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JunosOperating system · Juniper

CVE-2024-39561

MEDIUM · 5.8 CVSS v3.1 Published 2024-07-10
Fix available
A fix is available. Upgrade to 21.2 or later.
See remediation →
67/100
Remediation priority · Elevated
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 flow daemon (flowd) of Juniper Networks Junos OS on SRX4600 and SRX5000 Series allows an attacker to send TCP packets with SYN/FIN or SYN/RST flags, bypassing the expected blocking of these packets. A TCP packet with SYN/FIN or SYN/RST should be dropped in flowd. However, when no-syn-check and Express Path are enabled, these TCP packets are unexpectedly transferred to the downstream network. This issue affects Junos OS on SRX4600 and SRX5000 Series: * All versions before 21.2R3-S8, * from 21.4 before 21.4R3-S7, * from 22.1 before 22.1R3-S6, * from 22.2 before 22.2R3-S4, * from 22.3 before 22.3R3-S3, * from 22.4 before 22.4R3-S2, * from 23.2 before 23.2R2, * from 23.4 before 23.4R1-S1, 23.4R2.

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 vulnerability in Juniper Junos OS flow daemon (flowd) on SRX4600 and SRX5000 Series allows TCP packets with abnormal SYN/FIN or SYN/RST flag combinations to bypass filtering when no-syn-check and Express Path are enabled. These illegal packets should be dropped but are incorrectly forwarded downstream.

MitigationApply the appropriate Junos OS patch or upgrade to a fixed version (21.2R3-S8, 21.4R3-S7, 22.1R3-S6, 22.2R3-S4, 22.3R3-S3, 22.4R3-S2, 23.2R2, 23.4R1-S1, or 23.4R2) per the Juniper advisory. Test Express Path functionality post-update.

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= 21.2= 21.4= 22.1= 22.2= 22.3= 22.4= 23.2= 23.4

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
Changed
Confidentiality
None
Integrity
Low
Availability
None

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

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 model
    Run 'show chassis hardware' or 'show system information' to confirm the SRX model
    Affected if Device is NOT an SRX4600 or SRX5000 Series (vulnerability only applies to these models)
  2. Verify Junos OS version
    Run 'show version' and compare the installed version against the affected list: <21.2, 21.2, 21.4, 22.1, 22.2, 22.3, 22.4, 23.2, 23.4
    Affected if Version matches any of the affected versions listed in the CVE
  3. Confirm Express Path is enabled
    Run 'show security flow status' and look for Express Path or ESP acceleration status; alternatively check configuration for 'set security express-path'
    Affected if Express Path is enabled on the device
  4. Confirm no-syn-check is enabled
    Run 'show configuration security flow' and look for 'no-syn-check' under the flow settings, or check the config with 'show configuration | match no-syn-check'
    Affected if no-syn-check is configured in the security flow settings

User is affected only if: (1) device is SRX4600 or SRX5000 Series, (2) Junos version matches the affected list, AND (3) BOTH Express Path and no-syn-check are simultaneously enabled, allowing malformed SYN/FIN or SYN/RST TCP packets to bypass filtering.

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 21.2 or later
Fixed in 21.2
Interim mitigation

Apply the appropriate Junos OS patch or upgrade to a fixed version (21.2R3-S8, 21.4R3-S7, 22.1R3-S6, 22.2R3-S4, 22.3R3-S3, 22.4R3-S2, 23.2R2, 23.4R1-S1, or 23.4R2) per the Juniper advisory. Test Express Path functionality post-update.

Recommended fix High confidence

Upgrade to 21.2R3-S8, 21.4R3-S7, 22.1R3-S6, 22.2R3-S4, 22.3R3-S3, 22.4R3-S2, 23.2R2, or 23.4R2 (or later) depending on your current branch

  1. 1. Identify the current Junos OS version on the SRX4600 or SRX5000 Series device using 'show version'
  2. 2. Determine which upgrade path applies based on current version (see upgrade_path field)
  3. 3. Download the appropriate fixed Junos OS release from Juniper Networks support portal
  4. 4. Upload the Junos OS upgrade package to the device using 'file copy' or USB storage
  5. 5. Install the upgrade using 'request system software add <package-name>' command
  6. 6. Reboot the device after installation completes using 'request system reboot'
  7. 7. After reboot, verify the new version is running with 'show version' and confirm the fix is applied
  8. 8. If using no-syn-check and Express Path features, verify that SYN/FIN and SYN/RST packets are now properly dropped
Caveat Review Juniper Networks release notes for any compatibility concerns or configuration changes between your current version and the target fixed version

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

Fix this in Junos Scoped from the published advisory
  • Consultation6.0 h
  • Implementation8.0 h
  • Testing4.0 h
  • Review / QA2.0 h
20.0 hours of engineering $3,600
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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-2024-39561 in production — separate from our analysis above.

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What this is

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What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
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  • No spam, self-promotion, credentials, or personal data