Cisco Adaptive Security Appliance Software and Firewall Services Module Software Secure Shell Denial of Service Vulnerability

Summary

• A vulnerability in the implementation of the Secure Shell (SSH) function could allow an unauthenticated, remote attacker to deny SSH management access to legitimate users.
  
  The vulnerability is due to improper implementation of the logic to terminate SSH sessions. An attacker could exploit this vulnerability by starting an SSH session to the affected system. This vulnerability affects both SSHv1 and v2. An exploit could allow the attacker to deny SSH management access to legitimate users. The attacker can deny the access for 60 seconds, and then the affected system will release the resource and users will be able to use the SSH service again. However, repeated exploitation may lead to a denial of service condition.
  
  Cisco has confirmed the vulnerability in a security notice and software updates are available.
  
  An attacker can only deny access to a targeted system for 60 seconds, which then the affected system will release the resource to allow legitimate users SSH access again. However, repeated exploitation could lead to a sustained DoS condition.
  
  Customers are advised to review the bug report in the vendor announcements section for a current list of affected versions.
  
  Cisco indicates through the CVSS score that functional exploit code exists; however, the code is not known to be publicly available.

Affected Products

• Cisco has released a security notice for bug IDs CSCue63881, CSCuf51892, CSCue78671, and CSCug26937 at the following link: CVE-2013-1193

  Vulnerable Products

  At the time this alert was first published, the following versions of Cisco products were vulnerable. Later versions of Cisco products may also be affected.
  

  • Cisco ASA Software versions 8.4.6 and prior
  • Cisco FWSM versions 4.1.7 and prior

  
  

  Products Confirmed Not Vulnerable

  No other Cisco products are currently known to be affected by these vulnerabilities.

Workarounds

• Administrators are advised to apply the appropriate updates.
  
  Administrators may consider using IP-based access control lists (ACLs) to allow only trusted systems to access the affected systems.
  
  Administrators are advised to allow only privileged users to access administration or management systems.
  
  It is critical to prevent unauthorized direct communication to network devices. Restrict network traffic destined for the network infrastructure to protect against reconnaissance and DoS attacks. For configuration details, consult Protecting Your Core: Infrastructure Protection Access Control Lists.
  
  Administrators may consider using the HTTPS management interface instead of the SSH management access.
  
  Administrators are advised to monitor affected systems.

Fixed Software

• Cisco customers with active contracts should contact their Cisco support team for assistance in upgrading to a software version that includes fixes for this vulnerability. Cisco customers without contracts may contact the Cisco Technical Assistance Center at 1-800-553-2447 or 1-408-526-7209 or via e-mail at tac@cisco.com for assistance.

Exploitation and Public Announcements

• The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.

URL

• http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/Cisco-SA-20130412-CVE-2013-1193

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial Release	NA	Final	2013-Apr-12

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