An Improper Locking vulnerability in the SIP ALG of Juniper Networks Junos OS on MX Series with MS-MPC or MS-MIC card and SRX Series allows an unauthenticated, network-based attacker to cause a flow processing 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 occurs when SIP ALG is enabled and specific SIP messages are processed simultaneously. This issue affects: Juniper Networks Junos OS on MX Series and SRX Series 20.4 versions prior to 20.4R3-S4; 21.1 versions prior to 21.1R3-S3; 21.2 versions prior to 21.2R3-S2; 21.3 versions prior to 21.3R3; 21.4 versions prior to 21.4R3; 22.1 versions prior to 22.1R2. This issue does not affect Juniper Networks Junos OS versions prior to 20.4R1 on MX Series, or SRX Series.