Juniper Junos OS up to 25.2R1-S1 on SRX SIP Application Layer Gateway infinite loop

Summaryinfo

A vulnerability was found in Juniper Junos OS up to 25.2R1-S1 on SRX and classified as problematic. Affected is an unknown function of the component SIP Application Layer Gateway. Such manipulation leads to infinite loop. This vulnerability is referenced as CVE-2026-21905. It is possible to launch the attack remotely. No exploit is available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability was found in Juniper Junos OS up to 25.2R1-S1 on SRX. It has been rated as critical. Affected by this issue is an unknown code of the component SIP Application Layer Gateway. The manipulation with an unknown input leads to a infinite loop vulnerability. Using CWE to declare the problem leads to CWE-835. The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop. Impacted is availability. CVE summarizes:

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 advisory is available at supportportal.juniper.net. This vulnerability is handled as CVE-2026-21905 since 01/05/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. The technical details are unknown and an exploit is not available. The structure of the vulnerability defines a possible price range of USD $0-$5k at the moment (estimation calculated on 01/16/2026). This vulnerability is assigned to T1499 by the MITRE ATT&CK project.

Upgrading to version 21.2R3-S10, 21.4R3-S12, 22.4R3-S8, 23.2R2-S5, 23.4R2-S6, 24.2R2-S3, 24.4R2-S1, 25.2R1-S1 or 25.2R2 eliminates this vulnerability.

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Productinfo

Type

• Router Operating System

Vendor

• Juniper

Name

• Junos OS

Version

• 21.4R3-S11
• 22.4R3-S7
• 23.2R2-S4
• 23.4R2-S5
• 24.2R2-S2
• 24.4R2-S0
• 25.2R1
• 25.2R1-S1

License

• commercial

Website

• Vendor: https://www.juniper.net/

CPE 2.3info

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