Cisco Security Response

MD5 Hashes May Allow for Certificate Spoofing

Document ID: 575

http://tools.cisco.com/security/center/content/CiscoSecurityResponse/cisco-sr-20090115-md5

Revision 1.0

For Public Release 2009 January 15 16:00  UTC (GMT)


Contents

Response
Additional Information
Status of this Notice: Final
Revision History
Cisco Security Procedures

Cisco Response

This is the Cisco response to research done by Alexander Sotirov, Marc Stevens, Jacob Appelbaum, Arjen Lenstra, David Molnar, Dag Arne Osvik, and Benne de Weger pertaining to MD5 collisions in certificates issued by vulnerable certificate authorities.

Cisco has released an IntelliShield activity bulletin detailing the specifics of this issue. This bulletin is available at the following link: http://tools.cisco.com/security/center/viewAlert.x?alertId=17341.

The Cisco Adaptive Security Appliance (ASA) and IOS may both serve as certificate authorities and by default use the MD5 hashing algorithm in the digital signatures of certificates issued to end users and devices.

The hashing algorithm used in digital certificates on the Cisco ASA cannot be changed; however, the ASA is unlikely to be affected by the attacks described in this research due to the way certificates are generated on the device. Cisco recognizes the weaknesses in MD5 and plans to alter the signature algorithm used in digital certificates and modify the methods utilized in creation of CA and endpoint certificates. This will be addressed by Cisco Bug ID: CSCsw88068 ( registered customers only) .

The Cisco IOS CA may be vulnerable to the attack described in this research when configured to utilize MD5 hashes in endpoint certificates. This is the default behavior; however, the device can be reconfigured to utilize a more secure hashing algorithm. Cisco plans to change this default behavior and modify the methods utilized in creation of CA and endpoint certificates. This will be addressed by Cisco Bug ID: CSCsw90626 ( registered customers only) .

Additional Information

As a workaround, an administrator can configure IOS devices running 12.4(15)T and later to use a more secure algorithm with the hash command, as shown in the following example:

Router(config)#crypto pki server <NAME> 
Router(cs-server)#shutdown 
Certificate server 'shut' event has been queued for processing.
Router(cs-server)#hash sha1 
Router(cs-server)#no shutdown 
Certificate server 'no shut' event has been queued for processing.

Status of this Notice: Final

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Revision History

Revision 1.0

2009-January-15

Initial public release

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco products, obtaining assistance with security incidents, and registering to receive security information from Cisco, is available on Cisco's worldwide website at http://www.cisco.com/web/about/security/psirt/security_vulnerability_policy.html. This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at http://www.cisco.com/go/psirt.


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