CVE-2023-20109: Cisco IOS and IOS XE Group Encrypted Transport VPN Out-of-Bounds Write Vulnerability

CVE-2023-20109 is an out-of-bounds write vulnerability in the Group Encrypted Transport VPN (GET VPN) feature of Cisco IOS and IOS XE. An authenticated remote attacker who already holds administrative control of a group member or key server can abuse it to execute malicious code or crash the device. Network infrastructure that relies on GET VPN for encrypted group communications is therefore at risk of full compromise or denial of service once an attacker has obtained those elevated credentials.

Because the flaw sits in core routing and VPN software, successful exploitation can disrupt traffic, enable further lateral movement, or allow persistent control of the device. Teams should treat any device running the affected feature as high priority until the vendor fix is confirmed and applied.

How it works

The weakness is classified as CWE-787 (out-of-bounds write). In the GET VPN feature, certain processing of protocol data can write past the bounds of an allocated buffer. An attacker who already possesses administrative privileges on either a group member or a key server can supply crafted input that triggers this write. The result can be arbitrary code execution with the privileges of the affected process or an immediate device crash (denial of service). Exact packet formats, memory offsets, or trigger conditions are not detailed in the public summary; defenders must consult the Cisco advisory for any additional technical notes.

No unauthenticated remote path is described; the attacker must already control a legitimate administrative session on a participating GET VPN node. That prerequisite limits the initial attack surface but does not reduce the impact once the attacker is inside the administrative trust boundary.

Am I affected? How to find it in your systems

Cisco IOS and IOS XE are the operating systems used on a wide range of Cisco routers, switches, and related network appliances. GET VPN is an optional feature typically enabled on devices that participate in large-scale encrypted group overlays. Inventory every Cisco device running IOS or IOS XE, then determine whether GET VPN is configured (look for GDOI or related key-server/group-member statements in the running configuration).

• Collect software version strings from every device (show version or equivalent management API) and compare them against the fixed releases listed in the official Cisco advisory for CVE-2023-20109.
• Search configuration repositories and change-management records for any mention of GET VPN, GDOI, or key-server roles.
• Review device logs and SNMP traps for unexpected reloads, software exceptions, or crashes that coincide with administrative sessions on GET VPN participants.
• Monitor authentication and command-accounting logs for unusual administrative activity originating from group members or key servers.

If GET VPN is not in use, the vulnerability is not reachable; still confirm the software version so that future configuration changes do not re-introduce exposure.

How to remediate

The primary remediation is to install the software update that Cisco has released for this vulnerability. Obtain the correct image for each platform and train from the vendor advisory, then schedule a controlled upgrade window. After the upgrade, verify that GET VPN functionality continues to operate as expected and that the new version string matches the fixed release.

If the advisory lists additional configuration changes or feature-specific work-arounds, apply those as well. Where the product can no longer be supported, CISA guidance is to discontinue use. Beyond the patch, apply standard hardening for this class of flaw: restrict administrative access to GET VPN nodes with strong authentication and network ACLs, keep management planes on isolated networks, and ensure that only necessary devices hold group-member or key-server roles.

If you can't patch immediately

Until the vendor update can be installed, reduce risk with compensating controls:

• Segment GET VPN participants so that only authorized administrative hosts can reach their management interfaces; block all other sources.
• If operationally feasible, temporarily disable the GET VPN feature or remove devices from the group until they can be upgraded.
• Increase logging and monitoring of administrative sessions, GDOI protocol events, and device health; alert on crashes or unexpected reloads.
• Apply any virtual-patching or IPS signatures that the vendor or your security tools provide for this CVE, confirming coverage against the advisory.
• Enforce least-privilege administrative accounts and multi-factor authentication so that compromise of a single credential is harder.

These steps do not eliminate the vulnerability; they only shrink the window of exposure until the official fix is deployed.

If your data may have been exposed

Vulnerabilities that permit code execution on network devices can lead to broader breaches if an attacker uses the foothold to intercept traffic or pivot deeper into the environment. Ransomware use of this specific CVE is not documented. If you suspect compromise, isolate the affected devices, preserve logs, and follow your incident-response plan. As a quick additional check, you can run a free exposure scan of your email addresses against known breach data sets to see whether related credentials have already appeared in public dumps.