CVE-2021-34713
📋 TL;DR
An unauthenticated attacker on the same network segment can send specially crafted Ethernet frames to Cisco ASR 9000 routers running vulnerable IOS XR software, causing affected line cards to reboot due to a spin loop in the Layer 2 punt code. This vulnerability affects Cisco ASR 9000 Series Aggregation Services Routers with specific line cards. Network administrators with these devices in production should prioritize patching.
💻 Affected Systems
- Cisco ASR 9000 Series Aggregation Services Routers
📦 What is this software?
Ios Xr by Cisco
Ios Xr by Cisco
Ios Xr by Cisco
Ios Xr by Cisco
⚠️ Risk & Real-World Impact
Worst Case
An attacker could cause repeated line card reboots leading to sustained network outages, service disruption, and potential cascading failures in critical infrastructure.
Likely Case
Targeted line card reboots causing temporary network outages, packet loss, and service degradation for affected interfaces.
If Mitigated
With proper network segmentation and access controls, exploitation would be limited to authorized network segments only.
🎯 Exploit Status
Exploitation requires sending specific Ethernet frames to adjacent network segments. No authentication required.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Refer to Cisco Security Advisory for specific fixed releases for each IOS XR version
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-npspin-QYpwdhFD
Restart Required: Yes
Instructions:
1. Review Cisco Security Advisory for exact fixed releases. 2. Download appropriate IOS XR software version. 3. Schedule maintenance window. 4. Apply software update following Cisco IOS XR upgrade procedures. 5. Reboot affected line cards or entire router as required.
🔧 Temporary Workarounds
Access Control List (ACL) Filtering
allImplement ACLs to filter suspicious Ethernet frames on vulnerable interfaces
ipv4 access-list BLOCK-EXPLOIT
10 deny ip any any fragments
20 permit ip any any
interface GigabitEthernet0/0/0/0
ipv4 access-group BLOCK-EXPLOIT in
Network Segmentation
allIsolate vulnerable devices to trusted network segments only
vlan 100
name TRUSTED-SEGMENT
interface GigabitEthernet0/0/0/0
switchport access vlan 100
🧯 If You Can't Patch
- Implement strict network segmentation to limit adjacent attacker access
- Deploy intrusion detection/prevention systems to monitor for exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Check IOS XR version and line card model: 'show version' and 'show platform'Check Version:
show version | include Cisco IOS XRVerify Fix Applied:
Verify running IOS XR version is at or above fixed release from Cisco advisory📡 Detection & Monitoring
Log Indicators:
- Line card reboot events in system logs
- Unexpected line card state changes
- Network processor unresponsive errors
Network Indicators:
- Unusual Ethernet frame patterns targeting ASR 9000 devices
- Increased line card reboot events
SIEM Query:
source="cisco_ios_xr" AND ("line card reboot" OR "NP unresponsive" OR "spin loop")