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Category:   Application (Generic)  >   Telnet Vendors:   [Multiple Authors/Vendors]
(Caldera Issues Updated Linux Fix) Telnet Daemons May Give Remote Users Root Level Access Privileges
SecurityTracker Alert ID:  1002167
SecurityTracker URL:
CVE Reference:   GENERIC-MAP-NOMATCH   (Links to External Site)
Date:  Aug 9 2001
Impact:   Execution of arbitrary code via network, Root access via network, User access via network
Fix Available:  Yes  Vendor Confirmed:  Yes  

Description:   TESO reported that many BSD-derived Telnet daemons (servers) contain a vulnerability that may allow a remote user to obtain root level access on the server.

The vulnerability is reportedly due to a buffer overflow in the telnet option handling.

The following systems are reported to be vulnerable:

BSDI 4.x default, FreeBSD [2345].x default, IRIX 6.5, Linux netkit-telnetd < 0.14, NetBSD 1.x default, OpenBSD 2.x, Solaris 2.x sparc, and "almost any other vendor's telnetd".

A remote user can send a specially formatted option string to the remote telnet server and overwrite sensitive memory, causing arbitrary code to be executed with the privileges of the telnet server (which is typically root level privileges).

Telnet options are reportedly processed by the 'telrcv' function. The results of the parsing, which are to be send back to the client, are stored in the 'netobuf' buffer. It is apparently assumed that the reply data is smaller than the buffer size, so no bounds checking is performed. By using a combination of options, especially the 'AYT' Are You There option, it is possible for a remote user to append data to the buffer. It is reported that the characters that can be written to the buffer are limited, which makes constructing a successful exploit difficult.

The report states that a working exploit has been developed for BSDI, NetBSD and FreeBSD. However, the exploit was not released.

Impact:   A remote user can execute arbitrary code on the server with the privileges of the telnet server, which is typically root level privileges.
Solution:   The vendor has released a fix for Caldera Linux. See the Source Message for the vendor's advisory containing directions on how to obtain the appropriate fix.
Cause:   Boundary error
Underlying OS:  Linux (Caldera/SCO)
Underlying OS Comments:  many Linux and Unix OSs are vulnerable, but not all - see the Alert text for more information

Message History:   This archive entry is a follow-up to the message listed below.
Jul 18 2001 Telnet Daemons May Give Remote Users Root Level Access Privileges

 Source Message Contents

Subject:  Security Update: [CSSA-2001-SCO.10]: OpenServer: /etc/telnetd buffer overflow

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Resending because of previous mail problems

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            Caldera International, Inc. Security Advisory

Subject:                OpenServer 5.0.5, 5.0.6: telnetd buffer overflow
Advisory number:        CSSA-2001-SCO.10
Issue date:             2001 August 7
Cross reference:

1. Problem Description
        The telnet daemon /etc/telnetd is subject to a buffer
        overflow problem that could be used by a malicious user to
        gain unauthorized access to a system.

2. Vulnerable Versions

        Operating System        Version         Affected Files
        OpenServer 5            All             /etc/telnetd

3. Workaround


4. OpenServer

  4.1 Location of Fixed Binaries

  4.2 Verification

        md5 checksums:
        a3fead5326b361cc9a94f0376fdf0f10        telnetd.Z

        md5 is available for download from


  4.3 Installing Fixed Binaries

        Save the erg711793a.Z compressed tar archive into /tmp, and
        install/upgrade the affected binaries with the following commands:

        # cd /tmp
        # uncompress erg711793a.Z
        # tar xvf erg711793a
        # uncompress telnetd
        # mv /etc/telnetd /etc/telnetd.old
        # cp telnetd /etc
        # chown bin:bin /etc/telnetd
        # chmod 711 /etc/telnetd

        In addition, on any pre-5.0.6 system, or on any 5.0.6 system 
        without rs506a, execute the following commands:

        # uncompress
        # uncompress
        # cp /usr/lib
        # cp /usr/lib
        # chown bin:bin /usr/lib/
        # chown bin:bin /usr/lib/
        # chmod 555 /usr/lib/
        # chmod 555 /usr/lib/

5. References

6. Disclaimer

        Caldera International, Inc. is not responsible for the misuse
        of any of the information we provide on our website and/or
        through our security advisories. Our advisories are a service
        to our customers intended to promote secure installation and
        use of Caldera International products.


        Caldera International would like to thank Sebastian
        <> for his posting on bugtraq, and KF
        <> for reporting the problem to us.

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