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JunosOperating system · Juniper

CVE-2026-21918

HIGH · 7.5 CVSS v3.1 Published 2026-01-15
Fix available
A fix is available. Upgrade to 22.4 or later.
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
A Double Free vulnerability in the flow processing daemon (flowd) of Juniper Networks Junos OS on SRX and MX Series allows an unauthenticated, network-based attacker to cause a Denial-of-Service (DoS). On all SRX and MX Series platforms, when during TCP session establishment a specific sequence of packets is encountered a double free happens. This causes flowd to crash and the respective FPC to restart. This issue affects Junos OS on SRX and MX Series: * all versions before 22.4R3-S7, * 23.2 versions before 23.2R2-S3, * 23.4 versions before 23.4R2-S4, * 24.2 versions before 24.2R2.

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

A double free vulnerability exists in the flow processing daemon (flowd) of Juniper Junos OS on SRX and MX Series platforms. When a specific packet sequence is encountered during TCP session establishment, the flowd process experiences a double free condition, causing it to crash and triggering a restart of the Flexible PIC Concentrator (FPC). An unauthenticated, network-based attacker can exploit this to cause denial of service.

MitigationApply the vendor-supplied Junos OS updates: upgrade to 22.4R3-S7, 23.2R2-S3, 23.4R2-S4, or 24.2R2 or later. For systems that cannot be immediately updated, consider network segmentation to limit exposure to untrusted TCP traffic.

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:< 22.4= 22.4= 23.2= 23.4= 24.2

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. Identify platform type
    Execute 'show chassis hardware' or 'show version' and confirm the device is an SRX Series or MX Series router
    Affected if Device is not SRX or MX Series (vulnerability only affects these platforms)
  2. Check installed Junos version
    Run 'show version' and note the Junos OS version string (e.g., 22.4R1, 23.2R1, etc.)
    Affected if Version is < 22.4, or equals 22.4, 23.2, 23.4, or 24.2
  3. Verify flowd is running
    Run 'show system processes' or 'show system processes extensive' and confirm the flowd (flow-processing-daemon) process is active
    Affected if flowd is not running (vulnerability only applies when flowd is enabled)
  4. Check for recent FPC restarts or flowd crashes
    Run 'show log messages' and search for 'FPC' restart events or 'flowd' crash indicators using 'grep -i fpc' or 'grep -i flowd'
    Affected if Recent FPC restarts or flowd core dumps are present in logs

User is affected if running Junos version 22.4, 23.2, 23.4, or 24.2 (or any version below 22.4) on an SRX or MX Series device with flowd enabled.

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
Upgrade available Upgrade to 22.4 or later
Fixed in 22.4
Interim mitigation

Apply the vendor-supplied Junos OS updates: upgrade to 22.4R3-S7, 23.2R2-S3, 23.4R2-S4, or 24.2R2 or later. For systems that cannot be immediately updated, consider network segmentation to limit exposure to untrusted TCP traffic.

Recommended fix High confidence

Upgrade to 22.4R3-S7 or later, 23.2R2-S3 or later, 23.4R2-S4 or later, or 24.2R2 or later depending on the version branch

  1. Identify the current Junos OS version on the SRX/MX device using 'show version'
  2. Determine which version branch (22.4, 23.2, 23.4, or 24.2) the device is running
  3. Plan a maintenance window as the upgrade will require a reboot
  4. Download the appropriate fixed Junos OS version: 22.4R3-S7 or later (22.4 branch), 23.2R2-S3 or later (23.2 branch), 23.4R2-S4 or later (23.4 branch), or 24.2R2 or later (24.2 branch) from the Juniper support portal
  5. Transfer the Junos OS upgrade image to the device using SCP or USB
  6. Initiate the upgrade using 'request system software add <image> reboot' command
  7. After reboot, verify the new version is running with 'show version'
  8. Confirm flowd is running properly with 'show system processes' or similar process check
Caveat Review Juniper release notes for any configuration syntax changes or feature modifications between versions before upgrading; test in a lab environment first

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

Fix this in Junos Scoped from the published advisory
  • Consultation8.0 h
  • Implementation12.0 h
  • Testing6.0 h
  • Review / QA4.0 h
30.0 hours of engineering $5,380
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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-2026-21918 in production — separate from our analysis above.

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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