An Improper Check for Unusual or Exceptional Conditions vulnerability in the Routing Protocol Daemon (rpd) of Juniper Networks Junos OS and Junos OS Evolved allows an adjacent, unauthenticated attacker with an established ISIS adjacency to cause a Denial of Service (DoS). The rpd CPU spikes to 100% after a malformed ISIS TLV has been received which will lead to processing issues of routing updates and in turn traffic impact. This issue affects: Juniper Networks Junos OS 19.3 versions prior to 19.3R3-S4; 19.4 versions prior to 19.4R2-S6, 19.4R3-S6; 20.1 versions prior to 20.1R3-S2; 20.2 versions prior to 20.2R3-S3; 20.3 versions prior to 20.3R3-S1; 20.4 versions prior to 20.4R3; 21.1 versions prior to 21.1R3; 21.2 versions prior to 21.2R2. Juniper Networks Junos OS Evolved All versions prior to 20.4R3-S3-EVO; 21.2 versions prior to 21.2R2-EVO. This issue does not affect Juniper Networks Junos OS versions prior to 19.3R1.