An Improper Locking vulnerability in the SIP ALG of Juniper Networks Junos OS on MX Series and SRX Series allows an unauthenticated networked attacker to cause a flowprocessing daemon (flowd) crash and thereby a Denial of Service (DoS). Continued receipt of these specific packets will cause a sustained Denial of Service condition. This issue can occur in a scenario where the SIP ALG is enabled and specific SIP messages are being processed simultaneously. This issue affects: Juniper Networks Junos OS on MX Series and SRX Series 20.4 versions prior to 20.4R3-S1; 21.1 versions prior to 21.1R2-S2 21.1R3; 21.2 versions prior to 21.2R1-S2 21.2R2; 21.3 versions prior to 21.3R1-S1 21.3R2. This issue does not affect Juniper Networks Junos OS versions prior to 20.4R1.