A vulnerability in Juniper Networks Junos OS on SRX Series, allows a network-based unauthenticated attacker to cause a Denial of Service (DoS) by sending a specific fragmented packet to the device, resulting in a flowd process crash, which is responsible for packet forwarding. Continued receipt and processing of this specific packet will create a sustained DoS condition. This issue only affects SRX Series when 'preserve-incoming-fragment-size' feature is enabled. This issue affects Juniper Networks Junos OS on SRX Series: 18.3 versions prior to 18.3R3-S6; 18.4 versions prior to 18.4R3-S10; 19.1 versions prior to 19.1R3-S7; 19.2 versions prior to 19.2R3-S4; 19.3 versions prior to 19.3R3-S4; 19.4 versions prior to 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 21.1R2-S1, 21.1R3; 21.2 versions prior to 21.2R2. This issue does not affect Juniper Networks Junos OS prior to 17.3R1.