Junos OS (SRX firewalls)
CVE-2026-21920: SRX Series: If a specific request is processed by the DNS subsystem flowd will crash
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).
Published
Automatic summary from the vendor's CVE record. We haven't written a full analysis of this vulnerability yet.
Affected and fixed versions
| Product / branch | Fixed in |
|---|---|
| Junos OS (SRX Series) 23.4 | 23.4R2-S5 or later |
| Junos OS (SRX Series) 24.2 | 24.2R2-S1 or later |
| Junos OS (SRX Series) 24.4 | 24.4R2 or later |
| Junos OS (SRX Series) 0 | Not affected |
Always confirm against the vendor advisory, which lists every fixed hotfix.
What it is
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.
Am I affected?
A device is exposed to this issue when one of the following features are used / configurations are present:
- GeoIP:
[ security dynamic-address address-name <name> profile category GeoIP ... ]
- DNS security:
[ services security-metadata-streaming policy <name> dns detections ... ]
or
[ services security-intelligence profile <profile-name> category DNS ] [ services security-intelligence profile policy <policy-name> DNS <profile-name> ]
Workarounds
There are no known workarounds for this issue.
Exploitation
Juniper SIRT is not aware of any malicious exploitation of this vulnerability.
Sources
KEV status, EPSS score and vendor data refreshed automatically, last on 13 Oct 2026.