An Improper Validation of Syntactic Correctness of Input vulnerability in the Web-Filtering module of Juniper Networks Junos OS on SRX Series allows a...
Description
An Improper Validation of Syntactic Correctness of Input vulnerability in the Web-Filtering module of Juniper Networks Junos OS on SRX Series allows an unauthenticated, network-based attacker to cause a Denial-of-Service (DoS)
Remediation
Apply vendor security updates immediately. Monitor for exploitation attempts and review access logs.
Executive Summary:
A high-severity vulnerability has been identified in Juniper Networks Junos OS that affects SRX and MX Series devices. An unauthenticated attacker can remotely send specific network traffic that causes a critical process to crash, resulting in a complete denial of service (DoS) and disrupting all network traffic passing through the affected device.
Vulnerability Details
CVE-ID: CVE-2026-21905
Affected Software: Juniper Networks Junos OS
Affected Versions: See vendor advisory for specific affected versions
Vulnerability: This vulnerability is an infinite loop within the Session Initiation Protocol (SIP) Application Layer Gateway (ALG) component. An unauthenticated, network-based attacker can exploit this by sending a specially crafted sequence of SIP messages over TCP to a vulnerable device. The SIP ALG fails to properly handle these messages, causing the flow management process to enter an infinite loop, which consumes excessive CPU resources and ultimately leads to a process crash and a Denial of Service condition for the entire device.
Business Impact
This vulnerability is rated as High severity with a CVSS score of 7.5. Exploitation of this flaw will result in a Denial of Service (DoS), causing a complete outage of network traffic processed by the affected SRX or MX series device. The business impact includes significant operational disruption, unavailability of critical services (such as VoIP, internet access, or internal routing), potential financial loss due to downtime, and damage to the organization's reputation. The attack does not require authentication and can be launched remotely, increasing the risk profile.
Remediation Plan
Immediate Action: Apply the security updates provided by Juniper Networks to all affected devices immediately. Prioritize patching for internet-facing devices and critical infrastructure components to mitigate the risk of external attack.
Proactive Monitoring: Monitor network traffic for unusual or malformed SIP messages, particularly over TCP. System administrators should monitor CPU and memory utilization on the flow management process (flowd) on SRX and MX devices for abnormal spikes. Review system logs for entries related to SIP ALG errors or flowd process crashes.
Compensating Controls: If patching cannot be performed immediately, consider implementing the following controls:
Exploitation Status
Public Exploit Available: false
Analyst Notes: As of January 17, 2026, there is no known public proof-of-concept exploit code, and there are no reports of this vulnerability being actively exploited in the wild. However, given the low attack complexity, it is likely that threat actors could develop an exploit with minimal effort.
Analyst Recommendation
Due to the high severity (CVSS 7.5) and the potential for significant business disruption, it is strongly recommended that organizations prioritize the immediate application of vendor-supplied patches. Although this CVE is not currently listed on the CISA KEV catalog, its ability to be exploited by an unauthenticated remote attacker makes it a critical threat. Organizations should identify all vulnerable Juniper devices in their environment and proceed with the remediation plan without delay.