1. Introduction
The Multiple Mail Server EXPN/VRFY Information Disclosure vulnerability allows attackers to enumerate valid user names on a remote mail server. This can aid in phishing attacks and reconnaissance, potentially leading to account compromise. Systems running SMTP servers are usually affected. A successful exploit could lead to the disclosure of confidential information such as usernames, impacting confidentiality.
2. Technical Explanation
The vulnerability occurs when an SMTP server allows unauthenticated users to query user existence using the EXPN and VRFY commands. These commands were originally designed for mail delivery assistance but can be abused for enumeration. An attacker sends EXPN or VRFY requests with various usernames; a response indicates a valid account exists.
- Root cause: The SMTP server does not restrict access to the EXPN and/or VRFY commands, allowing remote users to query user accounts.
- Exploit mechanism: An attacker attempts to use the EXPN or VRFY command against the target mail server with a list of potential usernames. If the server responds with information about the username (e.g., full name, delivery address), it confirms the account’s existence. For example, an attacker might send “VRFY [email protected]” and if the server replies with “250 OK user is valid”, they know ‘testuser’ exists.
- Scope: Affected platforms are those running SMTP servers, including Sendmail versions that do not have appropriate privacy settings configured.
3. Detection and Assessment
You can confirm vulnerability by testing the EXPN and VRFY commands against your mail server. A thorough method involves attempting to enumerate a list of known users.
- Quick checks: Use telnet or netcat to connect to port 25 of your SMTP server and issue the VRFY command with a test username.
- Scanning: Nmap’s script engine can be used with the smtp-vrfy vulnerability scan script (example only).
- Logs and evidence: Examine mail server logs for EXPN or VRFY commands issued by remote hosts. Look for successful responses indicating user information disclosure.
telnet your.mail.server 25
VRFY [email protected]
4. Solution / Remediation Steps
The following steps disable the EXPN and VRFY commands on Sendmail servers to prevent user enumeration.
4.1 Preparation
- Take a backup of your
/etc/sendmail.cffile before making any changes. Stop the sendmail service if possible, depending on your system’s requirements. - Ensure you have access to edit the
/etc/sendmail.cfconfiguration file. A rollback plan is to restore the original/etc/sendmail.cffile and restart the sendmail service. - A change window may be required depending on your system’s uptime requirements; approval from a senior administrator might be needed.
4.2 Implementation
- Step 1: Open the
/etc/sendmail.cffile in a text editor with root privileges. - Step 2: Add the line
O PrivacyOptions=goawayinto the configuration file. - Step 3: Save the changes to the
/etc/sendmail.cffile. - Step 4: Restart the sendmail service for the changes to take effect.
4.3 Config or Code Example
Before
# No PrivacyOptions line present, or commented out.
After
O PrivacyOptions=goawayin
4.4 Security Practices Relevant to This Vulnerability
Several security practices can help prevent this type of vulnerability. Least privilege limits the impact if exploited, and safe defaults reduce the risk of misconfiguration.
- Practice 1: Implement least privilege principles by restricting access to sensitive services like SMTP only to authorized users and systems.
- Practice 2: Use safe defaults in your mail server configuration, disabling unnecessary features such as EXPN and VRFY commands by default.
4.5 Automation (Optional)
If you use a configuration management tool like Ansible, you can automate this change.
---
- hosts: mailservers
tasks:
- lineinfile:
path: /etc/sendmail.cf
regexp: '^O PrivacyOptions='
line: O PrivacyOptions=goawayin
state: present
- service:
name: sendmail
state: restarted
5. Verification / Validation
- Post-fix check: Use telnet or netcat to connect to port 25 of your SMTP server and issue the VRFY command with a test username; you should receive an error message (e.g., “500 Command not implemented”).
- Re-test: Re-run the earlier detection method (telnet/netcat) to confirm that the EXPN and VRFY commands are no longer responding with user information.
- Monitoring: Monitor mail server logs for any errors related to command restrictions, as an example.
telnet your.mail.server 25
VRFY [email protected]
500 Command not implemented
6. Preventive Measures and Monitoring
Update security baselines to include this configuration change, and add checks in CI/CD pipelines to prevent re-introduction of the vulnerability. For example, use a CIS control or GPO/Intune setting.
- Baselines: Update your server security baseline to require
O PrivacyOptions=goawayinin Sendmail configurations. - Pipelines: Add static analysis checks to your CI/CD pipeline to identify any instances where EXPN or VRFY commands are enabled without appropriate restrictions.
- Asset and patch process: Implement a regular review cycle for server configurations, including mail servers, to ensure compliance with security baselines.
7. Risks, Side Effects, and Roll Back
Disabling EXPN/VRFY may affect some legacy applications that rely on these commands. A rollback plan involves restoring the original /etc/sendmail.cf file.
- Risk or side effect 1: Some older mail clients or systems might not function correctly if they depend on EXPN or VRFY for address verification.
- Risk or side effect 2: Disabling these commands could potentially break some automated email processes that rely on them.
- Roll back: Step 1: Restore the original
/etc/sendmail.cffile from your backup. Step 2: Restart the sendmail service.
8. References and Resources
- Vendor advisory or bulletin: No specific vendor advisory available, as this is a configuration issue.
- NVD or CVE entry: No specific CVE assigned for this general configuration issue.
- Product or platform documentation relevant to the fix: Sendmail Documentation