Executing this code:

$mail = new Zend_Mail();
// ...
$mail->addCc('email@example.com', 'Injected email" <injected.email@example.com>, "Normal email');
$mail->send();

results in really sending an email with the following header:

Cc: "Injected email" <injected.email@example.com>, "Normal email" <email@example.com>

An even simpler way to add more recipients than expected:

$mail->addCc('email@example.com,another.email@example.com');

Same problem with $mail->addTo() or $mail->addBcc() .

I think that these methods should only add one single recipient, not more... (It would be a good protection from spam)

An easy way to correct the first problem should be by escaping (addcslashes()) the double-quote character (") with a backslash (\") when the recipient name needs to be quoted... This way the Cc header of the first example would be:
Cc: "Injected email\" <injected.email@example.com>, \"Normal email" <email@example.com>

For the second problem just checking for NO commas in the $email parameter should be ok.

Both these patches can be implemented within the method Zend_Mail::_addRecipientAndHeader().