CVE-2021-1621
📋 TL;DR
This vulnerability allows an unauthenticated attacker on the same network segment to send specially crafted Layer 2 frames to Cisco IOS XE routers, causing interface queues to become stuck and resulting in denial of service. The attack requires adjacency to the target router and affects specific Cisco IOS XE Software versions. Network administrators with vulnerable routers in production environments are affected.
💻 Affected Systems
- Cisco IOS XE Software
📦 What is this software?
Ios Xe by Cisco
Cisco IOS XE is Cisco's modern network operating system running on enterprise routers, switches, and wireless controllers deployed across corporate networks, data centers, branch offices, and service provider infrastructure worldwide. As the evolution of Cisco IOS, IOS XE provides a Linux-based modu...
Learn more about Ios Xe →Ios Xe by Cisco
Cisco IOS XE is Cisco's modern network operating system running on enterprise routers, switches, and wireless controllers deployed across corporate networks, data centers, branch offices, and service provider infrastructure worldwide. As the evolution of Cisco IOS, IOS XE provides a Linux-based modu...
Learn more about Ios Xe →Ios Xe by Cisco
Cisco IOS XE is Cisco's modern network operating system running on enterprise routers, switches, and wireless controllers deployed across corporate networks, data centers, branch offices, and service provider infrastructure worldwide. As the evolution of Cisco IOS, IOS XE provides a Linux-based modu...
Learn more about Ios Xe →Ios Xe by Cisco
Cisco IOS XE is Cisco's modern network operating system running on enterprise routers, switches, and wireless controllers deployed across corporate networks, data centers, branch offices, and service provider infrastructure worldwide. As the evolution of Cisco IOS, IOS XE provides a Linux-based modu...
Learn more about Ios Xe →Ios Xe by Cisco
Cisco IOS XE is Cisco's modern network operating system running on enterprise routers, switches, and wireless controllers deployed across corporate networks, data centers, branch offices, and service provider infrastructure worldwide. As the evolution of Cisco IOS, IOS XE provides a Linux-based modu...
Learn more about Ios Xe →⚠️ Risk & Real-World Impact
Worst Case
Critical network interfaces become completely unusable, causing widespread network outages and disrupting all traffic through affected routers.
Likely Case
Targeted interfaces experience packet loss and performance degradation, leading to localized network disruptions and connectivity issues.
If Mitigated
With proper network segmentation and access controls, only isolated segments would be affected with minimal business impact.
🎯 Exploit Status
Exploitation requires sending specific Layer 2 frames but does not require authentication. Attack complexity is low once attacker gains network access.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Fixed in Cisco IOS XE Software Releases 16.11.2, 16.12.2, 17.1.2, 17.2.2, 17.3.2 and later
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-quewedge-69BsHUBW
Restart Required: Yes
Instructions:
1. Download appropriate fixed software version from Cisco Software Center. 2. Backup current configuration. 3. Install new software version following Cisco IOS XE upgrade procedures. 4. Reboot router to activate new software.
🔧 Temporary Workarounds
Implement Layer 2 Access Controls
allRestrict which devices can send Layer 2 frames to router interfaces using port security, MAC address filtering, or 802.1X authentication.
interface GigabitEthernet0/0
switchport port-security maximum 1
switchport port-security violation restrict
switchport port-security mac-address sticky
Network Segmentation
allIsolate router interfaces from untrusted network segments using VLANs or physical separation to limit attack surface.
vlan 10
name TRUSTED_SEGMENT
interface GigabitEthernet0/0
switchport access vlan 10
🧯 If You Can't Patch
- Implement strict network segmentation to isolate routers from untrusted devices
- Deploy network monitoring to detect unusual Layer 2 traffic patterns
🔍 How to Verify
Check if Vulnerable:
Check IOS XE version with 'show version' command and compare against affected versions. Also check interface status for unusual queue behavior.Check Version:
show version | include VersionVerify Fix Applied:
After upgrade, verify version is 16.11.2 or later, 16.12.2 or later, 17.1.2 or later, 17.2.2 or later, or 17.3.2 or later. Monitor interface queues for normal operation.📡 Detection & Monitoring
Log Indicators:
- Interface resets
- Queue wedge errors in system logs
- Increased interface error counters
- Unexpected interface state changes
Network Indicators:
- Unusual Layer 2 frame patterns
- Sudden traffic drops on specific interfaces
- Increased broadcast/multicast traffic to router interfaces
SIEM Query:
source="router_logs" AND ("queue wedge" OR "interface reset" OR "%L2-*" OR "%LINEPROTO-5-UPDOWN")