☑️SMTP - 25,143,110,465,587,993,995
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 -t MX hackthebox.eudig mx plaintext.do | grep "MX" | grep -v ";"And then we can retrieve the IP address:
host -t A mail1.inlanefreight.htb.If we are targetting a custom mail server implementation, we can enumerate the following ports:
Port
Service
TCP/25
SMTP Unencrypted
TCP/143
IMAP4 Unencrypted
TCP/110
POP3 Unencrypted
TCP/465
SMTP Encrypted
TCP/587
SMTP Encrypted/STARTTLS
TCP/993
IMAP4 Encrypted
TCP/995
POP3 Encrypted
We can use nmap to enumerate these ports:
Interaction: we can use telnet to interact with mail servers.
Misconfigurations
A misconfiguration can happen when the SMTP service allows anonymous authentication or support protocols that can be used to enumerate valid usernames.
Username Enumeration
SMTP
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.
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."
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.
POP3
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.
Automating user enumeration
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.
Cloud Enumeration
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.
Now, we can attempt to identify usernames.
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.
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.
O365Spray - Password Spray
Protocol Specific Attacks
Email Spoofing - Open Relay
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.
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.
Next, we can use any mail client to connect to the mail server and send our email.
Latest 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.
The Concept of the Attack
As we already know, with the SMTP service, we can compose emails and send them to desired people. The vulnerability in this service lies in the program's code, namely in the function that records the sender's email address. This offers the possibility of escaping the function using a semicolon (;) and making the system execute arbitrary shell commands. However, there is a limit of 64 characters, which can be inserted as a command. The technical details of this vulnerability can be found here.
Here we need to initialize a connection with the SMTP service first. This can be automated by a script or entered manually. After the connection is established, an email must be composed in which we define the sender, the recipient, and the actual message for the recipient. The desired system command is inserted in the sender field connected to the sender address with a semicolon (;). As soon as we finish writing, the data entered is processed by the OpenSMTPD process.
Initiation of the Attack
Step
Remote Code Execution
Concept of Attacks - Category
1.
The source is the user input that can be entered manually or automated during direct interaction with the service.
Source
2.
The service will take the email with the required information.
Process
3.
Listening to the standardized ports of a system requires root privileges on the system, and if these ports are used, the service runs accordingly with elevated privileges.
Privileges
4.
As the destination, the entered information is forwarded to another local process.
Destination
Trigger Remote Code Execution
Step
Remote Code Execution
Concept of Attacks - Category
5.
This time, the source is the entire input, especially from the sender area, which contains our system command.
Source
6.
The process reads all the information, and the semicolon (;) interrupts the reading due to special rules in the source code that leads to the execution of the entered system command.
Process
7.
Since the service is already running with elevated privileges, other processes of OpenSMTPD will be executed with the same privileges. With these, the system command we entered will also be executed.
Privileges
8.
The destination for the system command can be, for example, the network back to our host through which we get access to the system.
Destination
An exploit has been published on the Exploit-DB platform for this vulnerability which can be used for more detailed analysis and the functionality of the trigger for the execution of system commands.
Boxes
As we've seen, email attacks can lead to sensitive data disclosure through direct access to a user's inbox or by combining a misconfiguration with a convincing phishing email. There are other ways to attack email services that can be very effective as well. A few Hack The Box boxes demonstrate email attacks, such as Rabbit, which deals with brute-forcing Outlook Web Access (OWA) and then sending a document with a malicious macro to phish a user, SneakyMailer which has elements of phishing and enumerating a user's inbox using Netcat and an IMAP client, and Reel which dealt with brute-forcing SMTP users and phishing with a malicious RTF file.
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