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JunosOperating system · Juniper

CVE-2024-39540

HIGH · 7.5 CVSS v3.1 Published 2024-07-11
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 Improper Check for Unusual or Exceptional Conditions vulnerability in the Packet Forwarding Engine (pfe) of Juniper Networks Junos OS on SRX Series, and MX Series with SPC3 allows an unauthenticated, network-based attacker to cause a Denial-of-Service (DoS). When an affected device receives specific valid TCP traffic, the pfe crashes and restarts leading to a momentary but complete service outage. This issue affects Junos OS: 21.2 releases from 21.2R3-S5 before 21.2R3-S6. This issue does not affect earlier or later releases.

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 Improper Check for Unusual or Exceptional Conditions vulnerability in the Packet Forwarding Engine (pfe) of Juniper Networks Junos OS on SRX Series and MX Series with SPC3 allows an unauthenticated, network-based attacker to cause a Denial-of-Service by sending specific valid TCP traffic that crashes and restarts the pfe, causing complete service outage.

MitigationApply Junos OS upgrade to 21.2R3-S6 or later (or a different unaffected release) following vendor documentation; implement network segmentation or ACLs to limit exposure to untrusted networks until patching is complete.

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:= 21.2

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 device hardware platform
    Run 'show chassis hardware' or check device model to confirm it is an SRX Series or MX Series with SPC3 service card
    Affected if Device is SRX Series or MX Series with SPC3 Service Processing Card
  2. Check installed Junos OS version
    Run 'show version' or 'show version detail' to identify the exact Junos OS version
    Affected if Version is 21.2 (any 21.2R0.x through 21.2Rx release, i.e., the 21.2 base train without S6 or later upgrades)
  3. Verify PFE status is active
    Run 'show pfe statistics' or 'show chassis fpc pic-status' to confirm the Packet Forwarding Engine is running
    Affected if PFE is active and processing traffic (vulnerable if exposed to untrusted networks)

Device is affected if it is an SRX or MX with SPC3, runs Junos OS version 21.2, and has an active PFE that could receive the malicious TCP traffic.

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

Apply Junos OS upgrade to 21.2R3-S6 or later (or a different unaffected release) following vendor documentation; implement network segmentation or ACLs to limit exposure to untrusted networks until patching is complete.

Recommended fix High confidence

21.2R3-S6 or later (any release after 21.2R3-S6)

  1. Confirm current Junos OS version by running 'show version' command
  2. Schedule a maintenance window as the upgrade will cause service interruption
  3. Download Junos OS 21.2R3-S6 or later from supportportal.juniper.net
  4. Transfer the upgrade package to the device using SCP or USB
  5. Initiate the upgrade using 'request system software add <package-name>' command
  6. Reboot the device using 'request system reboot' command to complete the installation
  7. After reboot, verify the new version with 'show version' and confirm the fix is applied by ensuring pfe stability

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

Fix this in Junos Scoped from the published advisory
  • Consultation4.0 h
  • Implementation4.0 h
  • Testing4.0 h
  • Review / QA2.0 h
14.0 hours of engineering $2,480
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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-2024-39540 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
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