SMTP Attacks
Default Configuration
Command
Description
AUTH PLAIN
AUTH is a service extension used to authenticate the client.
HELO
The client logs in with its computer name and thus starts the session.
MAIL FROM
The client names the email sender.
RCPT TO
The client names the email recipient.
DATA
The client initiates the transmission of the email.
RSET
The client aborts the initiated transmission but keeps the connection between client and server.
VRFY
The client checks if a mailbox is available for message transfer.
EXPN
The client also checks if a mailbox is available for messaging with this command.
NOOP
The client requests a response from the server to prevent disconnection due to time-out.
QUIT
The client terminates the session.
Telnet - HELO/EHLO
The command
VRFYcan be used to enumerate existing users on the system.However, this does not always work. Depending on how the SMTP server is configured, the SMTP server may issue
code 252and confirm the existence of a user that does not exist on the system.
Telnet - VRFY
Sometimes we may have to work through a web proxy. We can also make this web proxy connect to the SMTP server. The command that we would send would then look something like this:
CONNECT 10.129.14.128:25 HTTP/1.0
Send an Emails
All the commands we enter in the command line to send an email we know from every email client program like Thunderbird, Gmail, Outlook, and many others.
We specify the
subject, to whom the email should go, CC, BCC, and the information we want to share with others. Of course, the same works from the command line.
Key Points About Email Headers
Information Carrier: Contains valuable details about an email.
Included Information:
Sender (mandatory)
Recipient (mandatory)
Sending time (mandatory)
Arrival time (optional)
Email route (optional)
Content and format (optional)
Email Header Structure
https://datatracker.ietf.org/doc/html/rfc5322
Dangerous Settings
Relay Servers and Spam Filtering:
Circumventing Spam Filters: Some senders use trusted relay servers to bypass recipient spam filters.
Trusted Relay Servers: These servers are known and verified by other email servers.
Authentication Requirement: The sender typically needs to authenticate with the relay server.
Open Relay Configuration
With this setting, this SMTP server can send fake emails and thus initialize communication between multiple parties.
Another attack possibility would be to spoof the email and read it.
Footprinting the Service
Enumerate users on SMTP
DID not work need another tool
Last updated