MIME headers aren't encoded properly when using ISO-2022-JP (7bit) CharSet

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Problem description

In the PHPMailer.php, there is a code that set $charset = "us-ascii" if the input string does not have 8-bit characters. https://github.com/PHPMailer/PHPMailer/blob/3dffb3e4ee97b82a63eeac9ec2d03b164becc451/src/PHPMailer.php#L3282-L3286

In Japan, 7-bit charset ISO-2022-JP has been used historically. The has8bitChars($str) method returns false when the ISO-2022-JP string are passed.

<?php
require 'vendor/autoload.php';
use PHPMailer\PHPMailer\PHPMailer;

$mail = new PHPMailer();
$str = mb_convert_encoding('テスト', 'ISO-2022-JP');

var_dump($mail->has8bitChars($str)); // ==> false

As a result, PHPMailer generates us-ascii MIME headers when the ISO-2022-JP charset is set. For example:

Subject: =?us-ascii?Q?=1B$B%F%9%H=1B(B?=

Code to reproduce

<?php
require 'vendor/autoload.php';
use PHPMailer\PHPMailer\PHPMailer;

$mail = new PHPMailer();
$mail->CharSet = 'ISO-2022-JP';
$mail->setFrom('[email protected]');
$mail->addAddress('[email protected]');
$mail->Body = 'test';


$input = 'テスト';
$expected = mb_encode_mimeheader($input, $mail->CharSet);

if (isset($argv[1])) {
    $str = $input;
} else {
    $str = mb_convert_encoding($input, $mail->CharSet);
}

$mail->Subject = $str;

// Get whole message as string.
$mail->preSend();
$message = $mail->getSentMIMEMessage();
// echo $message;

preg_match('{^Subject: (.+?\n(?: =.+?\n)*)}m', $message, $m);
$encoded = $m[1];
$decoded = mb_decode_mimeheader($encoded);
$hex = preg_replace('{..}', '\0 ', strtoupper(bin2hex($decoded)));

echo "Expected: " . $expected . "\n";
echo "Encoded:  " . $encoded. "\n";
echo "Decoded(raw): " . $decoded. "\n";
echo "Decoded(hex): " . $hex . "\n";

The result is:

$ php test.php

Expected: =?ISO-2022-JP?B?GyRCJUYlOSVIGyhC?=
Encoded:  =?us-ascii?Q?=1B$B%F%9%H=1B(B?=

Decoded(raw): %F%9%H
Decoded(hex): 1B 24 42 25 46 25 39 25 48 1B 28 42

I believe that the mb_convert_encoding($str, 'ISO-2022-JP') is required for each header fields. For example:

$mail->Subject = mb_convert_encoding('テスト', 'ISO-2022-JP');

But it also doesn't work without the conversion.

$ php test.php --skip-conversion

Expected: =?ISO-2022-JP?B?GyRCJUYlOSVIGyhC?=
Encoded:  =?ISO-2022-JP?B?44OG44K544OI?=

Decoded(raw): ?????ｹ???
Decoded(hex): 3F 3F 3F 3F 3F EF BD B9 3F 3F 3F

Tested environment

• PHPMailer 6.1.8
• PHP 7.0.32

Related issues

• #1945