AV:R/AC:L/Au:NR/C:P/I:P/A:N/B:N/E:F/RL:O/RC:C
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The Intrusion Prevention System (IPS) feature set of Cisco IOS® contains several vulnerabilities. These include:
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Fragmented IP packets may be used to evade signature
inspection.
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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.
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Fragmented IP packets may be used to evade signature
inspection.
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Vulnerable Products
The following Cisco IOS release trains with the IPS feature set enabled are vulnerable to the fragmented packet evasion vulnerability:
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12.3T, except versions 12.3(2)T, 12.3(4)T, and
12.3(7)T
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12.4
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12.4T
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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:
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12.3T, except versions 12.3(2)T, 12.3(4)T, and
12.3(7)T
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12.3XQ, 12.3XR, 12.3XS, 12.3XW, 12.3XX, 12.3XY
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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
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12.4
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12.4MR
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12.4T
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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.
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12.3T, except versions 12.3(2)T, 12.3(4)T, and
12.3(7)T
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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
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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:
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.
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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
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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.
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To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
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