed25519-sha256; c=simple/simple;\n" +
" d=example.com dkim=pass header.d=example.net; s=sel;\n" +
" h=from; bh=AAAA; b=AAAA\n" +
"From: Joe SixPack <joe@example.com>\n" +
"\n" +
"Hi.\n")
res, err := VerifyMessage(ctx, message)
if err != nil {
t.Fatalf("VerifyMessage returned error: %v", err)
}
if res.Found != 1 || res.Valid != 0 {
t.Fatalf("wanted 1 found, 0 valid; got %v", res)
}
r := res.Results[0]
if r.State != PERMFAIL || !errors.Is(r.Error, errInvalidTag) {
t.Errorf("wanted PERMFAIL / %v; got %v / %v",
errInvalidTag, r.State, r.Error)
}
ar := res.AuthenticationResults()
if strings.Contains(ar, "example.net") {
t.Errorf("Authentication-Results contains the invalid d=: %q", ar)
}
}
func TestHeadersToInclude(t *testing.T) {
// Test that headersToInclude returns the expected headers.
cases := []struct {
sigH header
hTag []string
headers headers
want []header
}{
// Check that if a header appears more than once, we pick the latest
// first.
{
sigH: header{
Name: "DKIM-Signature",
Value: "v=1; a=rsa-sha256; s=brisbane; d=example.com;",
},
hTag: []string{"From", "To", "Subject"},
headers: headers{
{Name: "From", Value: "from1"},
{Name: "To", Value: "to1"},
{Name: "Subject", Value: "subject1"},
{Name: "From", Value: "from2"},
},
want: []header{
{Name: "From", Value: "from2"},
{Name: "To", Value: "to1"},
{Name: "Subject", Value: "subject1"},
},
},
// Check that if a header is requested twice but only appears once, we
// only return it once.
// This is a common technique suggested by the RFC to make signatures
// fail if a header is added.
{
sigH: header{
Name: "DKIM-Signature",
Value: "v=1; a=rsa-sha256; s=brisbane; d=example.com;",
},
hTag: []string{"From", "From", "To", "Subject"},
headers: headers{
{Name: "From", Value: "from1"},
{Name: "To", Value: "to1"},
{Name: "Subject", Value: "subject1"},
},
want: []header{
{Name: "From", Value: "from1"},
{Name: "To", Value: "to1"},
{Name: "Subject", Value: "subject1"},
},
},
// Check that if DKIM-Signature is included, we do *not* include the
// one we're currently verifying in the headers to include.
// https://datatracker.ietf.org/doc/html/rfc6376#section-3.7
{
sigH: header{
Name: "DKIM-Signature",
Value: "v=1; a=rsa-sha256; s=brisbane; d=example.com;",
},
hTag: []string{"From", "From", "DKIM-Signature", "DKIM-Signature"},
headers: headers{
{Name: "From", Value: "from1"},
{Name: "To", Value: "to1"},
{
Name: "DKIM-Signature",
Value: "v=1; a=rsa-sha256; s=sidney; d=example.com;",
},
{
Name: "DKIM-Signature",
Value: "v=1; a=rsa-sha256; s=brisbane; d=example.com;",
},
},
want: []header{
{Name: "From", Value: "from1"},
{
Name: "DKIM-Signature",
Value: "v=1; a=rsa-sha256; s=sidney; d=example.com;",
},
},
},
// Check that header names are compared case-insensitively, and that
// headers that are not present are skipped.
{
sigH: header{
Name: "DKIM-Signature",
Value: "v=1; a=rsa-sha256; s=brisbane; d=example.com;",
},
hTag: []string{"from", "X-Missing", "FROM", "subject"},
headers: headers{
{Name: "FROM", Value: "from1"},
{Name: "Subject", Value: "subject1"},
{Name: "From", Value: "from2"},
},
want: []header{
{Name: "From", Value: "from2"},
{Name: "FROM", Value: "from1"},
{Name: "Subject", Value: "subject1"},
},
},
}
for _, c := range cases {
got := headersToInclude(c.sigH, c.hTag, c.headers)
if diff := cmp.Diff(c.want, got); diff != "" {
t.Errorf("headersToInclude(%q, %v, %v) diff (-want +got):\n%s",
c.sigH, c.hTag, c.headers, diff)
}
}
}
func TestHeadersToIncludeLarge(t *testing.T) {
// Regression test for a DoS where headersToInclude was O(len(h=) *
// len(headers)), which with large (attacker-controlled) inputs could take
// hours. This takes milliseconds, while the quadratic implementation
// took minutes with this input. We don't check the timing, so a
// regression will show up as this test being very slow.
const n = 100_000
sigH := header{Name: "DKIM-Signature", Value: "v=1;"}
hTag := make([]string, n)
hdrs := make(headers, n)
for i := range n {
hTag[i] = "a"
hdrs[i] = header{Name: "a", Value: fmt.Sprint(i)}
}
got := headersToInclude(sigH, hTag, hdrs)
if len(got) != n {
t.Fatalf("wanted %d headers, got %d", n, len(got))
}
// Last instance first.
if got[0].Value != fmt.Sprint(n-1) || got[n-1].Value != "0" {
t.Errorf("unexpected order: first %q, last %q",
got[0].Value, got[n-1].Value)
}
}
func TestAuthenticationResults(t *testing.T) {
resBrisbane := &OneResult{
Domain: "football.example.com",
Selector: "brisbane",
B: "/gCrinpcQOoIfuHNQIbq4pgh9kyIK3AQUdt9OdqQehSwhEIug4D11" +
"BusFa3bT3FY5OsU7ZbnKELq+eXdp1Q1Dw==",
State: SUCCESS,
Error: nil,
}
resTest := &OneResult{
Domain: "football.example.com",
Selector: "test",
B: "F45dVWDfMbQDGHJFlXUNB2HKfbCeLRyhDXgFpEL8GwpsRe0IeIixNTe" +
"3DhCVlUrSjV4BwcVcOF6+FF3Zo9Rpo1tFOeS9mPYQTnGdaSGsgeefO" +
"sk2JzdA+L10TeYt9BgDfQNZtKdN1WO//KgIqXP7OdEFE4LjFYNcUxZ" +
"Q4FADY+8=",
State: SUCCESS,
Error: nil,
}
resFail := &OneResult{
Domain: "football.example.com",
Selector: "paris",
B: "slfkdMSDFeslif39seFfjl93sljisdsdlif923l",
State: PERMFAIL,
Error: ErrVerificationFailed,
}
resPermFail := &OneResult{
Domain: "football.example.com",
Selector: "paris",
// No B tag on purpose.
State: PERMFAIL,
Error: errMissingRequiredTag,
}
resTempFail := &OneResult{
Domain: "football.example.com",
Selector: "paris",
B: "shorty", // Less than 12 characters to check we include it well.
State: TEMPFAIL,
Error: &net.DNSError{
Err: "dns temp error (for testing)",
IsTemporary: true,
},
}
cases := []struct {
results *VerifyResult
want string
}{
{
results: &VerifyResult{},
want: ";dkim=none\r\n",
},
{
results: &VerifyResult{
Found: 1,
Valid: 1,
Results: []*OneResult{resBrisbane},
},
want: ";dkim=pass" +
" header.b=/gCrinpcQOoI header.d=football.example.com\r\n",
},
{
results: &VerifyResult{
Found: 2,
Valid: 2,
Results: []*OneResult{resBrisbane, resTest},
},
want: ";dkim=pass" +
" header.b=/gCrinpcQOoI header.d=football.example.com\r\n" +
";dkim=pass" +
" header.b=F45dVWDfMbQD header.d=football.example.com\r\n",
},
{
results: &VerifyResult{
Found: 2,
Valid: 2,
Results: []*OneResult{resBrisbane, resTest},
},
want: ";dkim=pass" +
" header.b=/gCrinpcQOoI header.d=football.example.com\r\n" +
";dkim=pass" +
" header.b=F45dVWDfMbQD header.d=football.example.com\r\n",
},
{
results: &VerifyResult{
Found: 2,
Valid: 1,
Results: []*OneResult{resFail, resTest},
},
want: ";dkim=fail reason=\"verification failed\"\r\n" +
" header.b=slfkdMSDFesl header.d=football.example.com\r\n" +
";dkim=pass" +
" header.b=F45dVWDfMbQD header.d=football.example.com\r\n",
},
{
results: &VerifyResult{
Found: 1,
Results: []*OneResult{resPermFail},
},
want: ";dkim=permerror reason=\"missing required tag\"\r\n" +
" header.d=football.example.com\r\n",
},
{
results: &VerifyResult{
Found: 1,
Results: []*OneResult{resTempFail},
},
want: ";dkim=temperror reason=\"lookup : dns temp error (for testing)\"\r\n" +
" header.b=shorty header.d=football.example.com\r\n",
},
}
for i, c := range cases {
got := c.results.AuthenticationResults()
if diff := cmp.Diff(c.want, got); diff != "" {
t.Errorf("case %d: AuthenticationResults() diff (-want +got):\n%s",
i, diff)
}
}
}
func FuzzVerifyMessage(f *testing.F) {
addTestdataCorpus(f)
// Return valid keys for every domain, so we can get past the key lookup
// and exercise the rest of the verification. The ed25519 key is the one
// used by testdata/01-rfc8463.
ctx := WithLookupTXTFunc(context.Background(),
func(ctx context.Context, domain string) ([]string, error) {
return []string{
"v=DKIM1; k=ed25519; p=" + exampleEd25519KeyB64,
"v=DKIM1; k=rsa; p=" + exampleRSAKeyB64,
}, nil
})
f.Fuzz(func(t *testing.T, msg string) {
res, err := VerifyMessage(ctx, msg)
if err != nil {
return
}
if res.Found != uint(len(res.Results)) {
t.Errorf("found %d signatures, but got %d results",
res.Found, len(res.Results))
}
if res.Valid > res.Found {
t.Errorf("%d valid signatures, but only %d found",
res.Valid, res.Found)
}
// This is always called on the result when receiving a message, so
// exercise it too.
res.AuthenticationResults()
})
}