CVE-2021-1611
📋 TL;DR
This vulnerability allows unauthenticated remote attackers to cause a denial of service by sending malformed Ethernet over GRE packets to affected Cisco wireless controllers. The device may reload, disrupting network services. Affected systems include Cisco Catalyst 9800 Family Wireless Controllers, Embedded Wireless Controllers, and Embedded Wireless on Catalyst 9000 Series Switches.
💻 Affected Systems
- Cisco Catalyst 9800 Family Wireless Controller
- Cisco Embedded Wireless Controller
- Cisco Embedded Wireless on Catalyst 9000 Series Switches
📦 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 →⚠️ Risk & Real-World Impact
Worst Case
Complete device reload causing extended network outage and service disruption
Likely Case
Device reload resulting in temporary DoS until system restarts
If Mitigated
No impact if patched or workarounds implemented
🎯 Exploit Status
Exploitation requires sending specially crafted EoGRE packets to vulnerable devices
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Cisco IOS XE Wireless Controller Software releases 17.3.4, 17.6.1, 17.7.1 and later
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ewlc-gre-6u4ELzAT
Restart Required: Yes
Instructions:
1. Download appropriate fixed software from Cisco Software Center. 2. Backup current configuration. 3. Install the update following Cisco IOS XE upgrade procedures. 4. Verify successful upgrade and functionality.
🔧 Temporary Workarounds
Disable EoGRE feature
allDisable Ethernet over GRE functionality if not required
configure terminal
no interface tunnel <tunnel-number>
end
write memory
Implement ACLs to block EoGRE traffic
allUse access control lists to restrict EoGRE traffic to trusted sources
configure terminal
access-list 100 permit gre host <trusted-source> any
access-list 100 deny gre any any
interface <interface-name>
ip access-group 100 in
end
write memory
🧯 If You Can't Patch
- Implement strict network segmentation to isolate affected devices
- Deploy intrusion prevention systems to detect and block malicious EoGRE packets
🔍 How to Verify
Check if Vulnerable:
Check IOS XE version with 'show version' command and compare to affected versionsCheck Version:
show version | include VersionVerify Fix Applied:
Verify version is 17.3.4, 17.6.1, 17.7.1 or later using 'show version' command📡 Detection & Monitoring
Log Indicators:
- Unexpected device reloads
- System crash logs
- EoGRE packet processing errors
Network Indicators:
- Spike in EoGRE traffic to wireless controllers
- Malformed GRE packets
SIEM Query:
source="cisco-ios" AND (event_type="crash" OR event_type="reload") AND device_type="wireless-controller"