Summary

• Multiple Cisco products are affected by a vulnerability in the generic routing encapsulation (GRE) tunnel decapsulation feature of the Snort 3 Detection Engine that could allow an unauthenticated, remote attacker to cause an unexpected restart of the Snort 3 Detection Engine.

  This vulnerability is due to improper handling of nested GRE traffic. An attacker could exploit this vulnerability by sending a crafted GRE payload through an affected device. A successful exploit could allow the attacker to trigger an unexpected reload of the Snort 3 Detection Engine, resulting in either a bypass or denial of service (DoS) condition, depending on device configuration. For more information, see the Details section of this advisory. The Snort 3 Detection Engine will restart automatically. No manual intervention is required.

  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-snort3-gre-JNpMHrTv

  This advisory is part of a group of advisories. For a complete list of the advisories and links to them, see Cisco Advance Notification for Publication of October 21, 2026, Security Advisories. In addition, for further documentation of improvements and fixes in Open Source Snort 3, see Multiple Cisco Products Snort 3 Security Hardening Release: October 2026.

Affected Products

• Vulnerable Products

  For information about which products are affected by this vulnerability at the time of publication, see the following sections.

  Open Source Snort 3

  At the time of publication, this vulnerability affected Open Source Snort 3.

  For information about which Snort releases were vulnerable at the time of publication, see the Fixed Software section of this advisory. For more information on Snort, see the Snort website.

  Cisco Secure Firewall Threat Defense Software

  At the time of publication, this vulnerability affected Cisco Secure Firewall Threat Defense (FTD) Software if Snort 3 and one of the following intrusion policies were configured:

  • Connectivity Over Security
  • Balanced Security and Connectivity
  • Security Over Connectivity
  • Maximum Detection
  • Custom policy with arbitrary UDP rule enabled

  For more information, see the Cisco Firepower Threat Defense Configuration Guide.

  For information about which Cisco Secure FTD Software releases are vulnerable, see the Fixed Software section of this advisory.

  Determine the Snort Configuration on Cisco Secure FTD Software

  On new installations of Cisco Secure FTD Software releases 7.0.0 and later, Snort 3 is running by default. On devices that were running Cisco Secure FTD Software Release 6.7.0 or earlier and were upgraded to Release 7.0.0 or later, Snort 2 is running by default.

  Snort 2 was deprecated in Cisco Secure FTD Software releases 7.7.0 and later.

  To determine if Snort 3 is running on Cisco Secure FTD Software, see Determine the Active Snort Version that Runs on FTD. Snort 3 must be active for this vulnerability to be exploited.

  Unified Threat Defense on Cisco IOS XE Software

  At the time of publication, this vulnerability affected the following Cisco products if they were running a vulnerable release of Unified Threat Defense (UTD) Snort IPS Engine for Cisco IOS XE Software or UTD Engine for Cisco IOS XE SD-WAN Software:

  • 1000 Series Integrated Services Routers (ISRs)
  • 4000 Series ISRs
  • 8100 Series Secure Routers
  • 8200 Series Secure Routers
  • 8300 Series Secure Routers
  • 8400 Series Secure Routers
  • Catalyst 8000V Edge Software
  • Catalyst 8200 Series Edge Platforms
  • Catalyst 8300 Series Edge Platforms
  • Catalyst 8500L Edge Platforms
  • Catalyst IR1800 Rugged Series Routers
  • Catalyst IR8340 Rugged Routers
  • Cloud Services Routers 1000V
  • Integrated Services Virtual Routers

  Note: UTD is not installed on these devices by default. If the UTD file is not installed, the device is not affected by these vulnerabilities.

  For information about which Cisco IOS XE Software releases were vulnerable at the time of publication, see the Fixed Software section of this advisory. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  Determine Whether UTD Is Enabled

  To determine whether UTD is enabled on a device, use the show utd engine standard status command. If the output shows a Yes under Running, UTD is enabled. If there is no output, the device is not affected. The following example shows the output on a device that has UTD enabled:

  Router# show utd engine standard status 
  Engine version       : 1.0.19_SV2.9.16.1_XE17.3
  Profile              : Cloud-Low
  System memory        :
               Usage  : 6.00 %
               Status : Green
  Number of engines    : 1

  
  

  Engine        Running    Health     Reason    
  ===========================================
  Engine(#1):   Yes        Green      None
  =======================================================
  .
  .
  .

   

  Cisco Meraki MX Security Appliances

  At the time of publication, this vulnerability affected the following products if they were running a vulnerable release of Cisco Meraki software:

  MX64 MX64W MX65 MX65W MX67 MX67C MX67W

  MX68 MX68CW MX75 MX84 MX85 MX95

  MX100 MX105 MX250 MX400 MX450 MX600

  For details about automatic upgrades of devices that are connected to the Cisco Meraki Dashboard, see the Fixed Software section of this advisory.

  Cisco Cyber Vision

  At the time of publication, this vulnerability affected Cisco Cyber Vision, regardless of device configuration.

  For information about which Cisco Cyber Vision releases were vulnerable at the time of publication, see the Fixed Software section of this advisory. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  Cisco Cloud-delivered Firewall and Cisco Secure Access

  At the time of publication, this vulnerability affected Cisco Cloud-delivered Firewall (CDFW) and Cisco Secure Access (CSA), regardless of device configuration.

  For information about which Cisco CDFW and CSA releases were vulnerable at the time of publication, see the Fixed Software section of this advisory. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  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 products:

  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
  • Cisco Secure Firewall Management Center (FMC) Software
  • Open Source Snort 2

  Note: Open Source Snort 2 has reached End of Life. For more information, see End of Life Announcement for Open Source Snort 2.

Details

• The following Cisco Secure FTD Software Snort 3 configuration parameters govern how traffic is handled if the Snort 3 process restarts, which could change how encrypted traffic is handled during an exploit of this vulnerability:

  • Snort Fail Open
  • snort preserve-connection

  For more information, see the "Snort Restart Traffic Behavior" section of the Cisco Secure Firewall Management Center Device Configuration Guide and of Cisco Security Cloud Control: Secure Firewall Threat Defense Management.

Workarounds

• There are no workarounds that address this vulnerability.

Fixed Software

• 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.

  Fixed Releases

  The Cisco Product Security Incident Response Team (PSIRT) validates only the affected and fixed release information that is documented in this advisory.

  For information about fixed releases, see the following sections.

  Open Source Snort 3

  At the time of publication, the release information in the following table was accurate.

  The left column lists Snort 3 software releases, and the right column indicates whether a release was affected by the vulnerability that is described in this advisory and which release included the fix for this vulnerability.

  Snort 3 Release	First Fixed Snort 3 Release
  3.x	3.12.4.0

  Note: This fix is also included as part of the broader hardening improvements that are disclosed in the security advisory Multiple Cisco Products Snort 3 Security Hardening Release: October 2026.

  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:

  1. Choose which advisories the tool will search—all advisories, only advisories with a Critical or High Security Impact Rating (SIR), or only this advisory.
  2. Choose the appropriate software.
  3. Choose the appropriate platform.
  4. 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.
  5. Click Check.

  Only this advisory All Critical and High advisories All advisories Cisco ASA Software Cisco FMC Software Cisco FTD Software Any Platform 3000 Series Industrial Security Appliances (ISA) ASA 5500-X Series Firewalls ASA Service Module Adaptive Security Virtual Appliance (ASAv) Firepower 1000 Series Firepower 2100 Series Firepower 4100 Series Firepower 9000 Series Secure Firewall 1200 Series Secure Firewall 3100 Series Secure Firewall 4200 Series

   

  For instructions on upgrading a Cisco Secure FTD device, see the appropriate Cisco Secure FMC upgrade guide.

  Additional Resources

  For help determining the best Cisco 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 Threat Defense Compatibility Guide

  UTD on Cisco IOS XE Software: CSCwu04383

  At the time of publication, the release information in the following table was accurate. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  In the following table, the left column lists Cisco IOS XE Software releases, and the right column indicates whether a release was affected by the vulnerability that is described in this advisory and which release included the fix for this vulnerability.

  Cisco IOS XE Software Release	First Fixed Release
  17.12	17.12.8
  17.15	17.15.6
  17.18	17.18.5
  26.1	26.1.2
  26.2	26.2.1

   

  Cisco Meraki MX Security Appliances

  Snort is integrated natively into Cisco Meraki security appliances as part of the threat protection services that are managed automatically through the Meraki Dashboard. No customer action is required to download Meraki firmware or the Snort 3 package for Meraki devices. As of October 21, 2026, all Meraki models listed in this advisory that are online and connected to the Cisco Meraki Dashboard have been automatically updated with the Snort 3 package.

  Cisco Cyber Vision: CSCwt83029

  At the time of publication, the release information in the following table was accurate. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  In the following table, the left column lists Cisco Cyber Vision releases, and the right column indicates whether a release was affected by the vulnerability that is described in this advisory and which release included the fix for this vulnerability.

  Cisco Cyber Vision Release	First Fixed Release
  5.4 and earlier	Migrate to a fixed release.
  5.5	5.5.0
  5.6	5.6.0

   

  Cisco CDFW and CSA: CSCwt83034

  Snort 3 is integrated natively into Cisco CDFW and CSA as part of the Snort 3 intrusion prevention system (IPS) technology to inspect traffic and block threats. No customer action is required. As of October 21, 2026, all Cisco CDFW and CSA cloud instances have been automatically updated with the Snort 3 package.

Exploitation and Public Announcements

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

Source

• This vulnerability was found during the resolution of a Cisco Technical Assistance Center (TAC) support case.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-snort3-gre-JNpMHrTv

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2026-OCT-21

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