CVE-2025-20146 Overview
A vulnerability exists in the Layer 3 multicast feature of Cisco IOS XR Software for Cisco ASR 9000 Series Aggregation Services Routers, ASR 9902 Compact High-Performance Routers, and ASR 9903 Compact High-Performance Routers. This flaw allows an unauthenticated, remote attacker to cause a line card to reset, resulting in a denial of service (DoS) condition that disrupts network traffic.
The vulnerability stems from incorrect handling of malformed IPv4 multicast packets received on line cards where the interface has either an IPv4 access control list (ACL) or a QoS policy applied. An attacker can exploit this by sending crafted IPv4 multicast packets through an affected device, causing line card exceptions or a hard reset. Traffic over the affected line card would be lost during the reload process.
Critical Impact
Successful exploitation causes line card resets on critical network infrastructure, resulting in traffic loss and potential network outages for enterprise and service provider environments.
Affected Products
- Cisco IOS XR Software versions 7.9.21, 7.10.2, 7.11.1, 7.11.2, 7.11.21, 24.1.1, 24.1.2, 24.2.1, 24.2.2, 24.3.1, 24.3.2
- Cisco ASR 9000 Series Aggregation Services Routers (ASR 9006, ASR 9010, ASR 9901, ASR 9904, ASR 9906, ASR 9910, ASR 9912, ASR 9922)
- Cisco ASR 9902 and ASR 9903 Compact High-Performance Routers
Discovery Timeline
- March 12, 2025 - CVE-2025-20146 published to NVD
- August 1, 2025 - Last updated in NVD database
Technical Details for CVE-2025-20146
Vulnerability Analysis
This vulnerability is classified under CWE-20 (Improper Input Validation), indicating that the affected software fails to properly validate or sanitize incoming IPv4 multicast packets. The flaw specifically manifests when line cards have IPv4 ACLs or QoS policies configured on their interfaces, creating a condition where malformed multicast packets can trigger unexpected behavior in the packet processing pipeline.
The vulnerability affects critical network infrastructure components used in enterprise and service provider environments. Cisco ASR 9000 Series routers are commonly deployed as core and edge routers in large-scale networks, making this vulnerability particularly impactful for organizations relying on these devices for high-availability network services.
Root Cause
The root cause is improper input validation in the Layer 3 multicast packet processing code path within Cisco IOS XR Software. When an interface has an IPv4 ACL or QoS policy applied, the packet processing logic encounters a condition where malformed IPv4 multicast packets are not properly handled, leading to memory corruption or exception conditions that trigger a line card reset.
The vulnerability specifically requires the presence of ACL or QoS configurations on the ingress interface, which creates an additional code path that fails to properly validate packet structures before processing.
Attack Vector
The attack can be executed remotely over the network without requiring authentication or user interaction. An attacker can exploit this vulnerability by:
- Identifying target Cisco ASR 9000 series routers running vulnerable IOS XR versions
- Crafting malformed IPv4 multicast packets designed to trigger the improper input validation flaw
- Sending these packets through the affected device toward interfaces with ACL or QoS policies configured
- Causing the line card to experience an exception or hard reset, disrupting all traffic flowing through that card
The attack has a changed scope, meaning the vulnerable component (IOS XR software) impacts resources beyond its security scope (network availability for all traffic traversing the line card). The exploitation mechanism targets the multicast packet processing subsystem, and successful attacks result in traffic loss while the line card reloads.
Detection Methods for CVE-2025-20146
Indicators of Compromise
- Unexpected line card resets or exceptions logged in system messages
- Repeated LC/0/x/CPU0 or similar line card crash events in syslog
- Unusual volumes of IPv4 multicast traffic from untrusted sources
- Traffic loss events correlating with multicast packet reception on ACL/QoS-enabled interfaces
Detection Strategies
- Monitor system logs for line card exception messages and unexpected reload events
- Implement network traffic analysis to identify anomalous IPv4 multicast packet patterns
- Deploy intrusion detection signatures targeting malformed multicast packet structures
- Correlate line card reset events with traffic patterns on interfaces with ACLs or QoS policies
Monitoring Recommendations
- Enable detailed logging for line card health and exception events on ASR 9000 series routers
- Configure SNMP traps for line card state changes and hardware exceptions
- Implement baseline monitoring for normal multicast traffic patterns to detect anomalies
- Review the Cisco Security Advisory for additional detection guidance
How to Mitigate CVE-2025-20146
Immediate Actions Required
- Review all deployed Cisco ASR 9000 series routers and identify those running vulnerable IOS XR versions
- Consult the Cisco Security Advisory for fixed software versions and upgrade guidance
- Prioritize patching for devices in critical network paths or those exposed to untrusted traffic
- Implement network segmentation to limit exposure of vulnerable devices to untrusted multicast traffic sources
Patch Information
Cisco has released security updates to address this vulnerability. Administrators should consult the official Cisco Security Advisory for specific fixed software versions and upgrade instructions. The advisory provides detailed information on affected releases and the recommended upgrade paths for each IOS XR version branch.
Organizations should prioritize patching based on the network criticality of affected devices and their exposure to potentially malicious traffic sources.
Workarounds
- Evaluate removing IPv4 ACLs or QoS policies from interfaces on vulnerable devices where feasible (may impact security or performance requirements)
- Implement upstream filtering to block malformed or suspicious IPv4 multicast traffic before it reaches vulnerable devices
- Deploy rate limiting on multicast traffic at network boundaries to reduce the impact of potential exploitation attempts
- Consider implementing network access controls to restrict which sources can send multicast traffic to affected interfaces
# Example: Review current ACL and QoS configurations on interfaces
show running-config interface | include access-group|service-policy
# Monitor line card status and health
show platform
# Review recent line card events and crashes
show logging | include LC.*reset|exception
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
