Cisco Security Advisory
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CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X
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Update for September 16, 2026: The original 1.0 version of this advisory was specific to the Cisco Adaptive Security Virtual Appliance (ASAv) and Cisco Secure Firewall Threat Defense Virtual (FTDv) models. However, it was later found that this vulnerability affects all Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software platforms.
A vulnerability in the VPN and management web servers of the Cisco Secure Firewall ASA Software and Cisco Secure FTD Software platforms could allow an unauthenticated, remote attacker to cause an affected device to run out of system memory or buffer blocks, which in turn could cause SSL VPN connection processing to slow down and eventually cease altogether.
This vulnerability is due to a lack of proper memory management for new incoming SSL/TLS connections. An attacker could exploit this vulnerability by sending a large number of new incoming SSL/TLS connections to the targeted device. A successful exploit could allow the attacker to deplete system memory or buffers, resulting in a denial of service (DoS) condition. The memory or buffers could be reclaimed slowly if the attack traffic is stopped, but a manual reload may be required to restore operations quickly.
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-asaftdvirtual-dos-MuenGnYRThis 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
This vulnerability affects Cisco products if they are running a vulnerable release of Cisco Secure Firewall ASA Software or Secure FTD Software and have one or both of the following features configured:- SSL VPN
- HTTP server used for the management interface
For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.
Determine Whether a Device Could Process SSL or TLS Messages
To determine whether a Cisco ASA or FTD device could process SSL or TLS packets, use the show asp table socket | include SSL|DTLS command and verify that it returns output. If this command returns empty output, the device is not affected. If this command returns any output, as shown in the following example, the device is affected:
asa# show asp table socket | include SSL|DTLS
SSL 0005aa68 LISTEN x.x.x.x:443 0.0.0.0:*
SSL 002d9e38 LISTEN x.x.x.x:8443 0.0.0.0:*
DTLS 0018f7a8 LISTEN 10.0.0.250:443 0.0.0.0:*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:
- Any hardware-based platforms that are running Secure Firewall ASA Software or Secure FTD Software
- Secure Firewall Management Center (FMC) Software, formerly Firepower Management Center Software
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A device may have been exploited by this vulnerability if the show blocks command indicates a leak of memory blocks. The device may stop passing traffic, or performance could degrade due to this memory leak. In this example, the memory blocks of 80 bytes are leaked, resulting in zero (0) blocks being available:
# show blocks
SIZE MAX LOW CNT
0 1450 1448 1450
4 100 99 99
80 1000 0 0
256 4148 3898 4040
1550 6279 6184 6258
2048 600 598 600
2560 164 164 164
4096 100 100 100
8192 100 100 100
9344 60000 58237 59837
16384 102 102 102
65536 16 16 16For more help to determine whether the device has been impacted by exploitation of this vulnerability, contact the Cisco Technical Assistance Center (TAC).
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There are no workarounds that address this vulnerability.
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Cisco considers any workarounds and mitigations (if applicable) to be temporary solutions until an upgrade to a fixed software release is available. To remediate this vulnerability and avoid future exposure as described in this advisory, Cisco strongly recommends that customers upgrade to the fixed software indicated in this advisory.
Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software
To help customers determine their exposure to vulnerabilities in Cisco Secure Firewall ASA, Secure FMC, and Secure 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, 9.20.3.4 for Cisco Secure Firewall ASA Software or 7.4.2 for Cisco Secure FTD Software.
- Click Check.
Note: For Cisco 3000 Series Industrial Security Appliances (ISAs) that are running Cisco Secure Firewall ASA Software, Cisco Secure Firewall 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 Secure Firewall ASA, Secure FMC, or Secure FTD Software release, see the following Recommended Releases documents. If a security advisory recommends a later release, Cisco recommends following the advisory guidance.
Cisco Secure Firewall 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 by Vivek Singh of Cisco during internal security testing.
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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.
For additional information on Cisco's vulnerability management, disclosure cadence, and software patching strategy, see Cisco's Transition to a Risk-Based Vulnerability Disclosure Model.
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Show LessVersion Description Section Status Date 2.0 Updated to indicate that this vulnerability affects all hardware platforms. Summary, Vulnerable Products, Indicators of Compromise Final 2026-SEP-16 1.0 Initial public release. — Final 2025-AUG-27
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