Advisory ID: cisco-sa-19980603-pix-key
For Public Release 1998 June 3 15:00 UTC (GMT)
PIX Private Link is an optional feature that can be installed in Cisco
PIX firewalls. PIX Private Link creates IP virtual private networks over
untrusted networks, such as the Internet, using tunnels encrypted with Data
Encryption Standard (DES). PIX Private Link in versions up to 4.1 uses DES in
ECB ("electronic codebook") mode.
An error in parsing of configuration file commands reduces the
effective key length for the PIX Private Link DES encryption to 48 bits from
the nominal 56 bits.
This advisory is posted at
This section provides details on affected products.
All users of the PIX Private Link encryption product with PIX software
versions earlier than the date of this notice are affected. This includes all
PIX Private Link software through version 4.1.6.
No other Cisco products are currently known to be affected by these
This vulnerability has been assigned Cisco bug ID CSCdk11848. The use
of ECB mode is Cisco bug ID CSCdj23353.
This vulnerability affects all released versions of PIX Private Link
software with version numbers up to and including 4.1.6, and all beta/interim
software released earlier than the date of this notice.
The first regular release containing a fix for this problem will be
version 4.2.1, which is tentatively scheduled for release in late June 1998.
This schedule is subject to change. Fixes for the 4.1 software release have not
yet been scheduled.
The 4.2.1 release also substitutes ECB mode with DES CBC mode.
Customers who need to upgrade immediately may contact Cisco's Technical
Assistance Center (TAC) to obtain interim software. Interim software has not
been subjected to full testing; it has a greater chance of containing serious
bugs than would regular released software.
Interim releases are available only by special request from the Cisco
TAC, not via the regular download channels. Cisco advises customers to install
interim releases only if absolutely necessary. Customers who choose to install
interim releases should plan to upgrade to the regular released software when
it becomes available.
When the fix is installed, it will be necessary to upgrade both ends of
each Private Link tunnel at the same time. This is because key the modified key
parsing algorithm will lead old and new versions to derive different encryption
keys from the same configuration file.
Software upgrades to correct this key-length problem will be offered
free of charge to all PIX Private Link customers, regardless of their service
contract status. Customers under contract may obtain upgrades through their
usual procedures. Customers not under contract should call the Cisco TAC.
Contact information for the TAC is in the "" section at the end of this message, and is available on
Cisco's Worldwide Web site at http://www.cisco.com/.
The use of ECB mode was a deliberate design decision for the PIX
Private Link product, and will not be changed. However, future IPSEC/IKE
products for the PIX platforms will use other encryption modes.
Cisco has provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in this Security Advisory is done in accordance with CVSS version 2.0.
CVSS is a standards-based scoring method that conveys vulnerability severity and helps determine urgency and priority of response.
Cisco has provided a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.
Cisco has provided an FAQ to answer additional questions regarding CVSS at
Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at
If attackers know the details of the key-parsing error in the PIX
Private Link software, they will know 8 bits of the key ahead of time. This
reduces the effective key length from the attacker's point of view from 56 to
48 bits. This reduction of the effective key length reduces the work involved
in a brute-force attack on the encryption by a factor of 256. That is,
knowledgeable attackers can, on the average, find the right key 256 times
faster than they would be able to find it with a true 56-bit key.
In addition to this key-length issue, some customers have expressed
concern over the use of DES ECB mode for PIX Private Link encryption. Although
the use of ECB mode is intentional, ECB is not generally considered to be the
best mode in which to employ DES, because it tends to simplify certain forms of
cryptanalysis and may permit certain replay attacks. Technical details of the
relative merits of various encryption modes are beyond the scope of this
document. Any interested reader should refer to a good cryptography text for
more information, such as Bruce Schneier's Applied
When considering software upgrades, also consult
and any subsequent advisories to determine exposure and a complete upgrade
In all cases, customers should exercise caution to be certain the
devices to be upgraded contain sufficient memory and that current hardware and
software configurations will continue to be supported properly by the new
release. If the information is not clear, contact the Cisco Technical
Assistance Center ("TAC") or your contracted maintenance provider for
There is no configuration workaround.
Cisco has made free software available to address this vulnerability
for affected customers. Prior to deploying software, customers should consult
their maintenance provider or check the software for feature set compatibility
and known issues specific to their environment.
Customers may only install and expect support for the feature sets they
have purchased. By installing, downloading, accessing or otherwise using such
software upgrades, customers agree to be bound by the terms of Cisco's software
license terms found at
or as otherwise set forth at Cisco.com Downloads at
Do not contact either "email@example.com" or "firstname.lastname@example.org"
for software upgrades.
Customers with contracts should obtain upgraded software through their
regular update channels. For most customers, this means that upgrades should be
obtained through the Software Center on Cisco's worldwide website at
Customers whose Cisco products are provided or maintained through prior
or existing agreement with third-party support organizations such as Cisco
Partners, authorized resellers, or service providers should contact that
support organization for guidance and assistance with the appropriate course of
action in regards to this advisory.
The effectiveness of any workaround or fix is dependent on specific
customer situations such as product mix, network topology, traffic behavior,
and organizational mission. Due to the variety of affected products and
releases, customers should consult with their service provider or support
organization to ensure any applied workaround or fix is the most appropriate
for use in the intended network before it is deployed.
Customers who purchase direct from Cisco but who do not hold a Cisco
service contract and customers who purchase through third-party vendors but are
unsuccessful at obtaining fixed software through their point of sale should get
their upgrades by contacting the Cisco Technical Assistance Center (TAC). TAC
contacts are as follows.
+1 800 553 2447 (toll free from within North America)
+1 408 526 7209 (toll call from anywhere in the world)
Have your product serial number available and give the URL of this
notice as evidence of your entitlement to a free upgrade. Free upgrades for
non-contract customers must be requested through the TAC.
for additional TAC contact information, including special localized telephone
numbers and instructions and e-mail addresses for use in various
Cisco has had no reports of malicious exploitation of this
Cisco knows of no public announcements of this vulnerability before the
date of this notice. This vulnerability was discovered by an engineering
analysis conducted by a Cisco customer at a security incident response
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This notice will be posted on Cisco's Worldwide Web site at
In addition to Worldwide Web posting, the initial version of this notice is
being sent to the following e-mail and Usenet news recipients:
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Future updates of this advisory, if any, will be placed on Cisco's
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newsgroups. Users concerned about this problem are encouraged to check the
above URL for any updates.
Update to reflect change in plan; ECB mode being changed to
Initial released version
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notices and of other Cisco security issues. This notice is copyright 1998 by
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