An Access of Uninitialized Pointer vulnerability in the SIP ALG of Juniper Networks Junos OS allows an unauthenticated network-based attacker to cause a Denial of Service (DoS). Continued receipt of these specific packets will cause a sustained Denial of Service condition. On all MX and SRX platforms if the SIP ALG is enabled an MS-MPC or MS-MIC or SPC will crash if it receives a SIP message with a specific contact header format. This issue affects Juniper Networks Junos OS on MX Series and SRX Series: 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 versions prior to 20.4R1.