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TESO Security Advisory
06/10/2001

Multiple vendor Telnet Daemon vulnerability

Summary
===================

    Within most of the current telnet daemons in use today there exist a buffer
    overflow in the telnet option handling. Under certain circumstances it may
    be possible to exploit it to gain root priviledges remotely.

Systems Affected
===================

    System | vulnerable | exploitable *
    ----------------------------------------+--------------+------------------
    BSDI 4.x default | yes | yes
    FreeBSD [2345].x default | yes | yes
    IRIX 6.5 | yes | no
    Linux netkit-telnetd < 0.14 | yes | ?
    Linux netkit-telnetd >= 0.14 | no |
    NetBSD 1.x default | yes | yes
    OpenBSD 2.x | yes | ?
    OpenBSD current | no |
    Solaris 2.x sparc | yes | ?
    <almost any other vendor's telnetd> | yes | ?
    ----------------------------------------+--------------+------------------

    * = From our analysis and conclusions, which may not be correct or we may
        have overseen things. Do not rely on this.

    Details about the systems can be found below.

Impact
===================

    Through sending a specially formed option string to the remote telnet
    daemon a remote attacker might be able to overwrite sensitive information
    on the static memory pages. If done properly this may result in arbitrary
    code getting executed on the remote machine under the priviledges the
    telnet daemon runs on, usually root.

Explanation
===================

    Within every BSD derived telnet daemon under UNIX the telnet options are
    processed by the 'telrcv' function. This function parses the options
    according to the telnet protocol and its internal state. During this
    parsing the results which should be send back to the client are stored
    within the 'netobuf' buffer. This is done without any bounds checking,
    since it is assumed that the reply data is smaller than the buffer size
    (which is BUFSIZ bytes, usually).

    However, using a combination of options, especially the 'AYT' Are You There
    option, it is possible to append data to the buffer, usually nine bytes
    long. To trigger this response, two bytes in the input buffer are
    necessary. Since this input buffer is BUFSIZ bytes long, you can exceed the
    output buffer by as much as (BUFSIZ / 2) * 9) - BUFSIZ bytes. For the
    common case that BUFSIZ is defined to be 1024, this results in a buffer
    overflow by up to 3584 bytes. On systems where BUFSIZ is defined to be
    4096, this is an even greater value (14336).

    Due to the limited set of characters an attacker is able to write outside
    of the buffer it is difficult - if not impossible on some systems - to
    exploit this buffer overflow. Another hurdle for a possible attacker may be
    the lack of interesting information to modify after the buffer.

    This buffer overflow should be considered serious nevertheless, since
    experience has shown that even complicated vulnerabilities can be
    exploited by skilled attackers, BIND TSIG and SSH deattack come to mind.

    We have constructed a working exploit for any version of BSDI, NetBSD and
    FreeBSD. Exploitation on Solaris sparc may be possible but if it is, it is
    very difficult involving lots of arcane tricks. OpenBSD is not as easily
    exploitable as the other BSD's, because they do compile with other
    options by default, changing memory layout.

Solution
===================

    The vendors have been notified of the problem at the same time as the
    general public, vendor patches for your telnet daemon that fix the bug will
    show up soon.

    Sometimes a fix might not be trivial and require a lot of changes to the
    source code, due to the insecure nature the 'nfrontp' pointer is handled.
    The best long term solution is to disable the telnet daemon at all, since
    there are good and free replacements.

Acknowledgements
===================

    The bug has been discovered by scut.

    The tests and further analysis were done by smiler, lorian, zip and scut.

Contact Information
===================

    The TESO crew can be reached by mailing to tesoteam-teso.net
    Our web page is at http://www.team-teso.net/

References
===================

    [1] TESO
        http://www.team-teso.net/

Disclaimer
===================

    This advisory does not claim to be complete or to be usable for any
    purpose. Especially information on the vulnerable systems may be inaccurate
    or wrong. Possibly supplied exploit code is not to be used for malicious
    purposes, but for educational purposes only.

    This advisory is free for open distribution in unmodified form.
    Articles that are based on information from this advisory should include
    link [1].

Exploit
===================

    Not this time. Not here.

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