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JunosOperating system · Juniper

CVE-2021-0254

CRITICAL · 9.8 CVSS v3.1 Published 2021-04-22
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
100/100
Remediation priority · Urgent
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 buffer size validation vulnerability in the overlayd service of Juniper Networks Junos OS may allow an unauthenticated remote attacker to send specially crafted packets to the device, triggering a partial Denial of Service (DoS) condition, or leading to remote code execution (RCE). Continued receipt and processing of these packets will sustain the partial DoS. The overlayd daemon handles Overlay OAM packets, such as ping and traceroute, sent to the overlay. The service runs as root by default and listens for UDP connections on port 4789. This issue results from improper buffer size validation, which can lead to a buffer overflow. Unauthenticated attackers can send specially crafted packets to trigger this vulnerability, resulting in possible remote code execution. overlayd runs by default in MX Series, ACX Series, and QFX Series platforms. The SRX Series does not support VXLAN and is therefore not vulnerable to this issue. Other platforms are also vulnerable if a Virtual Extensible LAN (VXLAN) overlay network is configured. This issue affects Juniper Networks Junos OS: 15.1 versions prior to 15.1R7-S9; 17.3 versions prior to 17.3R3-S11; 17.4 versions prior to 17.4R2-S13, 17.4R3-S4; 18.1 versions prior to 18.1R3-S12; 18.2 versions prior to 18.2R2-S8, 18.2R3-S7; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R1-S8, 18.4R2-S7, 18.4R3-S7; 19.1 versions prior to 19.1R2-S2, 19.1R3-S4; 19.2 versions prior to 19.2R1-S6, 19.2R3-S2; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R2-S4, 19.4R3-S1; 20.1 versions prior to 20.1R2-S1, 20.1R3; 20.2 versions prior to 20.2R2, 20.2R2-S1, 20.2R3; 20.3 versions prior to 20.3R1-S1.

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

Buffer overflow vulnerability in Juniper's overlayd service (handles VXLAN overlay OAM packets) due to improper buffer size validation. Unauthenticated remote attackers can send specially crafted UDP packets to port 4789 to trigger the overflow, potentially achieving remote code execution as root or causing partial DoS.

MitigationApply the appropriate Junos OS patch per the version matrix; for systems not requiring VXLAN, disable overlayd service. For SRX or non-VXLAN deployments, no action required.

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:= 15.1= 17.3= 17.4= 18.1= 18.2= 18.3= 18.4= 19.1= 19.2= 19.3= 19.4= 20.1

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
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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. Check Junos OS version
    Run 'show version' or 'system snapshot' to obtain the installed Junos version
    Affected if The installed version matches any of these: 15.1, 17.3, 17.4, 18.1, 18.2, 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, or 20.1
  2. Verify overlayd service status
    Run 'show system processes overlayd' or check service status via 'request system process restart overlayd' to determine if the overlayd process is active
    Affected if The overlayd service is running on the device
  3. Confirm VXLAN overlay configuration
    Run 'show configuration protocols evpn' and 'show configuration interfaces vlan' to look for VXLAN-related configuration, or use 'show vxlan' to display active VXLAN instances
    Affected if VXLAN overlay functionality is configured or active on the device
  4. Check UDP port 4789 accessibility
    Run 'show system services network-connections' or use 'netstat -an | grep 4789' to verify if UDP port 4789 is open and listening
    Affected if UDP port 4789 is open and listening, allowing external VXLAN OAM packets to reach the overlayd service

You are affected if your Junos version is one of the listed affected versions AND the overlayd service is running with VXLAN enabled and UDP port 4789 is exposed to untrusted networks.

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 the appropriate Junos OS patch per the version matrix; for systems not requiring VXLAN, disable overlayd service. For SRX or non-VXLAN deployments, no action required.

Recommended fix High confidence

Upgrade to the fixed version for your current Junos branch: 15.1R7-S9, 17.3R3-S11, 17.4R2-S13/17.4R3-S4, 18.1R3-S12, 18.2R2-S8/18.2R3-S7, 18.3R3-S4, 18.4R3-S7, 19.3R3-S1, 19.4R3-S1, 20.1R3, 20.2R3, or 20.3R1-S1

  1. 1. Identify the current Junos OS version on the device using 'show version' command
  2. 2. Determine which platform series the device is (MX, ACX, QFX, or other) - MX, ACX, and QFX are vulnerable by default; other platforms only if VXLAN overlay is configured
  3. 3. If the device is not MX/ACX/QFX and VXLAN is not configured, this vulnerability does not apply; otherwise, proceed with upgrade
  4. 4. Select the appropriate fixed version based on the current Junos branch: 15.1 series upgrade to 15.1R7-S9 or later, 17.3 series upgrade to 17.3R3-S11 or later, 17.4 series upgrade to 17.4R2-S13 or 17.4R3-S4 or later, 18.1 series upgrade to 18.1R3-S12 or later
  5. 5. For newer branches not listed in affected versions but still within the advisory scope (18.2, 18.3, 18.4, 19.x, 20.x), upgrade to the fixed versions mentioned in the advisory: 18.2R2-S8/18.2R3-S7, 18.3R3-S4, 18.4R3-S7, 19.3R3-S1, 19.4R3-S1, 20.1R3, 20.2R3, 20.3R1-S1
  6. 6. Schedule a maintenance window as upgrade will require a system reboot
  7. 7. Back up current configuration using 'request system configuration backup'
  8. 8. Load and install the appropriate fixed Junos OS image using 'request system software add <package>'
Caveat Standard Junos upgrade considerations apply - schedule maintenance window for reboot; verify configuration compatibility after upgrade; VXLAN overlay functionality continues to work after patch

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

Fix this in Junos Scoped from the published advisory
  • Consultation6.0 h
  • Implementation6.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,900
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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-2021-0254 in production — separate from our analysis above.

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