This vulnerability (CVE-2026-57023) in Juniper Networks Junos OS affects MX Series with SPC3 and SRX Series devices. It is caused by improper validation of specified quantity in input in the TCP proxy plugin. When TCP proxy is engaged to support ALGs, Advanced Anti-Malware, ICAP, or UTM, a TCP packet with a specifically malformed TCP header can cause the flow processing daemon (flowd) to crash and restart. This leads to a complete denial of service until the system recovers automatically. The affected versions are 23.4 prior to 23.4R2-S7, 24.2 prior to 24.2R2-S4, 24.4 prior to 24.4R2-S3, and 25.2 prior to 25.2R2. Releases before 23.4R1 are not vulnerable. The vulnerability is remotely exploitable without authentication and requires low attack complexity, with no impact on confidentiality or integrity but high impact on availability.

An unauthenticated, network-based attacker can cause a complete denial of service on affected Junos OS devices by sending a malformed TCP packet that crashes the flow processing daemon. This results in a service outage until the system automatically recovers. There is no impact on confidentiality or integrity, but availability is severely affected.

A patch is available for this vulnerability. The affected versions are fixed starting from 23.4R2-S7, 24.2R2-S4, 24.4R2-S3, and 25.2R2. Since this is a cloud service managed by Juniper Networks, the vendor typically handles remediation server-side. Users should verify with the vendor advisory for the latest patch status and apply updates accordingly to prevent denial of service.