Analyzing CVE-2025-21601: Juniper Junos OS Web Management DoS Vulnerability

Introduction

Network reliability is paramount, yet Juniper Networks' Junos OS faces a critical threat from CVE-2025-21601, a vulnerability enabling attackers to incapacitate devices through simple HTTP requests. This flaw, affecting web management interfaces, poses severe risks to network stability and operational continuity, demanding immediate attention from security teams.

Affected Systems and Versions

CVE-2025-21601 specifically impacts Juniper Networks Junos OS on the following series and versions:

• SRX Series: All versions before 21.4R3-S9, 22.2R3-S5, and 22.4R3-S4.
• EX Series: Versions before 23.2R2-S3 and 23.4R2-S3.
• MX240, MX480, MX960: Versions before 24.2R1-S1.
• QFX5120 Series: All versions prior to 24.2R2.

Technical Information

The vulnerability results from improper handling of HTTP requests by the Junos OS httpd process. Specifically, the system lacks adequate rate limiting and input validation, allowing attackers to send crafted requests that trigger recursive resource allocation. This leads to linear CPU consumption growth per request, ultimately exhausting available CPU resources and causing device unresponsiveness.

Attackers exploit this vulnerability by targeting exposed web management interfaces (J-Web, Captive Portal, JSC) with repeated HTTP requests. Sustained attack rates quickly escalate CPU usage to 100%, disrupting critical network functions.

Indicators of Compromise

High CPU usage of the httpd process is a clear indicator of exploitation:

show system processes extensive | match httpd
PID nobody     52   0   20M    191M select  2   0:01   80.00% httpd{httpd}

Patch Information

Juniper Networks has provided patches for all affected versions. Organizations should immediately upgrade to:

• SRX/EX/MX: Versions 21.4R3-S9, 22.2R3-S5, 22.4R3-S4, 23.2R2-S3, 23.4R2-S3, or 24.2R1-S1.
• QFX5120: Version 24.2R2.

For systems unable to patch immediately, restrict access to web management interfaces and disable unused services to mitigate risk.

Detection Methods

Monitor CPU usage of the httpd process via CLI diagnostics regularly. High sustained CPU usage (>70%) indicates potential exploitation:

show system processes extensive | match httpd

Vendor Security History

Juniper Networks has previously faced several critical vulnerabilities, notably CVE-2025-21590 and CVE-2024-39555, indicating recurring security challenges within Junos OS. Despite timely patch releases, the frequency of vulnerabilities suggests underlying issues in Juniper's software security lifecycle.

References

• Juniper Advisory JSA96452
• Stack Watch Juniper
• Security Online Juniper CVE-2025-21590
• Stack Watch Junos OS
• Bleeping Computer Juniper RCE
• Cybersecurity Dive Juniper Backdoor

Organizations must act swiftly to secure their Juniper infrastructure against this critical vulnerability, ensuring network resilience and operational integrity.