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CVE-2026-20074 Cisco CVE debrief

A vulnerability in the Intermediate System-to-Intermediate System (IS-IS) multi-instance routing feature of Cisco IOS XR Software could allow an unauthenticated, adjacent attacker to cause the IS-IS process to restart unexpectedly. This vulnerability is due to insufficient input validation of ingress IS-IS packets. An attacker could exploit this vulnerability by sending crafted IS-IS packets to an affected device after forming an adjacency. A successful exploit could allow the attacker to cause the IS-IS process to restart unexpectedly, resulting in a temporary loss of connectivity to advertised networks and a denial of service (DoS) condition.

Vendor
Cisco
Product
IOS XR Software
CVSS
HIGH 7.4
CISA KEV
Not listed in stored evidence
Original CVE published
2026-03-11
Original CVE updated
2026-07-08
Advisory published
2026-03-11
Advisory updated
2026-07-08

Who should care

Network administrators and security teams responsible for Cisco IOS XR Software deployments, particularly those using the IS-IS protocol, should be aware of this vulnerability and take necessary precautions to mitigate the risk.

Technical summary

The vulnerability is caused by insufficient input validation of ingress IS-IS packets in the IS-IS multi-instance routing feature of Cisco IOS XR Software. An unauthenticated, adjacent attacker can exploit this vulnerability by sending crafted IS-IS packets to an affected device after forming an adjacency, potentially causing the IS-IS process to restart unexpectedly and resulting in a temporary loss of connectivity to advertised networks and a denial of service (DoS) condition.

Defensive priority

High priority should be given to patching or mitigating this vulnerability, as it can be exploited by an unauthenticated, adjacent attacker to cause a denial of service (DoS) condition.

Recommended defensive actions

  • Apply the vendor-provided patch or update to affected Cisco IOS XR Software versions.
  • Implement compensating controls, such as ingress filtering, to prevent crafted IS-IS packets from reaching the affected device.
  • Monitor IS-IS process logs for unexpected restarts and investigate any suspicious activity.
  • Verify that IS-IS adjacencies are properly configured and limit the number of adjacencies to trusted sources.
  • Review network segmentation and isolation controls to prevent lateral movement.
  • Conduct regular security audits and vulnerability assessments.
  • Track and verify patch deployment and vulnerability mitigation progress.

Evidence notes

The CVE record was published on 2026-03-11T17:16:55.470Z and was last modified on 2026-07-08T19:07:12.303Z. The NVD entry is currently Analyzed. Cisco has provided a mitigation or vendor reference for this vulnerability.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-20074 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-20074

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-20074 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-20074

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.