smtpsrv: Always sanitize HELO/EHLO domain

Today we use the HELO/EHLO domain in a variety of places, which are
informational, but some have specific syntax (like the Received header).
Using the un-sanitized values can lead to some confusing and/or invalid
values. This is not very risky today, but it could be problematic in the
future if these strings are used in other contexts.

To prevent those problems, this patch sanitizes the HELO/EHLO domain
when receiving it.

Hooks used to receive the sanitized and raw variants; now, both are
sanitized (the raw one is kept for backwards compatibility).

If the sanitization leaves the string empty, we use the hard-coded
no-valid-characters.invalid as a placeholder, to make this situation
easier to identify when troubleshooting.

docs/hooks.md

@@ -42,9 +42,10 @@ The environment will contain the following variables:
  - `$PWD`: The working directory, which will be the config directory.
  - `$REMOTE_ADDR`: IP address of the remote side of the connection.
  - `$EHLO_DOMAIN`: EHLO/HELO domain, as given by the client; sanitized for
-   safety.
+   safety (only letters, digits, and `-.[]:` are kept). If there are no valid
- - `$EHLO_DOMAIN_RAW`: Same as `$EHLO_DOMAIN`, but not sanitized; be careful as
+   characters, it is `no-valid-characters.invalid`.
-   it can contain problematic characters.
+ - `$EHLO_DOMAIN_RAW`: Same as `$EHLO_DOMAIN`. Kept for backwards
+   compatibility; it used to be the unsanitized value.
  - `$MAIL_FROM`: MAIL FROM address.
  - `$RCPT_TO`: RCPT TO addresses, space separated.
  - `$AUTH_AS`: Authenticated user; empty if the connection has not

internal/smtpsrv/conn.go

@@ -356,7 +356,7 @@ func (c *Conn) HELO(params string) (code int, msg string) {
 	if len(strings.TrimSpace(params)) == 0 {
 		return 501, "Invisible customers are not welcome!"
 	}
-	c.ehloDomain = strings.Fields(params)[0]
+	c.setEHLODomain(params)
 
 	types := []string{
 		"general store", "used armor dealership", "second-hand bookstore",
@@ -374,7 +374,7 @@ func (c *Conn) EHLO(params string) (code int, msg string) {
 	if len(strings.TrimSpace(params)) == 0 {
 		return 501, "Invisible customers are not welcome!"
 	}
-	c.ehloDomain = strings.Fields(params)[0]
+	c.setEHLODomain(params)
 	c.isESMTP = true
 
 	buf := bytes.NewBuffer(nil)
@@ -871,11 +871,26 @@ func checkData(data []byte) error {
 	return nil
 }
 
+// Placeholder EHLO domain, used when the one given by the client has no valid
+// characters. The .invalid TLD is reserved for names that are guaranteed to
+// be invalid (RFC 6761).
+const invalidEHLODomain = "no-valid-characters.invalid"
+
+// setEHLODomain sets the EHLO domain from the (non-empty) HELO/EHLO
+// parameters, sanitizing it.
+func (c *Conn) setEHLODomain(params string) {
+	c.ehloDomain = sanitizeEHLODomain(strings.Fields(params)[0])
+	if c.ehloDomain == "" {
+		c.ehloDomain = invalidEHLODomain
+	}
+}
+
 // Sanitize HELO/EHLO domain.
 // RFC is extremely flexible with EHLO domain values, allowing all printable
 // ASCII characters. They can be tricky to use in shell scripts (commonly used
-// as post-data hooks), so this function sanitizes the value to make it
+// as post-data hooks), and in headers (e.g. "(", ")" and ";" have special
-// shell-safe.
+// meanings in the Received header), so this function sanitizes the value to
+// make it safe for both.
 func sanitizeEHLODomain(s string) string {
 	var n strings.Builder
 	for _, c := range s {
@@ -918,7 +933,10 @@ func (c *Conn) runPostDataHook(data []byte) ([]byte, bool, error) {
 		cmd.Env = append(cmd.Env, v+"="+os.Getenv(v))
 	}
 	cmd.Env = append(cmd.Env, "REMOTE_ADDR="+c.remoteAddr.String())
-	cmd.Env = append(cmd.Env, "EHLO_DOMAIN="+sanitizeEHLODomain(c.ehloDomain))
+	cmd.Env = append(cmd.Env, "EHLO_DOMAIN="+c.ehloDomain)
+
+	// The EHLO domain is already sanitized, so this is the same as
+	// EHLO_DOMAIN. We keep it for backwards compatibility.
 	cmd.Env = append(cmd.Env, "EHLO_DOMAIN_RAW="+c.ehloDomain)
 	cmd.Env = append(cmd.Env, "MAIL_FROM="+c.mailFrom)
 	cmd.Env = append(cmd.Env, "RCPT_TO="+strings.Join(c.rcptTo, " "))

internal/smtpsrv/conn_test.go

@@ -142,3 +142,29 @@ func TestSanitizeEHLODomain(t *testing.T) {
 		}
 	}
 }
+
+func TestEHLODomainIsSanitized(t *testing.T) {
+	cases := []struct {
+		params   string
+		expected string
+	}{
+		{"mx.example.com", "mx.example.com"},
+		{"[1.2.3.4] extra params", "[1.2.3.4]"},
+		{"evil;(x", "evilx"},
+		{"!!!", invalidEHLODomain},
+	}
+	for _, c := range cases {
+		for _, cmd := range []string{"HELO", "EHLO"} {
+			conn := &Conn{}
+			if cmd == "HELO" {
+				conn.HELO(c.params)
+			} else {
+				conn.EHLO(c.params)
+			}
+			if conn.ehloDomain != c.expected {
+				t.Errorf("%s %q: got EHLO domain %q, expected %q",
+					cmd, c.params, conn.ehloDomain, c.expected)
+			}
+		}
+	}
+}