CVE-2021-1623
📋 TL;DR
This vulnerability allows authenticated remote attackers to cause a denial of service (DoS) on Cisco cBR-8 routers by overwhelming the SNMP punt handling function with large numbers of SNMP requests. Organizations using affected Cisco cBR-8 routers with SNMP enabled are at risk. The attack requires authentication but can disrupt router operations.
💻 Affected Systems
- Cisco cBR-8 Converged Broadband Router
📦 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 router outage, disrupting all network traffic through the affected device, potentially affecting multiple services and users.
Likely Case
Degraded router performance, packet loss, and intermittent connectivity issues affecting services relying on the router.
If Mitigated
Minimal impact if SNMP access is restricted, rate-limited, or disabled on vulnerable devices.
🎯 Exploit Status
Exploitation requires SNMP authentication credentials but is technically simple once credentials are obtained.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Cisco IOS XE Software releases that include the fix (specific versions listed in Cisco advisory)
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-cbr8snmp-zGjkZ9Fc
Restart Required: Yes
Instructions:
1. Review Cisco advisory for fixed software versions. 2. Download appropriate fixed release from Cisco. 3. Backup current configuration. 4. Install update following Cisco upgrade procedures. 5. Reboot device. 6. Verify update and functionality.
🔧 Temporary Workarounds
Restrict SNMP Access
allLimit SNMP access to trusted management networks using access control lists (ACLs).
ip access-list standard SNMP-ACL
permit 192.168.1.0 0.0.0.255
snmp-server community <string> ro SNMP-ACL
Disable SNMP if Not Needed
allCompletely disable SNMP on devices where it's not required for operations.
no snmp-server community <string>
no snmp-server enable
🧯 If You Can't Patch
- Implement strict network segmentation to isolate SNMP traffic to management networks only.
- Deploy rate limiting on SNMP traffic using network security devices or router ACLs with rate limiting features.
🔍 How to Verify
Check if Vulnerable:
Check Cisco IOS XE version on cBR-8 router and compare against fixed releases in Cisco advisory. Verify SNMP is enabled.Check Version:
show versionVerify Fix Applied:
Confirm router is running a fixed Cisco IOS XE version from the advisory and test SNMP functionality remains operational under normal loads.📡 Detection & Monitoring
Log Indicators:
- High volume of SNMP requests in router logs
- SNMP-related error messages indicating processing issues
- System log entries showing punt path overload warnings
Network Indicators:
- Unusually high SNMP traffic to router management interfaces
- Increased packet loss or latency on router interfaces during SNMP floods
SIEM Query:
source="router_logs" AND ("SNMP" AND ("high volume" OR "overload" OR "punt"))