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add faq for smtp smuggling, fix bug around handling "\nX\n" for any X, reject bare carriage returns and possibly smtp-smuggling attempts

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mox was already strict in its "\r\n.\r\n" handling for end-of-message in an
smtp transaction.

due to a mostly unrelated bug, sequences of "\nX\n", including "\n.\n" were
rejected with a "local processing error".

the sequence "\r\n.\n" dropped the dot, not necessarily a big problem, this is
unlikely to happen in a legimate transaction and the behaviour not
unreasonable.

we take this opportunity to reject all bare \r.  we detect all slightly
incorrect combinations of "\r\n.\r\n" with an error mentioning smtp smuggling,
in part to appease the tools checking for it.

smtp errors are 500 "bad syntax", and mention smtp smuggling.
This commit is contained in:
Mechiel Lukkien
2024-01-01 18:30:31 +01:00
parent 4b8b53e776
commit 1f9b640d9a
10 changed files with 269 additions and 40 deletions
Download Patch File Download Diff File Expand all files Collapse all files

64
smtp/data_test.go
View File

@ -9,13 +9,24 @@ import (
)
func TestDataWrite(t *testing.T) {
if err := DataWrite(io.Discard, strings.NewReader("bad")); err == nil || !errors.Is(err, errMissingCRLF) {
t.Fatalf("got err %v, expected errMissingCRLF", err)
}
if err := DataWrite(io.Discard, strings.NewReader(".")); err == nil || !errors.Is(err, errMissingCRLF) {
t.Fatalf("got err %v, expected errMissingCRLF", err)
checkBad := func(s string, expErr error) {
t.Helper()
if err := DataWrite(io.Discard, strings.NewReader(s)); err == nil || !errors.Is(err, expErr) {
t.Fatalf("got err %v, expected %v", err, expErr)
}
}
checkBad("bad", errMissingCRLF)
checkBad(".", errMissingCRLF)
checkBad("bare \r is bad\r\n", ErrCRLF)
checkBad("bare \n is bad\r\n", ErrCRLF)
checkBad("\n.\nis bad\r\n", ErrCRLF)
checkBad("\r.\ris bad\r\n", ErrCRLF)
checkBad("\r\n.\ris bad\r\n", ErrCRLF)
checkBad("\r\n.\nis bad\r\n", ErrCRLF)
checkBad("\n.\ris bad\r\n", ErrCRLF)
checkBad("\n.\r\nis bad\r\n", ErrCRLF)
check := func(msg, want string) {
t.Helper()
w := &strings.Builder{}
@ -58,13 +69,15 @@ func TestDataReader(t *testing.T) {
return wrote, nil
}
check := func(data, want string) {
check := func(data, want string, expErr error) {
t.Helper()
s := &strings.Builder{}
dr := NewDataReader(bufio.NewReader(strings.NewReader(data)))
if _, err := io.Copy(s, dr); err != nil {
t.Fatalf("got err %v", err)
if expErr == nil || !errors.Is(err, expErr) {
t.Fatalf("got err %v, expected %v", err, expErr)
}
} else if got := s.String(); got != want {
t.Fatalf("got %q, expected %q, for %q", got, want, data)
}
@ -72,16 +85,43 @@ func TestDataReader(t *testing.T) {
s = &strings.Builder{}
dr = NewDataReader(bufio.NewReader(strings.NewReader(data)))
if _, err := smallCopy(s, dr); err != nil {
t.Fatalf("got err %v", err)
if expErr == nil || !errors.Is(err, expErr) {
t.Fatalf("got err %v, expected %v", err, expErr)
}
} else if got := s.String(); got != want {
t.Fatalf("got %q, expected %q, for %q", got, want, data)
}
}
check("test\r\n.\r\n", "test\r\n")
check(".\r\n", "")
check(".test\r\n.\r\n", "test\r\n") // Unnecessary dot, but valid in SMTP.
check("..test\r\n.\r\n", ".test\r\n")
check("test\r\n.\r\n", "test\r\n", nil)
check(".\r\n", "", nil)
check(".test\r\n.\r\n", "test\r\n", nil) // Unnecessary dot, but valid in SMTP.
check("..test\r\n.\r\n", ".test\r\n", nil)
check("..test\ntest.\n\r\n.\r\n", ".test\ntest.\n\r\n", nil) // Bare newlines are allowed.
check("..test\ntest\n", "", io.ErrUnexpectedEOF) // Missing end-of-message.
// Bare \r is rejected.
check("bare \r is rejected\r\n.\r\n", "", ErrCRLF)
check("bad:\r.\ris rejected\r\n.\r\n", "", ErrCRLF)
check("bad:\r.\nis rejected\r\n.\r\n", "", ErrCRLF)
// Suspicious bare newlines around a dot are rejected.
check("bad:\n.\nis rejected\r\n.\r\n", "", ErrCRLF)
check("bad:\n.\r\nis rejected\r\n.\r\n", "", ErrCRLF)
check("bad:\r\n.\nis rejected\r\n.\r\n", "", ErrCRLF)
// Suspicious near-smtp-endings at start of message.
check(".\ris rejected\r\n.\r\n", "", ErrCRLF)
check(".\nis rejected\r\n.\r\n", "", ErrCRLF)
check("\n.\ris rejected\r\n.\r\n", "", ErrCRLF)
check("\r.\ris rejected\r\n.\r\n", "", ErrCRLF)
check("\n.\nis rejected\r\n.\r\n", "", ErrCRLF)
check("\r.\nis rejected\r\n.\r\n", "", ErrCRLF)
check("\r.\r\nis rejected\r\n.\r\n", "", ErrCRLF)
check("\n.\r\nis rejected\r\n.\r\n", "", ErrCRLF)
check("\r\n.\ris rejected\r\n.\r\n", "", ErrCRLF)
check("\r\n.\nis rejected\r\n.\r\n", "", ErrCRLF)
s := &strings.Builder{}
dr := NewDataReader(bufio.NewReader(strings.NewReader("no end")))