CVE-2026-21920 Junos OS: SRX Series: If a specific request is processed by the DNS subsystem flowd will crash

High 8.7 Published 15 Jan 2026 Updated 15 Jan 2026

An unauthenticated attacker can crash the DNS flowd process on Juniper SRX devices configured for DNS processing, causing a denial of service.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

What is CVE-2026-21920?

CVE-2026-21920 is a high-severity denial of service vulnerability in Juniper Junos, published on 15 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

An Unchecked Return Value vulnerability in the DNS module of Juniper Networks Junos OS on SRX Series allows an unauthenticated, network-based attacker to cause a Denial-of-Service (DoS).

If an SRX Series device configured for DNS processing, receives a specifically formatted DNS request flowd will crash and restart, which causes a service interruption until the process has recovered.

This issue affects Junos OS on SRX Series:

* 23.4 versions before 23.4R2-S5, * 24.2 versions before 24.2R2-S1, * 24.4 versions before 24.4R2.

This issue does not affect Junos OS versions before 23.4R1.

Weakness: CWE-252

Which versions are affected by CVE-2026-21920?

It affects 1 product.

Products include Juniper Junos.

How do I fix CVE-2026-21920?

No fix has been published for CVE-2026-21920 yet. Limit exposure of the affected software until one is available.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-21920 being exploited?

There is no known exploitation of CVE-2026-21920 in the wild and no public exploit code.

All Juniper Junos vulnerabilities

Sources

Also known as GHSA-vj9c-h3r9-jwrf.