During a recent audit of a Tier-2 ISP’s infrastructure in Karnataka, we identified over 400 legacy GPON OLTs (Optical Line Terminals) with Telnet enabled on their management interfaces. These devices, primarily older models from vendors like D-Link and TP-Link, lacked SSH support and were exposed to the internal management VLAN without any form of encryption. Our initial scan using Nmap revealed not just open ports, but version strings indicating vulnerability to heap-based buffer overflows that have been documented in the NIST NVD for over a decade.
Introduction to Telnetd Buffer Overflow Vulnerabilities
What is Telnetd and Why is it Vulnerable?
The Telnet daemon (telnetd) is a legacy service that facilitates remote terminal access over a network. Unlike SSH, Telnet transmits all data—including usernames and passwords—in cleartext. The protocol was designed in an era when network security was secondary to connectivity. The telnetd binary is often written in C, a language that requires manual memory management.
Vulnerabilities in telnetd typically arise from how the daemon handles protocol options and sub-negotiations. When a client connects, the server and client exchange "options" to define terminal type, window size, and environment variables. If the code responsible for parsing these options does not strictly validate the length of the incoming data before copying it into a fixed-size buffer, an overflow occurs.
The Impact of Buffer Overflow Attacks on Network Security
A successful buffer overflow in telnetd often results in Remote Code Execution (RCE). By sending a specially crafted sequence of Telnet options, an attacker can overwrite the return address on the stack or corrupt the heap metadata. This allows the redirection of the execution flow to an attacker-controlled payload, such as a reverse shell.
In the context of Indian infrastructure, where many "Smart City" SCADA deployments rely on serial-to-ethernet converters, a compromised telnetd instance provides a pivot point into the OT (Operational Technology) network. This can lead to the unauthorized control of power grids, water treatment facilities, or traffic management systems.
Historical Context: Famous Telnetd Exploits
We frequently encounter two major CVEs during penetration tests of legacy environments. CVE-2011-4862 targets the encrypt_keyid function in telnetd. It allows an attacker to overflow a buffer by providing a long encryption key ID during the Telnet protocol's encryption option negotiation.
The second, more recent critical vulnerability is CVE-2020-10188. This is a heap-based buffer overflow in netkit-telnet. It exists in the client_reply function, where the daemon fails to check the bounds of a buffer when processing specific Telnet options. Despite being discovered in 2020, we still find unpatched instances in routers and switches deployed across rural broadband networks in India.
Technical Mechanics of a Telnetd Buffer Overflow
How Memory Corruption Occurs in Telnet Daemons
Memory corruption in telnetd usually happens during the negotiation phase. The Telnet protocol uses "Interpret as Command" (IAC) sequences, which start with the byte 0xFF. When the daemon encounters 0xFF, it enters a state machine to process the following bytes as commands or options.
If the daemon allocates a 256-byte buffer on the heap to store a specific option string but receives 512 bytes, the extra 256 bytes will overwrite adjacent memory. If that adjacent memory contains function pointers or heap headers, the attacker can manipulate the program's logic. We observed that many legacy MIPS-based embedded systems lack the memory protection features found in modern x86_64 servers, making exploitation significantly easier.
The Role of Unsafe C Functions in Legacy Code
The root cause of these overflows is the use of unsafe C library functions like strcpy(), sprintf(), and gets(). These functions do not check the size of the destination buffer. In the case of telnetd, the netkit source code often used manual pointer arithmetic to navigate the incoming byte stream.
Example of a conceptual exploit payload for a Telnet heap overflow
This simulates the IAC negotiation sequence
IAC = b'\xff' DONT = b'\xfe' DO = b'\xfd' WONT = b'\xfc' WILL = b'\xfb' SB = b'\xfa' SE = b'\xf0'
Crafting a payload that exceeds the expected buffer size
padding = b'A' * 512 shellcode = b'\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\xb0\x0b\xcd\x80' payload = IAC + SB + b'\x24' + b'\x00' + padding + shellcode + IAC + SE
Identifying Vulnerable Telnetd Versions
We use Nmap to fingerprint the service and determine if the version string matches known vulnerable releases. In many Indian enterprise environments, the banner might be suppressed, requiring more aggressive version detection.
$ nmap -sV -p 23 --script telnet-brute,telnet-encryption 192.168.1.1 -oN telnet_audit.txt
Sample Output
PORT STATE SERVICE VERSION
23/tcp open telnet Linux telnetd (netkit-telnet 0.17)
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel
If the version is netkit-telnet 0.17 or earlier, it is likely vulnerable to CVE-2020-10188. We also check for the presence of the TN3270 or TSPEED options, which are common vectors for these exploits.
Immediate Mitigation Strategies for Telnetd
Applying Critical Security Patches and Vendor Updates
The most direct mitigation is updating the telnetd package. For Linux distributions like Debian or RHEL, this is straightforward. However, for embedded devices (OLTs, DSLAMs), the firmware must be updated. We have observed that many vendors in the Indian market stop providing firmware updates after 3-5 years, leaving hardware permanently vulnerable.
Disabling the Telnet Service on Non-Essential Systems
If a device supports SSH, Telnet should be disabled immediately. On modern Linux systems, Telnet is often managed by systemd via a socket. We recommend masking the socket to prevent it from being started by any process or dependency.
Disabling telnet on a modern Linux system
sudo systemctl stop telnet.socket sudo systemctl disable telnet.socket sudo systemctl mask telnet.socket
Verifying the status
sudo systemctl status telnet.socket
On older systems using inetd or xinetd, you must edit the configuration file (usually /etc/inetd.conf) and comment out the line starting with telnet.
Restricting Access via Network Access Control Lists (ACLs)
When Telnet cannot be disabled due to legacy hardware requirements, it must be isolated. We implement strict ACLs on the upstream switch or the device itself. Access should only be permitted from a dedicated management Jump Host.
Example Cisco IOS ACL to restrict Telnet access
access-list 10 permit 10.10.50.5 # Management Jump Host IP line vty 0 4 transport input telnet access-class 10 in
System-Level Hardening and Exploit Prevention
Implementing Address Space Layout Randomization (ASLR)
ASLR makes it difficult for an attacker to predict the memory addresses of the stack, heap, and libraries. While most modern Linux kernels have ASLR enabled by default, many embedded systems used in Indian ISP networks have it disabled to save CPU cycles or because they use older kernels (e.g., 2.6.x).
To check and enable ASLR on a Linux-based management host:
Check ASLR status (0 = disabled, 1 = conservative, 2 = full)
cat /proc/sys/kernel/randomize_va_space
Enable full ASLR
echo 2 | sudo tee /proc/sys/kernel/randomize_va_space
Utilizing Data Execution Prevention (DEP) and NX Bits
DEP (or the NX bit) marks certain areas of memory, such as the stack and heap, as non-executable. This prevents an attacker from executing shellcode even if they successfully overflow a buffer and redirect execution to their payload. We verify NX support using readelf on the telnetd binary.
Checking for NX bit on the telnetd binary
readelf -l /usr/sbin/in.telnetd | grep STACK
Expected output: GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 RW
If 'E' is missing from 'RW', the stack is non-executable.
Stack Canaries and Compiler-Based Protections
Stack canaries are small values placed on the stack before the return address. If a buffer overflow occurs, the canary is overwritten first. The program checks the canary before returning; if it has changed, the program terminates to prevent exploitation. When compiling custom versions of network daemons, we always use the -fstack-protector-all flag in GCC.
Network-Layer Defenses Against Exploitation
Deploying Intrusion Detection and Prevention Systems (IDS/IPS)
An IDS like Suricata or Snort can detect the signature of a Telnet buffer overflow attempt. Specifically, we look for high frequencies of IAC sub-negotiation commands or sequences of 0xFF bytes that exceed typical protocol behavior, often integrated with a SIEM for real-time log monitoring.
Example Suricata rule for detecting CVE-2020-10188
alert tcp $EXTERNAL_NET any -> $HOME_NET 23 (msg:"ET EXPLOIT Possible netkit-telnetd Heap Overflow (CVE-2020-10188)"; flow:established,to_server; content:"|ff fa 18|"; depth:3; isdataat:500; classtype:attempted-admin; sid:1000001; rev:1;)
Using Firewalls to Block Port 23 Traffic
In a modern architecture, Port 23 should be blocked at the network perimeter. Within the internal network, it should be blocked between different VLANs. We use iptables or nftables on the local host to drop any Telnet traffic not originating from the secure management subnet.
Drop all Telnet traffic except from the management subnet
sudo iptables -A INPUT -p tcp -s 10.10.50.0/24 --dport 23 -j ACCEPT sudo iptables -A INPUT -p tcp --dport 23 -j DROP
Monitoring for Anomalous Telnet Traffic Patterns
We monitor for cleartext credential harvesting by capturing traffic on Port 23. If we see a high volume of traffic from a single IP attempting to authenticate via Telnet, it usually indicates a brute-force attack or an automated exploit scanner.
Monitoring Telnet traffic for cleartext passwords (for audit purposes only)
sudo tcpdump -i eth0 port 23 -A | grep -i "password"
The Ultimate Mitigation: Transitioning to Secure Protocols
Migrating from Telnet to SSH (Secure Shell)
SSH provides encrypted communication, strong authentication, and integrity checking. For Indian organizations falling under the DPDP Act 2023, migrating to secure SSH access for teams is not just a technical recommendation but a step toward compliance regarding the protection of administrative access credentials.
When migrating, we replace the telnetd package with openssh-server. We then generate strong host keys and disable legacy algorithms, adhering to OpenSSH security standards.
Configuring SSH for Maximum Security
A default SSH installation is better than Telnet, but it still requires hardening. We modify /etc/ssh/sshd_config to enforce key-based authentication and disable root login, following an SSH security hardening best practices guide.
/etc/ssh/sshd_config Hardening Snippet
Protocol 2 PermitRootLogin no MaxAuthTries 3 PubkeyAuthentication yes PasswordAuthentication no PermitEmptyPasswords no AllowTcpForwarding yes X11Forwarding no KexAlgorithms [email protected],diffie-hellman-group-exchange-sha256 Ciphers [email protected],[email protected]
After modifying the configuration, we validate it and restart the service:
sudo sshd -t sudo systemctl restart ssh
Phasing Out Legacy Hardware and Software Dependencies
For devices that absolutely cannot run SSH, such as old serial-to-ethernet converters in Indian power substations, we use SSH tunneling. This creates a secure "wrapper" around the Telnet traffic. The Telnet service is bound to 127.0.0.1 on the device (or restricted by a hardware firewall), and we tunnel into it from the Jump Host.
Creating an SSH tunnel to a remote Telnet service
This maps local port 5000 to the remote device's port 23
ssh -L 5000:127.0.0.1:23 user@jump-host -N -f
Connecting to the legacy device securely through the tunnel
telnet localhost 5000
Best Practices for Ongoing Vulnerability Management
Regular Security Audits and Penetration Testing
We perform quarterly audits using automated tools and manual verification. In the Indian SME sector, we often find that "shadow IT"—unauthorized devices added by technicians—reintroduces Telnet into the environment. A penetration test should specifically attempt to exploit telnetd to demonstrate the risk to stakeholders.
Automated Vulnerability Scanning for Network Daemons
We integrate tools like OpenVAS or Nessus into the CI/CD pipeline for infrastructure. These scanners are configured to flag any instance of Port 23 as a "Critical" finding. Under the DPDP Act 2023, maintaining a record of these scans and the subsequent remediation steps is vital for demonstrating "reasonable security practices."
Developing an Incident Response Plan for Buffer Overflow Attacks
If a buffer overflow is detected, the incident response plan must include:
- Immediate isolation of the affected segment.
- Capturing a memory dump of the
telnetdprocess for forensic analysis. - Reviewing logs for lateral movement (e.g., attempts to access
/etc/shadowor sensitive databases). - Notifying CERT-In if the breach affects critical information infrastructure.
When generating SSH keys for management, we use Ed25519 for the best balance of security and performance:
$ ssh-keygen -t ed25519 -a 100 -C "admin@research-lab"
The resulting public key should be deployed to the ~/.ssh/authorized_keys file on all managed devices, while the private key is stored on a hardware security module (HSM) or a dedicated, encrypted workstation, while also detecting malicious extensions on the administrator's machine.
Check the dmesg output for "segfault" errors associated with in.telnetd; this is often the first indicator that an automated exploit attempt has crashed the service due to an incorrect memory offset.