Port 2001 – Cisco Console (Terminal Server Reverse-Telnet)

Port 2001 is a chameleon. On a Cisco terminal or access server it’s the reverse-telnet port for the first asynchronous line — Cisco maps each console/aux line to TCP 2000 + line number, so line 1 answers on 2001 — and connecting to it gives out-of-band, cleartext console access to whatever router, switch, or serial device is cabled to that line. IANA registers 2001/tcp under the long-dead names dc / wizard, so scanners frequently label an open 2001 as dc, and it turns up in a handful of old malware port lists too. Whatever it turns out to be, if it’s a reverse-telnet line it is Telnet: the login, the password, and every keystroke cross the wire in the clear. One thing it is not is Secure Copy Protocol — SCP runs over SSH on port 22, not 2001, so the old “Cisco Secure Copy Protocol” label for this port is simply wrong.

Why It’s Open

A Cisco access server or terminal server is a router fitted with async modules (an old 2500-series access server, or a router with an NM-16A/NM-32A card) whose serial lines are cabled to the console ports of other devices. It presents each attached console on a TCP port equal to 2000 + the line number, so administrators reverse-telnet to 2001, 2002, 2003… to reach the consoles of racked routers, switches, firewalls, and other serial gear for out-of-band (OOB) management — the way you get to a box when the network path to it is down. An open 2001 therefore usually means a console/terminal server sitting in front of a rack.

Beyond the Cisco world, 2001 also shows up as a generic “alt-management/admin” listener and, historically, in malware port lists from the early-2000s trojan era — which is exactly why an unexpected 2001 is worth identifying rather than trusting. Modern out-of-band designs have moved to SSH-based console servers (Opengear, Lantronix, and friends) or SSH on the Cisco itself; a plain reverse-telnet line on 2001 is the legacy, cleartext way of doing the same job.

Common Risks

• No encryption. Reverse-telnet is Telnet. The attached device’s entire console session — its login prompt, its enable password, and every command — is transmitted in cleartext and can be read straight off the wire by anyone on the path.
• Unauthenticated console access. Async lines are very often configured with no login (or a single shared line password) so on-call engineers can jump onto a console quickly. Where that’s the case, simply reaching 2001 drops you onto the attached device’s console with whatever it grants — frequently enable or even ROMMON.
• Full device control. A console is the most privileged path into network gear: password recovery, full reconfiguration, and firmware-level access all run through it. Reverse-telnet hands that path to anyone who can reach the port.
• Session hijacking and stuck sessions. Reverse-telnet lines don’t time out on their own; an idle line can hold a pre-authenticated session that a later connector attaches to, and a crafted connection can wedge the device’s VTY pool (see CVE-2004-1464 below).
• Unknown-service ambiguity. Scanners label 2001 dc/wizard. An open 2001 could be a sanctioned console server, a forgotten admin daemon, or malware — fingerprint it before you trust it.

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Enumeration & Testing

Check if it’s open and identify the service

nmap -sV -p 2001 --script=banner <target>

Sweep a terminal server’s whole line range

nmap -sV -p 2000-2032 <target>

Several sequential open ports (2001, 2002, 2003…) is the tell-tale sign of a console/terminal server fronting multiple attached devices.

Raw banner grab with netcat

nc -nv <target> 2001

Interactive connection

telnet <target> 2001

If you land on a raw prompt, an IOS banner, or Press RETURN to get started, you’re on a reverse-telnet console — press Return and see what level of access the attached device gives you.

Brute-force a line password with Hydra

hydra -P passwords.txt telnet://<target>:2001 -t 4 -f

Reverse-telnet lines usually have no username — just a line password (or nothing at all) — so brute-force the password only, and watch for lines that accept an empty credential.

Capture every open port in the 2000-range and every console you can reach as you go, so the evidence lands in the pentest report instead of a scratch terminal you’ll lose.

What to Look For

Known CVEs and Exploits

There is no CVE for a service uniquely defined as “port 2001” — the dominant risk here is a design/configuration problem (cleartext, frequently unauthenticated console access to network gear), not a single patchable bug. The following are genuine, NVD-verified flaws on the Cisco Telnet / reverse-telnet surface that a reverse-telnet line on 2001 exposes:

• CVE-2004-1464 — Cisco IOS Telnet/reverse-telnet denial of service. A crafted TCP connection to a telnet or reverse-telnet port exhausts the device’s VTY resources so it can no longer accept telnet, reverse-telnet, RSH, SSH, or (in some cases) HTTP management sessions. Cisco’s advisory names the affected reverse-telnet ranges explicitly — 2001-2999, 3001-3099, 6001-6999, and 7001-7099 — making 2001 a textbook target. CVSS 5.0. See the Cisco advisory.
• CVE-2017-3881 — Cisco IOS/IOS XE Cluster Management Protocol (CMP) remote code execution. An unauthenticated attacker triggers it by sending malformed CMP-specific Telnet options while establishing a Telnet session, gaining RCE or reloading the device. CVSS 9.8. This is the “Vault 7” CMP-over-Telnet bug — direct proof that cleartext Telnet to Cisco gear is a real, unauthenticated RCE surface, not just an eavesdropping risk.
• CVE-2013-6979 — Cisco IOS XE VTY (Telnet) authentication bypass. The VTY auth implementation trusted the internal Linux-IOS 192.168.x.2 source address, letting a remote attacker reach the device without authenticating and act with the privileges configured on the vty line. CVSS 5.4.

Beware of pages that pin unrelated Cisco CVEs to this port — for example the IOS XE Web UI privilege-escalation bug (CVE-2023-20198) is an HTTP/HTTPS management-plane flaw (ports 80/443), not a port-2001/console issue, and doesn’t belong here.

Mitigation

• Don’t expose reverse-telnet/console-server lines to untrusted networks. Console access is the keys to the kingdom — TCP 2001 (and the whole 2000-20xx line range) should never be reachable from the internet or a general user VLAN. Put it on a dedicated out-of-band management network.
• Require authentication on every line. Never leave no login on a reverse-telnet line. Set a line password at minimum, and prefer AAA (login authentication) so console access is tied to real, auditable accounts.
• Prefer SSH-based OOB. Use SSH on port 22 on the device itself, or a modern console server that speaks SSH, instead of cleartext reverse-telnet — the same job, encrypted end to end. Reverse-telnet shares its entire risk profile with Telnet on port 23.
• ACL / VLAN-restrict TCP 2001. Apply a transit ACL (as Cisco recommends) so only the management network can reach the line ports.
• Identify what’s really on 2001. If the listener isn’t a sanctioned console server, fingerprint it — a dc/wizard service could be a forgotten admin daemon or malware. Close it if it’s unsanctioned.
• Patch IOS/IOS XE for the Telnet-surface CVEs above if reverse-telnet must remain enabled at all.

Real-World Example

In 2013, Rapid7’s HD Moore published Serial Offenders, a survey of internet-exposed serial-port/terminal servers. He found roughly 114,000 of them reachable online, and over 13,000 root shells, consoles, and admin interfaces that required no authentication at all — many already pre-authenticated by a legitimate user. The root cause is exactly the reverse-telnet model behind port 2001: the devices assume that anyone who can open the TCP session must be “physically” attached to the serial line, so they expose the connected console — routers, switches, SCADA/ICS gear, even POS terminals — straight over Telnet. Because those sessions never time out, an attacker can hijack an active console or simply wait for an idle, pre-authenticated shell and step into it. It’s the clearest illustration of why an open 2001 on an untrusted network is treated as a critical finding, not a curiosity.

FAQ

What is port 2001 used for?

On a Cisco terminal/access server, 2001 is the reverse-telnet port for the first asynchronous line (2000 + line 1), used for out-of-band console access to an attached device. IANA registers 2001/tcp under the obsolete names dc/wizard, and scanners often label an unknown 2001 listener dc.

Is port 2001 the Cisco Secure Copy Protocol?

No. Secure Copy (SCP) runs over SSH on port 22, not 2001. The “Cisco Secure Copy Protocol” label sometimes attached to this port is incorrect — 2001 is a Cisco reverse-telnet line, which is cleartext Telnet.

Is port 2001 dangerous?

On an untrusted network, yes. Reverse-telnet is cleartext, and the attached console is often unauthenticated, so anyone who can reach 2001 may gain privileged console access to network equipment. Restrict it to a management network or close it.

How is port 2001 different from port 2000?

Port 2000 is the base of the Cisco reverse-telnet range (and IANA’s SCCP/Skinny VoIP port); 2001 is 2000 + line 1, the first async console line. See our port 2000 page for the SCCP and Smart Install side of that number.

How do I close or secure port 2001?

Require authentication on the line (a line password or AAA), move out-of-band management to SSH, and ACL/VLAN-restrict TCP 2001 to a trusted management range. If the listener isn’t a sanctioned console server, identify and remove it, then rescan with nmap -p 2001 <target> to confirm it’s closed.

TL;DR

• Service: Cisco reverse-telnet console line (2000 + line#); IANA dc/wizard — not SCP
• Default port: 2001/TCP
• Biggest risk: cleartext + often unauthenticated console access to network gear
• Mitigation: require line auth/AAA, move OOB to SSH, ACL to a management network, and close it if it’s unsanctioned