Home > Security > From 0day to 0data: TelnetD

From 0day to 0data: TelnetD

Recently, it was found that BSD-derived Telnet implementations had a fairly straightforward vulnerability in their encryption handler. (Also, it was found that there was an encryption handler.) Telnet was the de facto standard protocol for remote administration of everything but Windows systems, so there’s been some curiosity in just how nasty this bug is operationally.

So, with the help of Fyodor and the NMAP scripting team, I set out to collect some data on just how widespread this bug is. First, I set out to find all hosts listening on tcp/23. Then, with Fyodor’s code, I checked for encryption support. Presence of support does not necessarily imply vulnerability, but absence does imply invulnerability.

The scanner’s pretty rough, and I’m doing some sampling here, but here’s the bottom line data:

Out of 156113 confirmed telnet servers randomly distributed across the Internet, only 429 exposed encryption support. By far my largest source of noise is the estimate of just how many telnet servers (as opposed to just TCP listeners on 23, or transient services) exist across the entire Internet. My conservative numbers place that count at around 7.8M. This yields an estimate of ~13785 ~21546 (or 15,600 to 23,500, at 95% confidence — thanks, David Horn!) potentially vulnerable servers.

Patched servers will still look vulnerable unless they remove encryption support entirely, so this isn’t something we can watch get fixed (well, unless we’re willing to crash servers, which I’m not, even if it’d be safe because these things are launched from inetd).

Here is a command line that will scan your networks. You may need nmap from svn.

/usr/local/bin/nmap -p23 -PS23 --script telnet-encryption -T5 -oA telnet_logs -iL list_of_ips -v

So, great bug, just almost certainly not widely distributed. I’ll document the analysis process when I’m not at con.

Categories: Security
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