CVE-2025-20146
📋 TL;DR
An unauthenticated remote attacker can cause denial of service on affected Cisco routers by sending crafted IPv4 multicast packets to line cards with ACLs or QoS policies applied. This vulnerability affects Cisco ASR 9000, ASR 9902, and ASR 9903 routers running vulnerable IOS XR software versions. Successful exploitation causes line card resets and traffic loss.
💻 Affected Systems
- Cisco ASR 9000 Series Aggregation Services Routers
- Cisco ASR 9902 Compact High-Performance Routers
- Cisco ASR 9903 Compact High-Performance Routers
📦 What is this software?
Ios Xr by Cisco
Ios Xr by Cisco
Ios Xr by Cisco
Ios Xr by Cisco
Ios Xr by Cisco
Ios Xr by Cisco
Ios Xr by Cisco
Ios Xr by Cisco
Ios Xr by Cisco
Ios Xr by Cisco
Ios Xr by Cisco
⚠️ Risk & Real-World Impact
Worst Case
Multiple line cards reset simultaneously, causing widespread network outages and extended service disruption across affected routers.
Likely Case
Targeted line card resets causing localized traffic loss and service disruption until the card reloads (typically minutes).
If Mitigated
Limited impact with proper network segmentation, traffic filtering, and monitoring in place to detect and block attack traffic.
🎯 Exploit Status
Requires crafting specific malformed IPv4 multicast packets and targeting interfaces with ACLs/QoS policies. No authentication needed.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Refer to Cisco Security Advisory for specific fixed releases
Vendor Advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-multicast-ERMrSvq7
Restart Required: No
Instructions:
1. Review Cisco Security Advisory for affected versions. 2. Upgrade to fixed IOS XR software releases. 3. No router reload required - only affected line cards may need reset after patch.
🔧 Temporary Workarounds
Remove ACLs/QoS from vulnerable interfaces
allRemove IPv4 ACLs and QoS policies from interfaces on vulnerable line cards to eliminate the attack vector
no ipv4 access-group [acl-name] [in|out]
no service-policy [policy-name] [input|output]
Filter multicast traffic
allImplement ingress filtering to block suspicious multicast traffic at network perimeter
ipv4 access-list BLOCK-MCAST deny ip any 224.0.0.0 15.255.255.255
ipv4 access-list BLOCK-MCAST permit ip any any
🧯 If You Can't Patch
- Implement strict network segmentation to isolate vulnerable routers from untrusted networks
- Deploy intrusion prevention systems (IPS) with signatures to detect and block crafted multicast packets
🔍 How to Verify
Check if Vulnerable:
Check IOS XR version with 'show version' and compare against affected versions in Cisco advisory. Check interface configurations with 'show running-config interface' for ACLs/QoS policies.Check Version:
show version | include Cisco IOS XRVerify Fix Applied:
Verify upgraded to fixed IOS XR version with 'show version'. Monitor line card stability and absence of resets after patch.📡 Detection & Monitoring
Log Indicators:
- Line card exception logs
- Line card reset/reload events
- Unexpected multicast traffic spikes
- CPU spikes on line cards
Network Indicators:
- Unusual multicast traffic patterns
- Crafted IPv4 multicast packets
- Line card interface flapping
SIEM Query:
source="router-logs" ("line card reset" OR "exception" OR "reload") AND ("multicast" OR "ACL" OR "QoS")