Public Vulnerability Reports

Multiple Telnet Client slc_add_reply() Buffer Overflow Vulnerability



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


Remote exploitation of an buffer overflow vulnerability error in
multiple telnet clients may allow execution of arbitrary commands.

THE vulnerability specifically exists in handling of the LINEMODE
suboptions, in that there is no size check made on the output, which is
stored in a fixed length buffer. By sending a specially constructed
reply containing a large number of SLC (Set Local Character) commands,
it is possible to overflow this buffer with server supplied data.


Successful exploitation would allow a remote attacker to execute
arbitrary code in the context of the user which initiated the telnet
process. In order to exploit this vulnerability, an attacker would need
to convince the user to connect to their malicious server. It may be
possible to automatically launch the telnet command from a webpage, for

<iframe src='telnet://malicious.server/'>

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


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


iDEFENSE is currently unaware of any effective workarounds for this


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


- ALT Linux
All supported ALT Linux distributions include telnet client derived from
OpenBSD 3.0. The slc_add_reply() buffer overflow vulnerability is
present in all our telnet clients.  Updated packages with fixes for
these issues will be released on March 28, 2005.

- 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

- FreeBSD
FreeBSD-SA-05:01.telnet security advisory:

- MIT (Kerberos)
This vulnerability is covered in the following upcoming advisory:
patch against krb5-1.4:

- Openwall Project
The bugs are fixed starting with telnet package version 3.0-owl2.

- Red Hat, Inc.
Red Hat Enterprise Linux ships 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
   Red Hat Enterprise Linux - krb5

- 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:
   Sun Alert #57755 
The Sun Alert will be updated with the patch information once it becomes
available. Sun patches are available from:

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 slc_add_reply() buffer overflow


The Common Vulnerabilities and Exposures (CVE) project has assigned the
names CAN-2005-0469 to these issues. This is a candidate for inclusion
in the CVE list (, which standardizes names for
security problems.


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


Gaël Delalleau credited with this discovery.

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