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Suppose a Java program wants to send email using the mail program. It might ask the user for an email address. The command might take the form:

mail <ADDRESS>

However, if an attacker supplies the following value for <ADDRESS>:

noboday@nowhere.com ; useradd attacker

the command executed is actually two commands:

mail noboday@nowhere.com ;

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useradd attacker

which causes a new account to be created for the attacker.

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String address = System.getProperty("email");
if (address == null) {
  // handle error
}

Runtime runtime = Runtime.getRuntime();
Process proc = runtime.exec("mail " + address); 
}

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Compliant Solution (Whitelisting)

This compliant solution sanitizes the email address by permitting only a handful of correct characters to appear, thus preventing command injection.

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