Multiple Telnet Client env_opt_add() Buffer Overflow Vulnerability

iDEFENSE Security Advisory 03.28.05
http://www.idefense.com/application/...ulnerabilities
March 28, 2005

I. BACKGROUND

The TELNET protocol allows virtual network terminals to be connected to=20
over the internet. The initial description of the telnet protocol was=20
given in RFC854 in May 1983. Since then there have been many extra=20
features added including encryption.=20

II. DESCRIPTION

Remote exploitation of a buffer overflow vulnerability in multiple
telnet clients could allow the execution of arbitrary code.

The vulnerability specifically exists in the env_opt_add() function of
telnet.c. A buffer of a fixed size (256 bytes) is allocated to store the
result of the processing this function performs on network input. If
this buffer is not large enough to contain the string, the buffer is=20
expanded by a further 256 bytes. This size is sufficient for most well=20
formed input, as the buffer passed as input to the affected function is=20
limited to the same size. However, due to the way the telnet protocol=20
escapes certain characters, it is possible to increase the length of the
output by including a large run of characters which need escaping. This=20
can allow the 256 byte input buffer to expand to a maximum of 512 bytes=20
in the allocated storage buffer. If, after expanding the buffer by 256=20
bytes, the buffer is still not large enough to contain the input, a heap
based buffer overflow occurs, which is exploitable on at least some=20
affected platforms.

III. ANALYSIS

Successful exploitation of this vulnerability could allow an attacker to
execute arbitrary commands in the context of the user who launched the=20
telnet client.

In order to exploit this vulnerability, an attacker would need to=20
convince the user to connect to their malicious server. It may be=20
possible to automatically launch the telnet command from a webpage, for=20
example:

<html><body>
<iframe src=3D'telnet://malicious.server/'>
</body>

On opening this page the telnet client may be launched and attempt to=20
connect to the host 'malicious.server'.

IV. DETECTION

iDEFENSE has confirmed the existance of the vulnerability in the telnet=20
client included in the Kerberos V5 Release 1.3.6 package and the client=20
included in the SUNWtnetc package of Solaris 5.9. It is suspected that=20
most BSD based telnet clients are affected by this vulnerability.

V. WORKAROUND

iDEFENSE is currently unaware of any effective workarounds for this=20
vulnerability.

VI. VENDOR RESPONSE

The following vendors have provided official responses related to this
vulnerability. Other vendors may be affected but have not provided an
official response.

Vulnerable:

- ALT Linux
All supported ALT Linux distributions include telnet client derived from
OpenBSD 3.0. The env_opt_add() buffer overflow vulnerability is present
in all our telnet clients. Updated packages with fixes for these issues
will be released on March 28, 2005.
http://lists.altlinux.ru/pipermail/s...rch/000287.ht=
ml

- Apple Computer, Inc.
Component: Telnet
Available for: Mac OS X 10.3.8, Mac OS X Server 10.3.8
This is fixed in Security Update 2005-003, which is available at
http://docs.info.apple.com/article.html?artnum=3D61798

- FreeBSD
FreeBSD-SA-05:01.telnet security advisory:
ftp://ftp.freebsd.org/pub/FreeBSD/CE...-05:01.telnet=
..asc

- MIT (Kerberos)
This vulnerability is covered in the following upcoming advisory:
MITKRB5-SA-2005-001:
http://web.mit.edu/kerberos/advisori...001-telnet.txt
patch against krb5-1.4:
http://web.mit.edu/kerberos/advisori...-patch_1.4.txt

- Openwall Project
The bugs are fixed starting with telnet package version 3.0-owl2.
http://www.openwall.com/Owl/CHANGES-current.shtml

- Red Hat, Inc.
Red Hat Enterprise Linux ship with telnet and krb5 packages vulnerable
to this issue. New telnet and krb5 packages are now available along
with our advisory at the URLs below and by using the Red Hat Network
'up2date' tool.
Red Hat Enterprise Linux - telnet
http://rhn.redhat.com/errata/RHSA-2005-330.html
Red Hat Enterprise Linux - krb5
http://rhn.redhat.com/errata/RHSA-2005-327.html

- Sun Microsystems Inc.
Sun confirms that the telnet(1) vulnerabilities do affect all
currently supported versions of Solaris:
Solaris 7, 8, 9 and 10
Sun has released a Sun Alert which describes a workaround until patches
are available at:
http://sunsolve.sun.com
Sun Alert #57755 =20
The Sun Alert will be updated with the patch information once it becomes
available. Sun patches are available from:
http://sunsolve.sun.com/securitypatch

Not Vulnerable:

- CyberSafe Limited
The CyberSafe TrustBroker products, version 3.0 or later, are not
vulnerable.

- Hewlett-Packard Development Company, L.P.
HP-UX and HP Tru64 UNIX are not vulnerable.

- InterSoft International, Inc.
InterSoft International, Inc. products NetTerm, SecureNetTerm and
SNetTerm are not affected by the env_opt_add() buffer overflow
conditions.

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2005-0468 to this issue. This is a candidate for inclusion
in the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

02/18/2005 Initial vendor notifications
03/28/2005 Coordinated public disclosure

IX. CREDIT

Ga=EBl Delalleau credited with this discovery.

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X. LEGAL NOTICES

Copyright =A9 2005 iDEFENSE, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
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