Cisco Security Advisory
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CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L/E:X/RL:X/RC:X
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A vulnerability in the SSH server of Cisco Adaptive Security Appliance (ASA) Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition for the SSH server of an affected device.
This vulnerability is due to a logic error when an SSH session is established. An attacker could exploit this vulnerability by sending crafted SSH messages to an affected device. A successful exploit could allow the attacker to exhaust available SSH resources on the affected device so that new SSH connections to the device are denied, resulting in a DoS condition. Existing SSH connections to the device would continue to function normally. The device must be rebooted manually to recover. However, user traffic would not be impacted and could be managed using a remote application such as Cisco Adaptive Security Device Manager (ASDM).
Cisco has released software updates that address this vulnerability. There are workarounds that address this vulnerability.
This advisory is available at the following link:
https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asa-ssh-dos-eEDWu5RMThis advisory is part of the October 2024 release of the Cisco ASA, FMC, and FTD Software Security Advisory Bundled Publication. For a complete list of the advisories and links to them, see Cisco Event Response: October 2024 Semiannual Cisco ASA, FMC, and FTD Software Security Advisory Bundled Publication.
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Vulnerable Products
At the time of publication, this vulnerability affected devices that were running Cisco ASA Software Release 9.16.4.67, 9.16.4.70, 9.18.4.40, or 9.20.3 if they had SSH enabled and were configured to use the proprietary ASA SSH stack.
For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.
Determine the Device Configuration
The SSH server is not enabled by default. The SSH server is enabled when the administrator allows SSH access from at least one host using the ssh { ip_address mask | ipv6_address/prefix } interface command.
If a device is running Cisco ASA Software Release 9.16.4.67 or 9.16.4.70 and has SSH enabled, it is affected by this vulnerability. If the device is running Cisco ASA Software Release 9.18.4.40 or 9.20.3, it is affected by this vulnerability only if the show running-config command returns no ssh stack ciscossh, as shown in the following example:
asa# show running-config ssh | include stack
no ssh stack ciscosshProducts 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:
- Firepower Threat Defense (FTD) Software
- Secure Firewall Management Center (FMC) Software, formerly Firepower Management Center Software
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Cisco ASA Software allows a maximum of five concurrent SSH connections per context. Once this limit is reached, no more connections are allowed. To determine the current number of active SSH sessions on a device, use the show resource usage command and inspect the SSH Server row. If the Current number is one more than the Limit, as shown in the following example, and this does not clear for a period of time, the device may have been exploited by this vulnerability:
asa# show resource usage
Resource Current Peak Limit Denied Context
SSH Server 6 6 5 0 System
ASDM 0 4 30 0 System
Syslogs [rate] 18 1536 N/A 0 System
Conns 8547 32540 500000 0 SystemThe device must be rebooted manually to clear this condition. Administrators could use SSH in another context or access the console port, but this is not always possible.
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There is no workaround for this vulnerability for devices that are running Cisco ASA Software Release 9.16.4.67 or 9.16.4.70.
For devices that are running Cisco ASA Software Release 9.18.4.40 or 9.20.3, use the ssh stack ciscossh CLI command to configure the device to use the CiscoSSH stack, which is not affected by this vulnerability. For more information about implementing this command, see the ssh stack ciscossh section of Cisco Secure Firewall ASA Series Command Reference.
While this workaround 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 ASA, FMC, and FTD Software
To help customers determine their exposure to vulnerabilities in Cisco ASA, FMC, and FTD Software, Cisco provides the Cisco Software Checker. This tool identifies any Cisco security advisories that impact a specific software release and the earliest release that fixes the vulnerabilities that are described in each advisory (“First Fixed”). If applicable, the tool also returns the earliest release that fixes all the vulnerabilities that are described in all the advisories that the Software Checker identifies (“Combined First Fixed”).
To use the tool, go to the Cisco Software Checker page and follow the instructions. Alternatively, use the following form to search for vulnerabilities that affect a specific software release. To use the form, follow these steps:
- Choose which advisories the tool will search—all advisories, only advisories with a Critical or High Security Impact Rating (SIR), or only this advisory.
- Choose the appropriate software.
- Choose the appropriate platform.
- Enter a release number—for example, 9.16.2.11 for Cisco ASA Software or 6.6.7 for Cisco FTD Software.
- Click Check.
Note: For Cisco 3000 Series Industrial Security Appliances (ISAs) that are running Cisco ASA Software, Cisco ASA Software Release 9.16.4.67 has been deferred and replaced by Release 9.16.4.70.
Additional Resources
For help determining the best Cisco ASA, FMC, or FTD Software release, see the following Recommended Releases documents. If a security advisory recommends a later release, Cisco recommends following the advisory guidance.
Cisco ASA Compatibility
Cisco Secure Firewall ASA Upgrade Guide
Cisco Secure Firewall Threat Defense Compatibility Guide
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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.
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This vulnerability was found during the resolution of a Cisco TAC support case.
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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 2024-OCT-23
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