Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
Sources: National Vulnerability Database, CISA KEV, Exploit-DB
CVE-2026-76505 Overview
CVE-2026-76505 is a denial-of-service vulnerability in the IPv4 packet-processing path of Cisco IOS XR Software. The flaw affects Cisco ASR 9000 and 9900 Series Aggregation Services Routers, along with the ASR 9902 and 9903 Compact High-Performance Routers. An unauthenticated, remote attacker can send crafted IPv4 packets to an affected line card interface and trigger network processor (NP) errors. The result is a line card reset or shutdown and a loss of traffic forwarding. The vulnerability is tracked under CWE-20: Improper Input Validation.
Critical Impact
Repeated exploitation can drive the network processor into a persistent shutdown state, requiring an explicit line card reload to restore traffic forwarding on affected ASR platforms.
Affected Products
- Cisco ASR 9000 and 9900 Series Aggregation Services Routers running vulnerable Cisco IOS XR Software
- Cisco ASR 9902 Compact High-Performance Router running vulnerable Cisco IOS XR Software
- Cisco ASR 9903 Compact High-Performance Router running vulnerable Cisco IOS XR Software
Discovery Timeline
- 2026-10-21 - CVE-2026-76505 published to NVD
- 2026-10-22 - Last updated in NVD database
Technical Details for CVE-2026-76505
Vulnerability Analysis
The vulnerability resides in the IPv4 packet-processing code path on affected line cards running Cisco IOS XR Software. When the line card receives a specially crafted IPv4 packet destined to the router itself (for-us traffic), the network processor mishandles the packet and generates an error. That error causes an NP reset. During the reset, the line card drops traffic until the processor recovers. Transit IPv4 traffic does not trigger the issue, and IPv6 traffic cannot trigger it at all. The flaw is network-reachable and requires no authentication or user interaction.
Repeated triggering compounds the impact. If the NP resets occur in rapid succession, the processor can enter a persistent shutdown state. In that condition, automatic recovery stops, and an operator must perform an explicit line card reload to restore forwarding. This escalates a transient outage into a sustained denial-of-service condition against production transit infrastructure.
Root Cause
The underlying defect is improper input validation [CWE-20] of IPv4 header fields during for-us packet processing on the ASR line card NP. Malformed field combinations are not rejected before reaching the processing logic that triggers the fault.
Attack Vector
An attacker only needs IP reachability to an interface on an affected line card. Because the trigger is for-us IPv4 traffic, control-plane and management addresses on the router are viable targets. Infrastructure Access Control Lists (iACLs) and Control Plane Policing (CoPP) can constrain exposure where properly deployed.
No verified proof-of-concept code is publicly available. See the Cisco Security Advisory on ASR9K for the authoritative technical details.
Detection Methods for CVE-2026-76505
Indicators of Compromise
- Line card syslog messages reporting network processor (NP) resets, errors, or shutdown events on ASR 9000, 9900, 9902, or 9903 platforms.
- Sudden traffic loss on a specific line card followed by recovery, correlated with inbound IPv4 traffic bursts to router-owned addresses.
- Persistent NP shutdown state requiring manual line card reload to restore forwarding.
Detection Strategies
- Monitor Cisco IOS XR syslog streams for NP error, reset, and shutdown signatures tied to the affected platforms.
- Correlate line card recovery events with ingress IPv4 for-us traffic patterns to identify repeat-trigger behavior.
- Alert on repeated resets of the same NP within a short interval, which indicates an active attempt to force persistent shutdown.
Monitoring Recommendations
- Ingest router syslog and SNMP traps into a centralized SIEM or data lake for correlation across the ASR fleet.
- Baseline normal rates of for-us IPv4 traffic to control-plane addresses and alert on deviations.
- Track line card and NP state transitions through streaming telemetry (model-driven telemetry) for near real-time visibility.
How to Mitigate CVE-2026-76505
Immediate Actions Required
- Review the Cisco Security Advisory on ASR9K and identify affected Cisco IOS XR releases in your environment.
- Schedule upgrades to a fixed Cisco IOS XR Software release as listed in the Cisco advisory.
- Audit infrastructure ACLs and CoPP policies to confirm they limit untrusted IPv4 traffic to router-owned addresses.
Patch Information
Cisco has published fixed software releases for this vulnerability. Consult the Cisco Security Advisory on ASR9K for the current list of fixed Cisco IOS XR versions applicable to ASR 9000, 9900, 9902, and 9903 platforms, and upgrade through standard change-management procedures.
Workarounds
- Apply infrastructure Access Control Lists (iACLs) to block untrusted IPv4 traffic destined to router infrastructure addresses on affected line card interfaces.
- Deploy Control Plane Policing (CoPP) to rate-limit for-us IPv4 traffic and reduce the probability of sustained NP reset conditions.
- Restrict management and routing-protocol peering to trusted sources only, limiting attacker reachability to router-owned IPv4 addresses.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
Sources: National Vulnerability Database, CISA KEV, Exploit-DB
