Check domain syntax after normalization to internationalized characte…

…rs as a precaution

Out of caution that normalization of the domain part to internationalized characters could turn a valid domain string into an invalid one, it is re-parsed at the end to ensure that it still is validated by the idna package. I could not find any examples where that was not already caught, however, since it seems like the existing IDNA calls already prevent it.

Some tests are added for invalid characters in the domain part which become invalid after Unicode NFC normalization. These were already handled. (The new code never raises an exception.)

See #142.

email_validator/syntax.py

@@ -446,7 +446,7 @@ class DomainNameValidationResult(TypedDict):
 def validate_email_domain_name(domain: str, test_environment: bool = False, globally_deliverable: bool = True) -> DomainNameValidationResult:
     """Validates the syntax of the domain part of an email address."""
 
-    # Check for invalid characters before normalization.
+    # Check for invalid characters.
     # (RFC 952 plus RFC 6531 section 3.3 for internationalized addresses)
     bad_chars = {
         safe_character_display(c)
@@ -466,8 +466,9 @@ def validate_email_domain_name(domain: str, test_environment: bool = False, glob
     # and converting all label separators (the period/full stop, fullwidth full stop,
     # ideographic full stop, and halfwidth ideographic full stop) to regular dots.
     # It will also raise an exception if there is an invalid character in the input,
-    # such as "⒈" which is invalid because it would expand to include a dot.
-    # Since several characters are normalized to a dot, this has to come before
+    # such as "⒈" which is invalid because it would expand to include a dot and
+    # U+1FEF which normalizes to a backtick, which is not an allowed hostname character.
+    # Since several characters *are* normalized to a dot, this has to come before
     # checks related to dots, like check_dot_atom which comes next.
     original_domain = domain
     try:
@@ -577,14 +578,23 @@ def validate_email_domain_name(domain: str, test_environment: bool = False, glob
     # but not be actual IDNA. For ASCII-only domains, the conversion out
     # of IDNA just gives the same thing back.
     #
-    # This gives us the canonical internationalized form of the domain.
+    # This gives us the canonical internationalized form of the domain,
+    # which we return to the caller as a part of the normalized email
+    # address.
     try:
         domain_i18n = idna.decode(ascii_domain.encode('ascii'))
     except idna.IDNAError as e:
         raise EmailSyntaxError(f"The part after the @-sign is not valid IDNA ({e}).") from e
 
-    # Check for invalid characters after normalization. These
-    # should never arise. See the similar checks above.
+    # Check that this normalized domain name has not somehow become
+    # an invalid domain name. All of the checks before this point
+    # using the idna package probably guarantee that we now have
+    # a valid international domain name in most respects. But it
+    # doesn't hurt to re-apply some tests to be sure. See the similar
+    # tests above.
+
+    # Check for invalid and unsafe characters. We have no test
+    # case for this.
     bad_chars = {
         safe_character_display(c)
         for c in domain
@@ -594,6 +604,13 @@ def validate_email_domain_name(domain: str, test_environment: bool = False, glob
         raise EmailSyntaxError("The part after the @-sign contains invalid characters: " + ", ".join(sorted(bad_chars)) + ".")
     check_unsafe_chars(domain)
 
+    # Check that it can be encoded back to IDNA ASCII. We have no test
+    # case for this.
+    try:
+        idna.encode(domain_i18n)
+    except idna.IDNAError as e:
+        raise EmailSyntaxError(f"The part after the @-sign became invalid after normalizing to international characters ({e}).") from e
+
     # Return the IDNA ASCII-encoded form of the domain, which is how it
     # would be transmitted on the wire (except when used with SMTPUTF8
     # possibly), as well as the canonical Unicode form of the domain,

tests/test_syntax.py

@@ -392,6 +392,12 @@ def test_domain_literal() -> None:
         ('me@⒈wouldbeinvalid.com',
          "The part after the @-sign contains invalid characters (Codepoint U+2488 not allowed "
          "at position 1 in '⒈wouldbeinvalid.com')."),
+        ('me@\u037e.com',
+         "The part after the @-sign is invalid (Codepoint U+003B at position 1 "
+         "of ';' not allowed)."),
+        ('me@\u1fef.com',
+         "The part after the @-sign is invalid (Codepoint U+0060 at position 1 "
+         "of '`' not allowed)."),
         ('@example.com', 'There must be something before the @-sign.'),
         ('white space@test', 'The email address contains invalid characters before the @-sign: SPACE.'),
         ('test@white space', 'The part after the @-sign contains invalid characters: SPACE.'),