Editing Telnet (section)

== Security vulnerabilities ==
Telnet is vulnerable to [[Cyberattack|network-based cyberattacks]], such as [[packet sniffing]] sensitive information including passwords and [[Fingerprint (computing)|fingerprinting]].<ref name="das2015" />
<ref name="shimonski2002">{{Citation |last1=Shimonski |first1=Robert J. |title=Chapter 11 - Detecting and Performing Security Breaches with Sniffer Pro |date=2002-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9781931836579500150 |work=Sniffer Pro Network Optimization and Troubleshooting Handbook |pages=513–565 |editor-last=Shimonski |editor-first=Robert J. |place=Burlington |publisher=Syngress |language=en |isbn=978-1-931836-57-9 |access-date=2023-01-12 |last2=Eaton |first2=Wally |last3=Khan |first3=Umer |last4=Gordienko |first4=Yuri |editor2-last=Eaton |editor2-first=Wally |editor3-last=Khan |editor3-first=Umer |editor4-last=Gordienko |editor4-first=Yuri |doi=10.1016/B978-193183657-9/50015-0}}</ref>
<ref name="samtani2016">{{Cite book |last1=Samtani |first1=Sagar |last2=Yu |first2=Shuo |last3=Zhu |first3=Hongyi |last4=Patton |first4=Mark |last5=Chen |first5=Hsinchun |title=2016 IEEE Conference on Intelligence and Security Informatics (ISI) |chapter=Identifying SCADA vulnerabilities using passive and active vulnerability assessment techniques |date=2016 |chapter-url=https://ieeexplore.ieee.org/document/7745438 |pages=25–30 |doi=10.1109/ISI.2016.7745438|isbn=978-1-5090-3865-7 |s2cid=11741873 }}</ref>
Telnet services can also be exploited to leak information about the server (such as hostnames, IP addresses and brand) by packet sniffing the banner. This information can then be searched to determine if a Telnet service accepts a connection without [[authentication]]. Telnet is also frequently exploited by [[malware]] due to being improperly configured.<ref name="srinivasa2021" /> In fact, Telnet is targeted by attackers more frequently than other common protocols, especially when compared to [[Universal Plug and Play|UPnP]], [[CoAP]], [[MQTT]], [[Advanced Message Queuing Protocol|AMQP]] and [[XMPP]] {{Citation needed|date=July 2024}}. Common devices targeted are [[Internet of things|Internet of things devices]], routers and modems.

The [[SANS Institute]] recommends that the use of Telnet for remote logins should be discontinued under normal circumstances for the following reasons:<ref>{{Cite web |last=Kirk |first=Jeremy |date=2007-02-12 |title=Zero-day flaw in Solaris allows remote attacks |url=https://www.networkworld.com/article/2295079/zero-day-flaw-in-solaris-allows-remote-attacks.html |access-date=2023-01-12 |website=Network World |language=en}}</ref>

* Telnet, by default, does not [[encryption|encrypt]] any data sent over the connection (including passwords), and so it is often feasible to eavesdrop on the communications and use the password later for malicious purposes; anybody who has access to a [[Router (computing)|router]], [[network switch|switch]], [[network hub|hub]] or [[Gateway (computer networking)|gateway]] located on the network between the two hosts where Telnet is being used can intercept the packets passing by and obtain login, password and whatever else is typed with a [[packet analyzer]].<ref name="samtani2016" />
* Most Telnet implementations lack authentication. An estimated 22,887 Telnet-enabled devices found by security researchers not only lacked authentication but also provided [[Root privileges|unrestricted access]] to the system.<ref name="srinivasa2021" />
* Most Telnet authentication mechanisms are vulnerable to being intercepted by [[Man-in-the-middle attacks]].<ref name="samtani2016" />
Extensions to Telnet provide [[Transport Layer Security]] (TLS) security and [[Simple Authentication and Security Layer]] (SASL) authentication that address the above concerns.<ref name="mahmood2003">{{Cite book |last=Mahmood |first=H.B. |title=9th Asia-Pacific Conference on Communications (IEEE Cat. No.03EX732) |chapter=Transport layer security protocol in Telnet |date=2003 |chapter-url=https://ieeexplore.ieee.org/document/1274255 |volume=3 |pages=1033–1037 Vol.3 |doi=10.1109/APCC.2003.1274255|isbn=0-7803-8114-9 |s2cid=56798078 }}</ref>  However, most Telnet implementations do not support these extensions; and they do not address other vulnerabilities such as parsing the banner information.<ref name="samtani2016" /> Telnet over [[VPN]] is a viable option if SSHv2 is not supported, or a VPN is already used to securely tunnel other application data to the remote network the Telnet server is present in. However, precautions must be taken: ideally the VPN should terminate on the Telnet server itself, unless the [[LAN]] has additional security measures against eavesdropping and modification by other devices such as additional encryption and/or [[VLANs]]. This is because Telnet traffic leaves the VPN server in its insecure plaintext form after it is decrypted. The VPN software should be a trusted one that is heavily audited (e.g. [[OpenVPN]], [[WireGuard]], [[IPsec]]), using preferably certificate-based/public key [[mutual authentication]].

IBM 5250 or 3270 workstation emulation is supported via custom telnet clients, [[IBM 5250#Telnet 5250|TN5250]]/[[Telnet 3270|TN3270]], and [[IBM i]] systems. Clients and servers designed to pass [[IBM 5250]] data streams over Telnet generally do support [[Secure Sockets Layer|SSL]] encryption, as SSH does not include 5250 emulation. Under [[IBM i]] (also known as OS/400), port 992 is the default port for TelnetS (Telnet over SSL/TLS).<ref>{{cite web |title=TCP/IP Ports Required for IBM i Access and Related Functions |url=http://www-01.ibm.com/support/docview.wss?uid=nas8N1019667 |url-status=live |archive-url=https://web.archive.org/web/20160918095706/http://www-01.ibm.com/support/docview.wss?uid=nas8N1019667 |archive-date=2016-09-18 |publisher=IBM Support |access-date=2016-09-07}}</ref>