Summary

• The Intrusion Prevention System (IPS) feature set of Cisco IOSŪ contains several vulnerabilities. These include:

  • Fragmented IP packets may be used to evade signature inspection.
    
  • IPS signatures utilizing the regular expression feature of the ATOMIC.TCP signature engine may cause a router to crash resulting in a denial of service.
    

  There are mitigations and workarounds for these vulnerabilities. Cisco has made free software available to address these vulnerabilities for affected customers.

  This advisory is posted at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20070213-iosips.

Affected Products

• 

  Vulnerable Products

  The following Cisco IOS release trains with the IPS feature set enabled are vulnerable to the fragmented packet evasion vulnerability:

  • 12.3T, except versions 12.3(2)T, 12.3(4)T, and 12.3(7)T
    
  • 12.4
    
  • 12.4T
    
  • 12.4XE
    

  The following Cisco IOS release trains with the IPS feature set enabled are vulnerable to the ATOMIC.TCP regular expression denial of service vulnerability:

  • 12.3T, except versions 12.3(2)T, 12.3(4)T, and 12.3(7)T
    
  • 12.3XQ, 12.3XR, 12.3XS, 12.3XW, 12.3XX, 12.3XY
    
  • 12.3YA, 12.3YD, 12.3YG, 12.3YH, 12.3YI, 12.3YJ, 12.3YK, 12.3YM, 12.3YQ, 12.3YS, 12.3YT, 12.3YX, 12.3YZ
    
  • 12.4
    
  • 12.4MR
    
  • 12.4T
    
  • 12.4XA, 12.4XB
    

  To determine if the IPS feature set is active on an IOS device, use the show ip ips configuration command. This command will list the interfaces configured to use IPS inspection. You will then need to further check the status of each interface to confirm if they are enabled or not.

  router#show ip ips configuration
  Configured Config Locations: -none-
  Last signature default load time: 18:46:50 UTC Jan 5 2007
  Last signature delta load time: -none-
  Last event action (SEAP) load time: -none-
  IPS Auto Update is not currently configured
  IPS fail closed is disabled
  Fastpath ips is enabled
  Quick run mode is enabled
  Event notification through syslog is enabled
  Event notification through SDEE is enabled
  Total Active Signatures: 85
  Total Inactive Signatures: 61
  IPS Rule Configuration
   IPS name test
  IPS Category CLI is not configured
  Interface Configuration
   Interface FastEthernet0/0
    Inbound IPS rule is test
    Outgoing IPS rule is not set
  
  router#show ip interface FastEthernet0/0
  FastEthernet0/0 is up, line protocol is up

  In the above example, interface FastEthernet0/0 is configured to use IPS and is shown to be enabled.

  Products Confirmed Not Vulnerable

  No other Cisco products are currently known to be affected by this vulnerability.

Details

• Cisco IOS Intrusion Prevention System (IPS) is an inline, deep-packet inspection-based feature that enables Cisco IOS software to mitigate network attacks. Cisco IOS IPS enables the network to defend itself with the intelligence to identify, classify, and stop or block certain malicious or damaging traffic in real time. The IOS IPS feature set contains multiple vulnerabilities. Only IOS images containing the IPS feature set are affected by these vulnerabilities.

  Fragmented Packet Evasion Vulnerability

  Some of the IPS signatures utilize regular expressions. Due to a vulnerability, an attacker can evade those IPS signatures by sending malicious network traffic as IP fragments. This may result in potential malicious traffic bypassing signature inspection and possibly allow the exploitation of protected systems. IPS signatures which do not utilize regular expressions are not affected by this vulnerability. All IP protocols (e.g. TCP, UDP, ICMP) are affected by this vulnerability. There is a mitigation for this vulnerability. This vulnerability is documented in Cisco Bug ID CSCsg15598 ( registered customers only) .

  ATOMIC.TCP Regular Expression Denial of Service Vulnerability

  Certain network traffic can trigger IPS signatures which use the regular expression feature of the ATOMIC.TCP signature engine which may cause the IOS IPS device to crash. This may cause a denial of service resulting in disruption network traffic. Signature 3123.0 (Netbus Pro Traffic) has been demonstrated to trigger this vulnerability. There is a workaround for this vulnerability. This vulnerability is documented in Cisco Bug ID CSCsa53334 ( registered customers only) .

  Vulnerability Scoring Details

  Cisco is providing scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS).

  Cisco will provide a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.

  Cisco PSIRT will set the bias in all cases to normal. Customers are encouraged to apply the bias parameter when determining the environmental impact of a particular vulnerability.

  CVSS is a standards based scoring method that conveys vulnerability severity and helps determine urgency and priority of response.

  Cisco has provided an FAQ to answer additional questions regarding CVSS at http://www.cisco.com/web/about/security/intelligence/cvss-qandas.html.

  Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at https://sec.cloudapps.cisco.com/security/center/cvssCalculator.x.

  CSCsg15598 ( registered customers only) - DYIDS: Fragmentation prevents signature recognition Calculate the environmental score of CSCsg15598
  CVSS Base Score - 4.7
  Access Vector	Access Complexity	Authentication	Confidentiality Impact	Integrity Impact	Availability Impact	Impact Bias
  Remote	Low	Not Required	Partial	Partial	None	Normal
  CVSS Temporal Score - 3.9
  Exploitability		Remediation Level		Report Confidence
  Functional		Official Fix		Confirmed

  CSCsa53334 ( registered customers only) - bus error in single_pkt_regex Calculate the environmental score of CSCsa53334
  CVSS Base Score - 3.3
  Access Vector	Access Complexity	Authentication	Confidentiality Impact	Integrity Impact	Availability Impact	Impact Bias
  Remote	Low	Not Required	None	None	Complete	Normal
  CVSS Temporal Score - 2.7
  Exploitability		Remediation Level		Report Confidence
  Functional		Official Fix		Confirmed

Workarounds

• The effectiveness of any mitigation 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 mitigation or fix is the most appropriate for use in the intended network before it is deployed.

  Additional mitigations that can be deployed on Cisco devices within the network are available in the Cisco Applied Mitigation Bulletin companion document for this advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoAppliedMitigationBulletin/cisco-amb-20070213-iosips

  Fragmented Packet Evasion Vulnerability

  There is a mitigation for the fragmented packet evasion vulnerability. The fragments keyword of IOS transit Access Control Lists (ACL) can be used to prohibit fragmented IP packets from transiting an IOS device. More information about filtering IP fragments can be found here:

  • http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a00801afc76.shtml#frag
    

  Blocking IP fragments may have adverse affects on some protocols (like HTTP, FTP and Kerberos/Active Directory), so this workaround should be used with caution.

  access-list 100 deny tcp any 10.1.1.0 0.0.0.255 fragments
  access-list 100 deny udp any 10.1.1.0 0.0.0.255 fragments

  ATOMIC.TCP Regular Expression Denial of Service Vulnerability

  There is a workaround for the ATOMIC.TCP regular expression denial of service vulnerability by deleting IPS signature 3123.0 from the IOS IPS Signature Definition File (SDF). Disabling signature 3123.0 is alone not sufficient for the workaround to be effective. The following commands will delete signature 3123.0 from an IOS IPS device.

  router#configure terminal
  router(config)#ip ips signature 3123 delete
  %IPS Signature 3123:0 is marked for deletion
  %IPS The signature will be deleted when signatures are reloaded or saved
  router(config)#interface FastEthernet0/0
  router(config)#no ip ips test in     
  router(config)#ip ips test in
  router(config)#exit
  

  In the above example, signature 3123.0 is first deleted from the Signature Definition File, then the IPS instance running on interface FastEthernet0/0 is stopped and started to reinitialize the IPS state to reflect the signature deletion. If the IPS feature set is configured on multiple interfaces, then these steps must be completed for each affected interface.

  To determine if signature 3123 is active, use the show ip ips signature command.

  router#show ip ips signatures | include 3123
  3123:0      N   A      MED     0      0     0   100    30 FA  N    S46    

  In the command output above, the N after 3123.0 indicates that the signature is present in the configuration but not enabled. Once the signature has been deleted, rerunning the show ip ips signature command shows:

  router#show ip ips signatures | include 3123
  3123:0      N*   A      MED     0      0     0   100    30 FA  N    S46    

  In the command output above, the N* after 3123.0 indicates that the signature has been deleted from the configuration. The IPS feature set must be restarted on each interface configured for IPS to complete the workaround. Once completed, the output of the show ip ips signature command will show:

  router#show ip ips signatures | include 3123
  
  router#

  This vulnerability may affect any IPS signature using the regular expression functionality of the ATOMIC.TCP engine. Currently, Cisco only ships one signature configured this way (3123.0). If custom signatures have been added to an IOS IPS device configured use the ATOMIC.TCP engine with a regular expression, these signatures must also be deleted from the IPS configuration to ensure the effectiveness of the workaround.

Fixed Software

• When considering software upgrades, also consult http://www.cisco.com/go/psirt and any subsequent advisories to determine exposure and a complete upgrade solution.

  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 assistance.

  Each row of the Cisco IOS software table (below) describes a release train and the platforms or products for which it is intended. If a given release train is vulnerable, then the earliest possible releases that contain the fix (the "First Fixed Release") and the anticipated date of availability for each are listed in the "Rebuild" and "Maintenance" columns. A device running a release in the given train that is earlier than the release in a specific column (less than the First Fixed Release) is known to be vulnerable. The release should be upgraded at least to the indicated release or a later version (greater than or equal to the First Fixed Release label).

  For more information on the terms "Rebuild" and "Maintenance," consult the following URL: http://www.cisco.com/warp/public/620/1.html

  Major Release	Availability of Repaired Releases
  Affected 12.3-Based Release	Rebuild	Maintenance
  12.3T	All 12.3(2)T, 12.3(4)T, and 12.3(7)T releases are not vulnerable
  	All 12.3(8)T releases are vulnerable
  	12.3(11)T10
  	12.3(14)T7
  	Limited platform support is available: contact TAC
  	Please migrate to 12.4(12) or later
  12.3XQ	Vulnerable; migrate to 12.4(12) or later
  12.3XR	Vulnerable; contact TAC
  12.3XS	Vulnerable; migrate to 12.4(12) or later
  12.3XW	Vulnerable; migrate to 12.3(11)YF or later
  12.3XX	Vulnerable; migrate to 12.4(12) or later
  12.3XY	Vulnerable; migrate to 12.4(12) or later
  12.3YA	Vulnerable; contact TAC
  12.3YD	Vulnerable; migrate to 12.4(2)T3 or later
  12.3YG	Vulnerable; migrate to 12.4(2)T3 or later
  12.3YH	Vulnerable; migrate to 12.4(2)T3 or later
  12.3YI	Vulnerable; migrate to 12.4(2)T3 or later
  12.3YJ	Vulnerable; migrate to 12.3(14)YQ8 or later
  12.3YK	Vulnerable; migrate to 12.4(4)T or later
  12.3YM	12.3(14)YM5
  12.3YQ	12.3(14)YQ8
  12.3YS	Vulnerable; migrate to 12.4(4)T or later
  12.3YT	Vulnerable; migrate to 12.4(4)T or later
  12.3YX	12.3(14)YX3
  12.3YZ	12.3(11)YZ
  Affected 12.4-Based Release	Rebuild	Maintenance
  12.4	12.4(1c)
  	12.4(3b)	12.4(5)
  	12.4(7e); available 26-Mar-07
  	All 12.4(8) releases are vulnerable
  	12.4(10b)	12.4(12)
  12.4MR	12.4(6)MR1
  12.4T	12.4(2)T3	12.4(4)T
  		12.4(6)T
  	12.4(9)T3; available 9-Apr-07
  	12.4(11)T1
  12.4XA	12.4(2)XA2
  12.4XB	12.4(2)XB3
  12.4XE	Vulnerable; contact TAC

Exploitation and Public Announcements

• The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerabilities described in this advisory.

  The fragmented packet evasion vulnerability was discovered internally by Cisco.

  The ATOMIC.TCP regular expression denial of service vulnerability was reported to Cisco by a customer.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20070213-iosips

Revision History

• Revision 1.0

  2007-February-13

  Initial public release.

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