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JunosOperating system · Juniper

CVE-2026-21920

HIGH · 7.5 CVSS v3.1 Published 2026-01-15
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

An unchecked return value vulnerability in the DNS module of Juniper Junos OS on SRX Series allows an unauthenticated attacker to send a specifically formatted DNS request that causes the flowd process to crash and restart, resulting in denial of service.

MitigationUpgrade Junos OS on SRX Series devices to version 23.4R2-S5 or later for 23.4 branches, 24.2R2-S1 or later for 24.2 branches, or 24.4R2 or later for 24.4 branches. Alternatively, disable DNS processing on affected SRX devices if upgrade is not immediately feasible.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
JunosOperating system
Affected:= 23.4= 24.2= 24.4

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Confirm device is SRX Series
    Run 'show chassis hardware' or 'show system information' and verify the model is an SRX device (e.g., SRX210, SRX240, SRX3400, SRX5400, etc.)
    Affected if The device is not an SRX Series device - this CVE only affects SRX Series appliances
  2. Check installed Junos OS version
    Run 'show version' or 'show system information' and locate the Junos version in the output
    Affected if The installed version exactly matches 23.4, 24.2, or 24.4 (these are the exact affected versions per the CVE)
  3. Verify DNS processing is enabled
    Check DNS proxy or DNS inspection configuration with 'show configuration | display set | match dns' or review security zone/policy DNS settings
    Affected if DNS processing is enabled on the device - the vulnerability exists in the DNS module, so DNS must be active for exploitation
  4. Confirm flowd process is running
    Run 'show system processes' or 'show system processes extensive | match flowd' to verify the flowd daemon is active
    Affected if flowd process is running - this is the process that will crash when the specially crafted DNS request is received

A user is affected if they have an SRX Series device running exactly version 23.4, 24.2, or 24.4 with DNS processing enabled, as the vulnerability resides in the DNS module and causes the flowd process to crash.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade Junos OS on SRX Series devices to version 23.4R2-S5 or later for 23.4 branches, 24.2R2-S1 or later for 24.2 branches, or 24.4R2 or later for 24.4 branches. Alternatively, disable DNS processing on affected SRX devices if upgrade is not immediately feasible.

Recommended fix High confidence

23.4R2-S5, 24.2R2-S1, or 24.4R2 (or later stable releases within each branch)

  1. 1. Identify the current Junos OS version on the SRX device using 'show version'
  2. 2. Determine the appropriate upgrade path based on current version (23.4.x -> 23.4R2-S5 or later, 24.2.x -> 24.2R2-S1 or later, 24.4.x -> 24.4R2 or later)
  3. 3. Schedule a maintenance window as the upgrade will cause a brief service interruption
  4. 4. Download the appropriate fixed Junos OS image from the Juniper support portal
  5. 5. Upload the software package to the SRX device using 'file copy' or USB
  6. 6. Install the software using 'request system software add <package> reboot'
  7. 7. After reboot, verify the new version is running with 'show version'
  8. 8. Verify DNS processing is functioning normally and no crashes occur
Caveat Standard Junos upgrade considerations apply - review release notes for any configuration changes between major versions

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Junos Scoped from the published advisory
  • Consultation4.0 h
  • Implementation6.0 h
  • Testing3.0 h
  • Review / QA2.0 h
15.0 hours of engineering $2,690
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-21920 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data