A Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in the SIP application layer gateway (ALG) of Juniper Networks Junos OS on SRX Series and MX Series with MX-SPC3 or MS-MPC allows an unauthenticated network-based attacker sending specific SIP messages over TCP to crash the flow management process, leading to a Denial of Service (DoS).

On SRX Series, and MX Series with MX-SPC3 or MS-MPC service cards, receipt of multiple SIP messages causes the SIP headers to be parsed incorrectly, eventually causing a continuous loop and leading to a watchdog timer expiration, crashing the flowd process on SRX Series and MX Series with MX-SPC3, or mspmand process on MX Series with MS-MPC.

This issue only occurs over TCP. SIP messages sent over UDP cannot trigger this issue.

This issue affects Junos OS on SRX Series and MX Series with MX-SPC3 and MS-MPC:

• all versions before 21.2R3-S10, 
• from 21.4 before 21.4R3-S12, 
• from 22.4 before 22.4R3-S8, 
• from 23.2 before 23.2R2-S5, 
• from 23.4 before 23.4R2-S6, 
• from 24.2 before 24.2R2-S3, 
• from 24.4 before 24.4R2-S1, 
• from 25.2 before 25.2R1-S1, 25.2R2.

The SIP ALG must be enabled to be vulnerable to this issue. The SIP ALG is enabled by default on MX-SPC3 and some SRX Series platforms. For other platforms, the following configuration is required:

[security alg sip enable]

On SRX Series and MX Series with MX-SPC3, the SIP ALG status can be confirmed via the 'show security alg status' command:

user@junos> show security alg status
ALG Status:
...
  SIP     : Enabled

Juniper SIRT is not aware of any malicious exploitation of this vulnerability.

This issue was seen during production usage.

The following software releases have been updated to resolve this specific issue: 21.2R3-S10, 21.4R3-S12, 22.4R3-S8, 23.2R2-S5, 23.4R2-S6, 24.2R2-S3, 24.4R2-S1, 25.2R1-S1, 25.2R2, 25.4R1, and all subsequent releases.

This issue is being tracked as 1876029 which is visible on the Customer Support website.

Note: Juniper SIRT's policy is not to evaluate releases which are beyond End of Engineering (EOE) or End of Life (EOL).

Where practical, use a firewall filter to limit SIP traffic over TCP.

Alternately, if the SIP ALG is not required, it can be explicitly disabled on SRX Series and MX Series with MX-SPC3 via:

# set security alg sip disable

2026-01-14: Initial Publication

• KB16613: Overview of the Juniper Networks SIRT Quarterly Security Bulletin Publication Process [juniper.net]
• KB16765: In which releases are vulnerabilities fixed? [juniper.net]
• KB16446: Common Vulnerability Scoring System (CVSS) and Juniper's Security Advisories [juniper.net]
• Report a Security Vulnerability - How to Contact the Juniper Networks Security Incident Response Team