Quick Summary
Open SMTP Relay Misconfiguration is a critical mail server configuration issue where the Simple Mail Transfer Protocol (SMTP) service allows unauthenticated users to relay emails to external domains. This may allow attackers to abuse the mail server for spam distribution, phishing campaigns, malware delivery, and domain reputation damage.
Vulnerability Classification
| Field | Value |
|---|---|
| Vulnerability Type | Mail Server Misconfiguration |
| CWE ID | CWE-284 – Improper Access Control |
| CVE ID | N/A (Configuration Issue) |
| CVSS 4.0 Base Score | 8.7 (High) |
| CVSS Vector | AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:H |
| OWASP Category | A05:2021 – Security Misconfiguration |
| Attack Surface | External / Internal Network |
Affected Asset / Environment
- Service: SMTP
- Default Port: 25 (SMTP), 587 (Submission)
- Platforms: Postfix, Sendmail, Exchange, Exim
- Testing Method: Black-box / External Assessment
- Tools Used: Nmap, Telnet, Netcat
Description
The assessor observed that the SMTP server accepts email relay requests from unauthenticated users and allows messages to be sent to external domains. During testing, it was possible to send email messages without valid credentials.
An open relay allows attackers to use the organization’s mail server to distribute spam, phishing emails, or malicious attachments. This may lead to domain blacklisting and email delivery disruption.
Mail servers should only allow relay from authenticated users or trusted internal IP addresses.
Root Cause
The issue occurs due to improper SMTP configuration where relay restrictions are not enforced.
Common root causes include:
- Relay restrictions not properly defined
- Misconfigured access control rules
- Default configuration not hardened
- Failure to restrict relay to authenticated users
- Missing IP-based filtering
Business Impact
Exploitation of this vulnerability may allow attackers to send large volumes of spam or phishing emails through the organization’s mail server. This may lead to domain blacklisting, loss of email reputation, and service disruption.
Additionally, misuse of the SMTP server may expose the organization to regulatory penalties and reputational damage.
Technical Impact
An attacker can:
- Relay emails to external domains
- Conduct phishing campaigns
- Distribute malware
- Abuse infrastructure for spam distribution
- Trigger denial-of-service through mail flooding
Although not directly resulting in data compromise, open relay can significantly impact availability and reputation.
Proof of Concept (PoC)
Step1: Identify SMTP Service
nmap -sV -p 25 <target-ip>
If port 25 is open and identified as SMTP, proceed to relay testing.
Step2: Test SMTP Relay Using Telnet
telnet <target-ip> 25
Once connected:
HELO attacker.com
MAIL FROM:<attacker@external.com>
RCPT TO:<victim@externaldomain.com>
DATA
Subject: Test RelayThis is a relay test.
.
QUIT
If the server accepts the RCPT TO for an external domain without authentication, it is configured as an open relay.
Step3: Automated Relay Test Using Nmap Script
nmap --script smtp-open-relay -p 25 <target-ip>
If the script reports relay allowed, the misconfiguration is confirmed.
Exploitation Prerequisites
- Network access to port 25
- SMTP service running
- No authentication required for relay
- No IP-based relay restriction
Remediation
It is recommended that SMTP relay be restricted immediately.
Recommended actions:
- Allow relay only for authenticated users
- Restrict relay to trusted internal IP addresses
- Configure proper relay access rules
- Enable SMTP authentication
- Implement SPF, DKIM, and DMARC
- Regularly audit mail server configuration
After applying changes, verify that external relay attempts are rejected.
Detection and Monitoring
- Monitor mail logs for unusual outbound volume
- Alert on high-volume relay attempts
- Check domain reputation status regularly
- Conduct periodic relay testing