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CERT* Vendor-Initiated Bulletin VB-97.09
October 2, 1997
Topic: Vulnerabilities in Cisco CHAP Authentication
Source: Cisco Systems
To aid in the wide distribution of essential security
information, the
CERT Coordination Center is forwarding the following
information from
Cisco Systems. Cisco urges you to act on this information
as soon as
possible. Cisco contact information is included in the
forwarded text
below; please contact them if you have any questions or
need further
information.
=======================FORWARDED TEXT STARTS
HERE============================
Cisco Systems Field Notice:
Vulnerabilities in Cisco CHAP Authentication
October 1, 1997, 09:00 AM US/Pacific, Revision 4
-----------------------------------------------------------
-------------
Summary
-------
A serious security vulnerability (bug ID CSCdi91594)
exists in PPP CHAP
authentication in all "classic" Cisco IOS software
versions (the software
used on Cisco non-switch products with product numbers
greater than or equal
to 1000, on the AGS/AGS+/CGS/MGS, and on the CS-500, but
not on Catalyst
switches or on 7xx or 9xx routers) starting with the
introduction of CHAP
support in release 9.1(1). The vulnerability permits
attackers with
appropriate skills and knowledge to completely circumvent
CHAP
authentication. Other PPP authentication methods are not
affected.
A related vulnerability exists in Cisco IOS/700 software
(the software used
on 7xx routers). A configuration workaround exists for
IOS/700, and a
complete fix for 76x and 77x routers will be included in
software version
4.1(2), due to be released by December, 1997. A fix for
75x routers is
scheduled for the first half of 1998.
This problem has been corrected in the following classic
Cisco IOS software
releases:
Major First Repaired Recommended
Maintenance Releases
Release Maintenance Release For
Installation
------- -------------------
----------------
Cisco IOS
10.3 10.3(19a) 10.3(19a)
Cisco IOS
11.0 11.0(17), 11.0(17)BT 11.0(17),
11.0(17)BT
Cisco IOS 11.1(13), 11.1(13)AA, 11.1(14),
11.1(14)AA, 11.1(14)CA,
11.1 11.1(13)CA, 11.1(13)IA 11.1(14)IA
Cisco IOS 11.2(8), 11.2(8)P, 11.2(8),
11.2(8)P, 11.2(4)F1.
11.2 11.2(4)F1 (replaces 11.2(9) not
recommended for CHAP
11.2(4)F) users.
Cisco Systems strongly recommends that all customers using
classic IOS PPP
with CHAP authentication upgrade to one of these or to a
newer release, and
that all users of IOS/700 PPP with CHAP authentication
install the
configuration workarounds described in this document.
The 11.2(4)F1 release will be available by Monday, October
6, 1997. Users of
11.2F releases are encouraged to move to 11.2 or 11.2P
releases if at all
possible. All the other releases mentioned above are
available immediately
as of the release of this notice.
The recommended release numbers listed above are expected
to be the best
choices for most common situations, but it's very
important that customers
evaluate their network configurations and other needs
before choosing which
releases to use.
Cisco is offering free software upgrades to all classic
IOS PPP users in
order to address this vulnerability. Upgrade details are
at the end of this
notice. Free upgrades will be offered to IOS/700 users
upon release of
IOS/700 version 4.1(2).
Impact
------
A moderately sophisticated programmer with appropriate
knowledge can set up
an unauthorized PPP connection to any system that is
running vulnerable
software, and that depends on CHAP for authentication. To
gain this
unauthorized access, an attacker must have the following:
* Knowledge of the details of this vulnerability
* Access to modifiable code (generally meaning source
code) for a
PPP/CHAP implementation, and sufficient programming
skill to make
simple changes to that code. Note that such source
code is widely
available on the Internet.
* A modest amount of information about the
configuration of the network
to be attacked, including such things as usernames
and IP addresses.
This vulnerability cannot be exploited by an attacker who
is using an
unmodified, properly functioning PPP/CHAP implementation;
the attacker must
make modifications to his or her software to exploit this
vulnerability.
Who is Vulnerable
-----------------
All systems running "classic" Cisco IOS Software releases
older than those
listed above, and which rely on CHAP for PPP
authentication, are vulnerable.
Cisco believes that the greatest practical risk is to
dialin services using,
for example, ISDN or POTS modems.
Systems running IOS/700 software are vulnerable to a
related attack if they
are using CHAP bidirectionally to authenticate both
calling and called
systems.
Systems using PAP for PPP authentication are not
vulnerable. Systems not
configured for PPP are not vulnerable. If the keywords
"ppp" and "chap" do
not both appear in your system configuration file, you are
not vulnerable.
Workarounds - Classic IOS
-------------------------
Cisco knows of no generally usable workarounds for the
classic IOS
vulnerability. Affected users who wish to protect
themselves must upgrade
their software or stop using CHAP authentication.
Alternatives to CHAP
authentication include PAP authentication and reliance on
"Caller ID"
information. The security differences between these
methods are complex and
situation dependent, and are beyond the scope of this
document.
Workarounds - IOS/700
---------------------
The IOS/700 vulnerability may be avoided by making any of
the following
configuration changes:
* Prevent the routers in question from receiving any
incoming calls,
perhaps by changing the ISDN switch configuration, or
by relying on
caller ID and using the "set callerid" and "set
callidreceive" commands
* Prevent routers that receive calls from
authenticating themselves to
the calling systems using CHAP. You can effectively
do this by using
the "set ppp secret client" command to set the CHAP
secret that would
be used for such authentication to some randomly
chosen "garbage"
value.
* Configure the routers such that different CHAP
secrets are used in each
direction on each link. You can do this using the
"set ppp secret
client" and "set ppp secret host" commands. Note that
this method
cannot be used on 7xx routers that need to
communicate with classic IOS
routers, since classic IOS does not support
asymmetric CHAP secrets.
Any one of these changes should be sufficient. The changes
may be removed
after the release and installation of IOS/700 software
version 4.1(2).
Classic IOS Software Upgrade Notes
----------------------------------
You should upgrade your classic Cisco IOS software to one
of the releases
mentioned in the first section of this notice, or to a
later release.
Instructions for obtaining the new software are at the end
of this notice.
Instructions for installing upgraded software are in the
standard system
documentation.
Before installing any Cisco IOS software upgrade, you
should always verify
that the new software is compatible with your hardware.
It's especially
important to make sure that you have enough memory to do
the upgrade.
General assistance and full system documentation are
available via the
Internet's Worldwide Web at .
Before installing any upgrade of any description, it's
always wise to make
sure that the version you're installing has no bugs that
will negatively
impact your configuration. Please check Cisco's Web site
for more
information and advice on software upgrades in general.
The new software has been changed in a number of ways in
order to make it
more resistant to CHAP-related attacks. Some of those
changes may cause CHAP
authentication to fail in certain customer networks. Cisco
believes the
affected configurations to be rare. If you install
upgraded software, and
legitimate CHAP connections stop working, please see the
paragraphs
immediately following this one, which we believe describe
the failures that
are likely to be be seen in real networks. If you still
can't get CHAP
working after reading the paragraphs below, please call
the Cisco TAC for
assistance in reconfiguring your software.
The fix for this vulnerability was released in Cisco IOS
software version
10.3(19), but an error in the implementation of the fix
caused almost all
CHAP authentication between 10.3(19) systems to fail. This
error is
corrected in 10.3(19a). 10.3(19) may be safely used if the
command "no ppp
chap wait" is configured for each interface on which CHAP
is used. Because
multiple fixes have been introduced for the potential
attack against which
the modified behavior guards, using "no ppp chap wait"
will not appreciably
increase your system's vulnerability.
If an intermediate device, such as an ISDN switch,
establishes incoming
calls to two separate systems running the modified IOS
software, and then
places those two systems in contact with one another, CHAP
authentication
between the two systems may fail. This is because each
system "thinks" that
it's receiving a call, and neither system "thinks" that it
originated the
call. If this is a problem in your configuration, use the
command "ppp
direction dedicated" on the affected interfaces of both
systems.
Exploitation
------------
Cisco is not aware of these vulnerabilities having been
exploited by "system
crackers", nor of any publicly available exploitation
code. Cisco does not
believe that the details of the vulnerabilities are widely
understood in the
cracker community. The theoretical possibility of these
vulnerabilities has,
however, been discussed fairly openly among PPP security
professionals.
Even though Cisco does not know of active exploitation of
these
vulnerabilities, Cisco expects that the cracker community
will eventually
"discover" them, and that the issuance of this notice will
tend to
accelerate that process. Vulnerable customers should
upgrade or install
workarounds with all possible speed.
Details of the Vulnerabilities
------------------------------
Cisco will not release any further details of these
vulnerabilities at this
time. Further details will be available to interested
parties after March
31, 1998.
The Cisco bug tracking number for the Cisco IOS software
vulnerability is
CSCdi91594. The bug tracking number for the error in the
10.3(19) fix is
CSCdj37314.
Upgrades
--------
Cisco customers and service partners can obtain the latest
releases of Cisco
IOS software from the Software Center within Cisco
Connection Online (CCO),
Cisco's Internet customer support service. CCO is located
at
"".
1. Existing Registered Users for CCO
Customers and Partners with Cisco IOS software
service agreements who
are already registered for CCO may proceed directly
to the Cisco IOS
Software Center to obtain a new software release to
solve this issue.
The Software Center includes Software Upgrade
Planners to inform you
about new features, additional caveats, release
notes, and
compatibility requirements to ensure a successful
upgrade. The Cisco
IOS Software Center is located at
"".
2. New Registered Users for CCO
Customers and Partners who have a software service
contracts directly
with Cisco or a Cisco Partner, but have not yet
registered for Cisco
Connection Online, and who know their contract
number, can proceed
directly to register online at
"".
Online registration
takes effect immediately, after which customers may
proceed directly to
the Software Center to obtain an upgrade.
3. Assistance in Registering on CCO
Customers and Partners who have a software service
contract, but need
to confirm their contract number to register for CCO,
or require any
other assistance registering for CCO access should
contact Cisco's
Global Technical Assistance Center (TAC) at
1-800-553-2447,
1-408-526-7209, or email "tac@cisco.com". Additional
worldwide contacts
for Cisco support can be found at
""
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