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Home : Advisories : Windwos 2000 Telnet Service DoS

Title: Windwos 2000 Telnet Service DoS
Released by: Alexander Ivanchev
Date: 30th November 2000
Printable version: Click here
I'd like to report the following issue with Windows 2000's Telnet Service


1. System Environment:

Windows 2000 Professional Final Build [Version 5.00.2195]

Service Pack 1, All latest windowsupdate.com security updates (as of


Telnet Service Build 5.00.99201.1

2. Classification

Denial of Service, possible code problems

3. Details

a. DoS - The Telnet Service in question is vulnerable to a simple Denial

of Service attack. The problem apparently lies within the login routine of

the daemon. The problem can be demonstrated by telneting to a machine

running the specified version of the Telnet Service and waiting at the

login/password prompt until a session timeout takes place. However, after

it does time out the connection is not reset by the daemon until the user

presses a key. In Windows 2000 Professional, due to the fact, it allows

only one telnet connection per host, this will effectively disable access

for the authorized user. We did not test the problem with Server/Advanced

Server/Datacenter but I believe that by establishing the maximum number of

allowed connections and not terminating them would result in the same

problem. Thus, this constitutes a Denial of Service attack. Theoretically,

it is also quite possible to exhaust server side sockets if there is not a

limit imposed on the maximum number of telnet sessions.

b. Possible code problem - On the Windows 2000 Professional test machine

the above vulnerability was tested, the following strange behavior of the

telnet service was observed:

By establishing a telnet session, and not terminating it, during the wait

interval, attempts to establish a different telnet session fail with the

following message:

Microsoft Windows Workstation allows only 1 Telnet Client License

Server has closed connection

Connection to host lost.

However, when a connection is attempted AFTER the session had timed out,

but it is still not reset, SOMETIMES the following return message would



Microsoft Windows Workstation allows only 1 Telnet Client License

Server has closed connection

Connection to host lost.

Where ECHELON is the hostname of the machine.

Needless to say, this does not seem right.

c. Note: When a machine comes under the above-described attack, the 'List

the current users' telnet admin option will NOT report established

connections, since a login would not have taken place, even though the

number of allowed connections could have been reached. (This of course

could be easily discovered using netstat or an equivalent utility)

4. Credits

The above problems were discovered and tested by Alexander Ivanchev of

Breach Technologies.

Alexander Ivanchev can be reached at ai@bulinfo.net or pr@breachtec.org

5. Disclosure Policy

Breach Technologies will wait for 21 days since the message is sent,

before disclosing details about the above matter. If an official

advisory/patch is/are released before this period ends, disclosure will

take place earlier

6. Feedback Policy

Please feel free to contact us with questions, corrections and comments

regarding these issues.

Thank you for your time and attention!

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