Quick Summary
POP3 Service Running Without SSL/TLS is a security misconfiguration where the Post Office Protocol version 3 (POP3) service is enabled without encryption. This allows credentials and email content to be transmitted in cleartext, exposing them to interception through packet sniffing or man-in-the-middle attacks.
Vulnerability Classification
| Field | Value |
|---|---|
| Vulnerability Type | Cleartext Protocol Exposure |
| CWE ID | CWE-319 – Cleartext Transmission of Sensitive Information |
| CVE ID | N/A (Configuration Issue) |
| CVSS 4.0 Base Score | 8.0 (High) |
| CVSS Vector | AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:L |
| OWASP Category | A05:2021 – Security Misconfiguration |
| Attack Surface | Internal / External Network |
Affected Asset / Environment
- Service: POP3
- Default Ports: 110 (POP3), 995 (POP3S – Secure)
- Platforms: Exchange, Dovecot, Postfix, Courier
- Testing Method: Black-box / Internal Assessment
- Tools Used: Nmap, Telnet, OpenSSL
Description
The assessor observed that the POP3 service is accessible on port 110 without enforcing SSL/TLS encryption. During testing, the server accepted plaintext authentication commands over an unencrypted channel.
POP3 is used by email clients to retrieve messages from mail servers. When encryption is not enabled, usernames, passwords, and email content are transmitted in cleartext.
Attackers positioned within the same network segment may capture credentials and email data using packet capture tools.
Root Cause
The issue occurs due to improper mail server configuration where SSL/TLS is not enforced for POP3 connections.
Common root causes include:
- POP3 service enabled without secure variant (POP3S)
- TLS not configured or certificate not installed
- Legacy mail client compatibility
- Failure to enforce encrypted connections
Business Impact
Exploitation of this vulnerability may allow attackers to capture valid email credentials. Compromised email accounts may lead to unauthorized access to sensitive communications, password reset abuse, and business email compromise.
Interception of email content may also expose confidential information and increase risk of phishing or fraud.
In regulated environments, unencrypted transmission of sensitive data may violate compliance requirements.
Technical Impact
An attacker can:
- Capture usernames and passwords in cleartext
- Intercept email content
- Replay authentication credentials
- Conduct man-in-the-middle attacks
- Use compromised credentials for further access
This significantly weakens email security controls.
Proof of Concept (PoC)
Step1: Identify POP3 Service
nmap -sV -p 110 <target-ip>
If port 110 is open and identified as POP3, proceed to validation.
Step2: Connect via Telnet
telnet <target-ip> 110
If server responds with a POP3 banner and allows command interaction without encryption, exposure exists.
Step3: Attempt Authentication
Within session:
USER testuser
PASS testpassword
If credentials are transmitted in cleartext, the service is insecure.
Step4: Verify Lack of TLS Support
openssl s_client -connect <target-ip>:110
If TLS negotiation fails or is not supported, encryption is not enforced.
Exploitation Prerequisites
- Network access to port 110
- POP3 service enabled
- SSL/TLS not enforced
- Ability to capture network traffic
Remediation
It is recommended that POP3 over SSL/TLS (POP3S) be enforced.
Recommended actions:
- Disable plaintext POP3 (port 110)
- Enable POP3S on port 995
- Install valid TLS certificates
- Enforce encrypted connections only
- Update mail client configurations
- Restrict mail service exposure via firewall
After implementing changes, verify that plaintext authentication is blocked.
Detection and Monitoring
- Monitor network traffic for POP3 plaintext sessions
- Alert on login attempts over port 110
- Conduct periodic encryption compliance checks
- Enforce secure mail server configuration policies