CVE-2025-20191
📋 TL;DR
This vulnerability allows an unauthenticated attacker on the same network segment to send a specially crafted DHCPv6 packet to Cisco networking devices, causing them to crash and reload. This results in a denial of service (DoS) condition affecting Cisco IOS, IOS XE, NX-OS, and AireOS WLC software. Network administrators using these Cisco products should prioritize patching.
💻 Affected Systems
- Cisco IOS Software
- Cisco IOS XE Software
- Cisco NX-OS Software
- Cisco Wireless LAN Controller (WLC) AireOS Software
⚠️ Manual Verification Required
This CVE does not have specific version information in our database, so automatic vulnerability detection cannot determine if your system is affected.
Why? The CVE database entry doesn't specify which versions are vulnerable (no version ranges provided by the vendor/NVD).
🔒 Custom verification scripts are available for registered users. Sign up free to download automated test scripts.
- Review the CVE details at NVD
- Check vendor security advisories for your specific version
- Test if the vulnerability is exploitable in your environment
- Consider updating to the latest version as a precaution
⚠️ Risk & Real-World Impact
Worst Case
Critical network infrastructure devices become completely unavailable, causing widespread network outages and service disruption until devices reboot.
Likely Case
Targeted devices experience periodic reloads causing intermittent connectivity issues and service degradation for connected clients.
If Mitigated
With proper network segmentation and access controls, only authorized DHCPv6 traffic reaches vulnerable devices, preventing exploitation.
🎯 Exploit Status
Exploitation requires sending crafted DHCPv6 packets from adjacent network position. No authentication needed.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check Cisco advisory for specific fixed versions per product line
Vendor Advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-sisf-dos-ZGwt4DdY
Restart Required: Yes
Instructions:
1. Review Cisco advisory for affected versions
2. Download appropriate fixed software from Cisco.com
3. Schedule maintenance window for device reload
4. Apply update following Cisco upgrade procedures
5. Verify device is running patched version
🔧 Temporary Workarounds
Disable DHCPv6 processing on vulnerable interfaces
allPrevent processing of DHCPv6 packets on interfaces where not required
interface <interface_name>
no ipv6 dhcp server
no ipv6 dhcp relay
Implement DHCPv6 guard or rate limiting
allUse access control to restrict DHCPv6 traffic to trusted sources
ipv6 dhcp guard policy POLICY_NAME
device-role server
trusted-port
apply to affected interfaces
🧯 If You Can't Patch
- Implement strict network segmentation to isolate vulnerable devices from untrusted networks
- Deploy intrusion prevention systems (IPS) with signatures to detect and block malicious DHCPv6 packets
🔍 How to Verify
Check if Vulnerable:
Check device version against Cisco advisory and verify SISF is enabled with 'show running-config | include sisf'Check Version:
show version | include VersionVerify Fix Applied:
Verify device is running patched version with 'show version' and confirm no unexpected reloads in logs📡 Detection & Monitoring
Log Indicators:
- Unexpected device reloads
- DHCPv6 packet processing errors
- SISF process crashes
- System watchdog timeouts
Network Indicators:
- Spike in DHCPv6 traffic to network devices
- Malformed DHCPv6 packets from single source
- Network devices becoming unresponsive
SIEM Query:
source="network_device" AND (event_type="reload" OR event_type="crash") AND process="SISF"