Cisco Security Advisory
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CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N/E:X/RL:X/RC:X
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A vulnerability in the H.323 application level gateway (ALG) used by the Network Address Translation (NAT) feature of Cisco IOS XE Software could allow an unauthenticated, remote attacker to bypass the ALG.
This vulnerability is due to insufficient data validation of traffic that is traversing the ALG. An attacker could exploit this vulnerability by sending crafted traffic to a targeted device. A successful exploit could allow the attacker to bypass the ALG and open connections that should not be allowed to a remote device located behind the ALG.
Note: This vulnerability has been publicly discussed as NAT Slipstreaming.
Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability.
This advisory is available at the following link:
https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-iosxe-h323alg-bypass-4vy2MP2QThis advisory is part of the September 2021 release of the Cisco IOS and IOS XE Software Security Advisory Bundled Publication. For a complete list of the advisories and links to them, see Cisco Event Response: September 2021 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.
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Vulnerable Products
At the time of publication, this vulnerability affected Cisco devices if they were running a vulnerable release of Cisco IOS XE Software, were configured for NAT, and had the H.323 ALG enabled. The H.323 ALG is enabled by default when NAT is configured.
For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.
Determine Whether a Device is Configured to Perform NAT
Administrators can determine whether NAT is active on the device (preferred) or whether NAT commands are present in the device configuration.
Determine Whether NAT is Active
To determine whether NAT is active on a device, log in to the device and issue the show ip nat statistics command in the CLI. If NAT is active, the Outside interfaces and Inside interfaces sections of the command output will include at least one interface.
The following example shows the output of the show ip nat statistics command for a device on which NAT is active:
Router# show ip nat statistics Total active translations: 1 (0 static, 1 dynamic; 0 extended)
Outside interfaces:
GigabitEthernet0/0/3
Inside interfaces:
GigabitEthernet0/0/1If the output of the show ip nat statistics command does not list any interfaces, NAT is not active on the device.
Determine Whether NAT Commands are Present
To determine whether NAT commands are present in the device configuration, issue the show running-config command in the CLI. If NAT is active on the device, the output will include the ip nat inside and ip nat outside interface commands. In the case of the NAT Virtual Interface, the ip nat enable interface command will be present.
Determine Whether H.323 ALG is Disabled in the NAT Configuration
To determine whether the H.323 ALG is disabled in the NAT configuration, use the show running-config | include ip nat service H225 privileged EXEC command. The presence of no ip nat service H225 indicates that the H.323 ALG is disabled in the NAT configuration.
The following example shows the output of show running-config | include ip nat service H225 in Cisco IOS XE Software that has the H.323 ALG disabled in the NAT configuration:
Router#show running-config | include ip nat service H225
no ip nat service H225If no ip nat service H225 does not appear in the output of show running-config | include ip nat service H225, and the device runs an affected version of Cisco IOS XE Software with NAT enabled, that configuration is affected by this vulnerability.
Products Confirmed Not Vulnerable
Only products listed in the Vulnerable Products section of this advisory are known to be affected by this vulnerability.
Cisco has confirmed that this vulnerability does not affect the following Cisco products:
- IOS Software
- IOS XR Software
- Meraki products
- NX-OS Software
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There are no workarounds that address this vulnerability. However, a mitigation is available.
Administrators may mitigate this vulnerability by disabling the NAT ALG for H.323 packets. However, this action may negatively impact normal operation of any device that sends or receives traffic through the affected device and, consequently, may disrupt normal network operations.
Administrators should verify that their network environment does not require use of a NAT ALG for H.323 packets before they disable this functionality. To disable use of the NAT ALG for H.323 packets, use the no ip nat service H225 command in global configuration mode.
While this mitigation has been deployed and was proven successful in a test environment, customers should determine the applicability and effectiveness in their own environment and under their own use conditions. Customers should be aware that any workaround or mitigation that is implemented may negatively impact the functionality or performance of their network based on intrinsic customer deployment scenarios and limitations. Customers should not deploy any workarounds or mitigations before first evaluating the applicability to their own environment and any impact to such environment.
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When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories page, to determine exposure and a complete upgrade solution.
In all cases, customers should ensure that the devices to be upgraded contain sufficient memory and confirm that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, customers are advised to contact the Cisco Technical Assistance Center (TAC) or their contracted maintenance providers.
Cisco IOS and IOS XE Software
To help customers determine their exposure to vulnerabilities in Cisco IOS and IOS XE Software, Cisco provides the Cisco Software Checker to identify any Cisco Security Advisories that impact a specific software release and the earliest release that fixes the vulnerabilities described in each advisory (“First Fixed”). If applicable, the tool also returns the earliest release that fixes all the vulnerabilities described in all the advisories identified (“Combined First Fixed”).
Customers can use the Cisco Software Checker to search advisories in the following ways:
- Choose the software and one or more releases
- Upload a .txt file that includes a list of specific releases
- Enter the output of the show version command
After initiating a search, customers can customize the search to include all Cisco Security Advisories, a specific advisory, or all advisories in the most recent bundled publication.
Customers can also use the following form to determine whether a release is affected by any Cisco Security Advisory by entering a Cisco IOS or IOS XE Software release—for example, 15.1(4)M2 or 3.13.8S:
By default, the Cisco Software Checker includes results only for vulnerabilities that have a Critical or High Security Impact Rating (SIR). To include results for Medium SIR vulnerabilities, customers can use the Cisco Software Checker on Cisco.com and check the Medium check box in the drop-down list under Impact Rating when customizing a search.
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The Cisco Product Security Incident Response Team (PSIRT) is aware that proof-of-concept exploit code is available for the vulnerability described in this advisory.
The Cisco PSIRT is not aware of any malicious use of the vulnerability that is described in this advisory.
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Cisco was made aware of this vulnerability through public discussion of the NAT Slipstreaming attacks.
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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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Show LessVersion Description Section Status Date 1.0 Initial public release. — Final 2021-SEP-22
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