Cisco IOS and IOS XE UDP Processing Vulnerability: Remote Denial of Service (DoS) Analysis and Mitigation Guide (CVE-2017-6627)
Executive & SecOps Summary
SECOPS ASSESSMENTCisco IOS 15.1, 15.2, 15.4 and IOS XE versions 3.14 through 3.18.
An unauthenticated, remote attacker can send UDP packets with a destination port of 0 to cause an interface queue wedge, leading to a complete Denial of Service (DoS) condition.
Upgrade to the latest recommended Cisco IOS/IOS XE software releases, or implement Infrastructure Access Control Lists (iACLs) to drop UDP packets targeting destination port 0.
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Administrators should upgrade to a fixed Cisco IOS/IOS XE software release as recommended by Cisco. Below is an example of upgrading the IOS image via TFTP:
# Verify current IOS version and available flash space
show version
show file systems
# Download the patched IOS image from a secure TFTP server
copy tftp: flash:
Address or name of remote host? 192.168.1.100
Source filename? c3900e-universalk9-mz.SPA.154-3.M10.bin
Destination filename [c3900e-universalk9-mz.SPA.154-3.M10.bin]?
# Configure the system to boot from the new image
configure terminal
no boot system
boot system flash c3900e-universalk9-mz.SPA.154-3.M10.bin
end
write memory
# Reload the device to apply the upgrade
reload2. Temporary Workaround & Hardening Configuration
If an immediate upgrade is not feasible, apply an Access Control List (ACL) at the network boundary or on the device itself to drop incoming UDP packets with a destination port of 0.
configure terminal
# Define an extended ACL to block UDP destination port 0
ip access-list extended BLOCK_UDP_PORT_ZERO
deny udp any any eq 0
permit ip any any
exit
# Apply the ACL to the external-facing interface
interface GigabitEthernet0/0
ip access-group BLOCK_UDP_PORT_ZERO in
end
write memory3. Log Audit & IoC Verification Commands
Monitor the input queue status and active sockets to detect potential queue wedging or abnormal UDP packet accumulation.
# Check interface input queue statistics and drops
show interfaces | include queue|Input queue
# Inspect active UDP sockets and port allocations
show sockets
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Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2017-6627) exists in the UDP packet processing engine of Cisco IOS and IOS XE software. The root cause is attributed to design flaws in certain Cisco IOS application components that instantiate UDP sockets but fail to properly close them, leaving them in an idle state indefinitely. This leads to resource exhaustion and socket leaks within the operating system (CWE-399, CWE-404).
2. Exploitation Mechanics & Attack Scenario
An unauthenticated remote attacker can exploit this vulnerability by crafting and transmitting malicious UDP packets with the destination port explicitly set to '0' toward the target device. When the vulnerable device receives these packets, the flawed socket handling logic fails to release the associated buffers, causing the packets to be held in the input interface queue. Once the input queue accumulates 250 of these packets, it triggers an 'interface queue wedge'. Consequently, the interface stops accepting any subsequent legitimate UDP traffic, resulting in a persistent Denial of Service (DoS) state.
3. Infrastructure & Business Impact
As a vulnerability listed in the CISA KEV (Known Exploited Vulnerabilities) catalog, this flaw poses a severe operational risk. Successful exploitation allows an attacker to disable critical network infrastructure, such as core routers or enterprise switches, without any authentication. Because only 250 crafted packets are required to completely wedge the interface queue, this vulnerability represents a highly efficient and low-cost vector for targeted network disruption.
Official Vendor Advisories & References
A vulnerability in the UDP processing code of Cisco IOS 15.1, 15.2, and 15.4 and IOS XE 3.14 through 3.18 could allow an unauthenticated, remote attacker to cause the input queue of an affected system to hold UDP packets, causing an interface queue wedge and a denial of service (DoS) condition. The vulnerability is due to Cisco IOS Software application changes that create UDP sockets and leave the sockets idle without closing them. An attacker could exploit this vulnerability by sending UDP packets with a destination port of 0 to an affected device. A successful exploit could allow the attacker to cause UDP packets to be held in the input interfaces queue, resulting in a DoS condition. The input interface queue will stop holding UDP packets when it receives 250 packets. Cisco Bug IDs: CSCup10024, CSCva55744, CSCva95506.
Frequently Asked SecOps Questions (CVE-2017-6627)
FAQQ1. How critical is CVE-2017-6627 and is it actively exploited in the wild?
CVE-2017-6627 is rated CVSS 7.5 (HIGH) with a 30-day EPSS exploitation probability of 6.2%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2017-6627?
Primary impacted vendors include cisco across ios, ios_xe. Scope summary: Cisco IOS 15.1, 15.2, 15.4 and Cisco IOS XE 3.14, 3.15, 3.16, 3.17, 3.18
Q3. What temporary mitigations or workarounds exist for CVE-2017-6627 before patching?
[Mitigation] Upgrade to the latest recommended Cisco IOS/IOS XE software releases, or implement Infrastructure Access Control Lists (iACLs) to drop UDP packets targeting destination port 0. Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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