Quick Summary
Telnet Service Enabled on Production Server is a network service misconfiguration where the Telnet protocol is enabled and accessible on a production environment. Telnet transmits credentials and session data in cleartext, making it vulnerable to interception, credential theft, and unauthorized access.
Vulnerability Classification
| Field | Value |
|---|---|
| Vulnerability Type | Insecure Network Protocol Exposure |
| CWE ID | CWE-319 – Cleartext Transmission of Sensitive Information |
| CVE ID | N/A (Configuration Issue) |
| CVSS 4.0 Base Score | 8.2 (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: Telnet
- Default Port: 23
- Common Platforms: Linux, Unix, Network Devices, Legacy Windows Systems
- Testing Method: Black-box Network Assessment
- Tools Used: Nmap, Netcat, Telnet Client, Wireshark
Description
The assessor observed that the Telnet service is enabled and accessible on the production server via port 23. Telnet is a legacy remote administration protocol that does not encrypt traffic.
During testing, the service responded to connection attempts and presented a login prompt. Because Telnet transmits credentials in plaintext, any attacker positioned within the same network segment may intercept authentication details using packet capture tools.
Production systems should not expose Telnet due to its inherent lack of encryption and susceptibility to man-in-the-middle attacks.
Root Cause
The issue occurs due to legacy configuration practices or failure to disable outdated remote administration services.
Common root causes include:
- Telnet enabled by default on network devices
- Failure to replace Telnet with SSH
- Inadequate hardening of production servers
- Lack of secure remote access policy enforcement
Business Impact
Exploitation of this vulnerability may allow attackers to intercept administrator credentials in transit. Compromised credentials may lead to unauthorized system access, lateral movement, data exfiltration, or service disruption.
If exposed externally, the server may become a target for brute-force attacks, increasing the risk of system compromise and regulatory non-compliance.
Technical Impact
An attacker can:
- Capture usernames and passwords in cleartext
- Perform credential harvesting using packet sniffing
- Execute brute-force attacks against Telnet login
- Gain unauthorized shell access
- Escalate privileges if weak credentials are used
This significantly weakens remote access security controls.
Proof of Concept (PoC)
Step1: Identify Telnet Service
nmap -sV -p 23 <target-ip>If output shows port 23 open and service as telnet, proceed to validation.
Step2: Confirm Service Accessibility
telnet <target-ip> 23If the server responds with a login banner or prompt, the service is active.
Step3: Enumerate Banner Information
nmap -sV --script banner -p 23 <target-ip>If version details or system information are disclosed, the exposure risk increases.
Step4: Capture Cleartext Credentials (Lab Validation Only)
Using Wireshark on the same network segment, filter traffic:
tcp.port == 23Attempt login via Telnet and observe credentials transmitted in plaintext.
Step5: Test for Weak Authentication (If Authorized)
hydra -l admin -P passwords.txt telnet://<target-ip>Successful login confirms exploitable authentication weakness.
Exploitation Prerequisites
- Network access to port 23
- Telnet service enabled
- Valid or weak credentials
- No IP-based restriction
Remediation
It is recommended that Telnet service be disabled immediately on production systems.
Recommended actions:
- Disable Telnet daemon or service
- Replace Telnet with SSH (Secure Shell)
- Restrict remote management to secure protocols only
- Implement firewall rules to block port 23
- Enforce strong authentication and access control
- Regularly audit network services for legacy protocols
Detection and Monitoring
- Monitor firewall logs for port 23 access attempts
- Alert on Telnet login attempts
- Conduct periodic port scans to identify legacy services
- Implement IDS/IPS signatures for Telnet traffic