Chapter 9. Sending Email
One of the most common tasks your CGI scripts need to perform is sending email. Email is a popular method for exchanging information between people, whether that information comes from other people or from automated systems. You may need to send email updates or receipts to visitors of your web site. You may need to notify members of your organization about certain events like a purchase, a request for information, or feedback about your web site. Email is also a useful tool to notify you when there are problems with your CGI scripts. When you write subroutines that respond to errors in your CGI scripts, it is a very good idea to include code to notify whomever is responsible for maintaining the site about the error.
There are several ways to send email from an application, including using an external mail client, such as sendmail or mail, or by directly communicating with the remote mail server via Perl. There are also Perl modules that make sending mail especially easy. We'll explore all these options in this chapter by building a sample application that provides a web front end to an emailer.
Since the subject of security is still fresh in our minds, however, we should take a moment to review security as it relates to email. Sending email is probably one of the largest causes of security errors in CGI scripts.
9.1.1. Mailers and Shells
Most CGI scripts open a pipe to an external mail client such as sendmail and mail, and pass the email address through the shell as a parameter. Passing any user data through a shell is a very bad thing as we saw in the previous chapter (if you skipped ahead to this chapter, it would be wise to go back and review Chapter 8, "Security", before continuing). Unless you like living dangerously, you should never pass an email address to an external application via a shell. It is not possible to verify that email addresses contain only certain safe characters either. Contrary to what you may expect, a proper email address can contain any valid ASCII character, including control characters and all those troublesome characters that have special meaning in the shell. We'll review what comprises a valid email address in the next section.
9.1.2. False Identities
You have likely received email claiming to be from someone other than the true sender. It happens all the time with unsolicited bulk email (spam). Falsifying the return address in an email message is very simple to do, and can even be quite useful. You probably would rather have email messages sent by your web server appear to come from actual individuals or groups within your company than the user (e.g., nobody) that the web user runs as. We'll see how to do this in our examples later in this chapter.
So how does this relate to security? Say, for example, you create a web form that allows users to send feedback to members of your organization. You decide to generalize the CGI script responsible for this so you don't have to update it when internal email addresses change. Instead, you insert the email addresses into hidden fields in the feedback form since they're easier to update there. However, you do take security precautions. Because you recognize that it's possible for a cracker to change hidden fields, you are careful not to pass the email addresses through a shell, and you treat them as tainted data. You handled all the details correctly, but you still have a potential security problem -- it's just at a higher level.
If the user can specify the sender, the recipient, and the body of the message, you are allowing them to send any message to anyone anywhere, and the resulting message will originate from your machine. Anyone can falsify the return address in an email message, but it is very difficult to try to mask the message's routing information. A knowledgeable person can look at the headers in an email message and see where that message truly originated, and all the email messages your web server sends out will clearly originate from the machine hosting it.
Thus this feedback page is a security problem because crackers given this much freedom could send damaging or embarrassing email to whomever they wanted, and all the messages would look like they are from your organization. Although this may not seem as serious as a system breach, it is still something you probably would rather avoid.
Spam, of course, refers to unsolicited junk email. It's those messages that you get from someone you've never heard of advertising weight loss plans, get-rich schemes, and less-than-reputable web sites. None of us like spam, so be certain your web site doesn't contribute to the problem. Avoid creating CGI scripts that are so flexible that they allow the user to specify the recipient and the content of the message. The previous example of the feedback page illustrates this. As we saw in the last chapter, it is not difficult to create a web client with LWP and a little bit of Perl code. Likewise, it would not be difficult for a spammer to use LWP to repeatedly call your CGI script in order to send out numerous, annoying messages.
Of course, most spammers don't operate this way. The big ones have dedicated equipment, and for those who don't, it's much more convenient to hijack an SMTP server, which is designed to send mail, than having to pass requests through a CGI script. So even if you do create scripts that are wide open to hijacking, the chances that someone will exploit it are slim ... but what if it does happen? You probably do not want to face the mass of angry recipients who have tracked the routing information back to you. When it comes to security, it's always better to play it safe.
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