CVE-2017-6627: Cisco IOS Software and Cisco IOS XE Software UDP Packet Processing Denial-of-Service Vulnerability

CVE-2017-6627 is a denial-of-service vulnerability in the UDP packet processing code of Cisco IOS and IOS XE Software. An unauthenticated remote attacker can cause an affected device to hold UDP packets in an interface input queue, creating a queue wedge that disrupts normal traffic handling and can take the interface or device out of service. For network teams this matters because IOS and IOS XE commonly run on routers, switches, and other infrastructure that sit in the path of critical traffic; a successful attack can interrupt connectivity without any credentials.

Public detail is limited to the vendor and CISA descriptions. Confirm exact affected releases, fixed versions, and any platform-specific notes directly against the Cisco advisory before acting.

How it works

The weakness is classified as CWE-399 (resource management errors). In this case the flaw lies in how the software handles certain UDP packets. When those packets are processed, the device can fail to release or properly drain them from the interface input queue. Over time the queue fills and becomes wedged, so legitimate traffic that should be forwarded or delivered is blocked. The attacker needs only network reachability to the vulnerable UDP processing path; no authentication is required. The result is a denial-of-service condition rather than code execution or data theft. Exact packet characteristics and trigger conditions are not provided in the summary material and must be taken from the vendor advisory if needed for detection engineering.

Am I affected? How to find it in your systems

Cisco IOS and IOS XE are typically found on enterprise and service-provider routers, Layer-3 switches, and certain security or aggregation appliances. Inventory every device running either operating system:

• Use configuration management databases, network management platforms, or direct CLI commands (for example, show version) to list IOS and IOS XE images and feature sets.
• Compare each image against the affected-product list in the Cisco advisory for CVE-2017-6627; do not rely on version ranges stated elsewhere.
• Note which interfaces are configured to accept or process UDP traffic from untrusted networks, because those are the most exposed attack surface.

Telemetry signs of exploitation or the resulting condition include sudden growth in input-queue counters, interfaces reporting queue wedges, unexplained drops or loss of forwarding on UDP-bearing interfaces, and device or interface-level denial-of-service symptoms without a corresponding hardware failure. Capture show interface, show buffers, and related queue statistics before and during any suspected event, and retain logs for later correlation. Because public exploit details are sparse, treat any unexplained UDP-related queue exhaustion on an unpatched device as suspicious until proven otherwise.

How to remediate

Patch first. Apply the software updates Cisco released for this vulnerability, following the exact image and upgrade path named in the vendor advisory. CISA’s required action is to apply updates per vendor instructions; schedule the upgrade in a maintenance window that includes verification of interface and routing health afterward.

After patching, perform basic hardening appropriate to this class of flaw:

• Restrict UDP exposure on control- and management-plane interfaces with infrastructure ACLs or control-plane policing so that only necessary sources can reach the device.
• Disable any unused UDP services or features that are not required for production traffic.
• Ensure logging and SNMP or telemetry alerts are enabled for interface queue depth and drop counters so recurrence is visible quickly.

Re-inventory after the change window to confirm every previously vulnerable device now runs a fixed release.

If you can't patch immediately

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

• Segment vulnerable devices so that untrusted or Internet-facing networks cannot send arbitrary UDP packets to them; place strict ACLs or firewall rules at the edge.
• Apply control-plane policing or rate limiting focused on UDP to limit the rate at which packets can fill input queues.
• If a particular UDP-based feature is the exposure point and is not business-critical, disable it temporarily.
• Increase monitoring of interface input-queue depths, wedge flags, and related syslog messages; alert on thresholds that indicate a queue is not draining.
• Virtual patching via a network firewall or IPS may be possible if the vendor or a security vendor publishes specific signatures; validate any such rule against the Cisco advisory so it does not block legitimate traffic.

These steps only buy time; they do not replace the software fix.

If your data may have been exposed

This vulnerability is a denial-of-service issue; the public record does not document direct data exfiltration or ransomware use tied to CVE-2017-6627. Nonetheless, any successful attack on network infrastructure can be a precursor to broader compromise. If you have evidence that devices were wedged or that an attacker maintained presence afterward, follow your incident-response process, preserve logs, and check for secondary access. You can also run a free exposure scan of your email addresses against known breach data sets to see whether credentials or other information have appeared in unrelated breaches while you complete containment and patching.