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JunosOperating system · Juniper

CVE-2021-0240

MEDIUM · 6.5 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 →
70/100
Remediation priority · Elevated
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
On Juniper Networks Junos OS platforms configured as DHCPv6 local server or DHCPv6 Relay Agent, the Juniper Networks Dynamic Host Configuration Protocol Daemon (JDHCPD) process might crash if a malformed DHCPv6 packet is received, resulting in a restart of the daemon. The daemon automatically restarts without intervention, but continued receipt and processing of this packet will create a sustained Denial of Service (DoS) condition. This issue only affects DHCPv6. DHCPv4 is not affected by this issue. This issue affects Juniper Networks Junos OS: 17.3 versions prior to 17.3R3-S12; 17.4 versions prior to 17.4R3-S5; 18.1 versions prior to 18.1R3-S13; 18.2 versions prior to 18.2R3-S8; 18.3 versions prior to 18.3R3-S5; 18.4 versions prior to 18.4R1-S8, 18.4R3-S7; 19.1 versions prior to 19.1R3-S5; 19.2 versions prior to 19.2R3-S2; 19.3 versions prior to 19.3R3-S2; 19.4 versions prior to 19.4R3-S2; 20.1 versions prior to 20.1R3; 20.2 versions prior to 20.2R2-S3, 20.2R3; 20.3 versions prior to 20.3R2; 20.4 versions prior to 20.4R2.

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

On Juniper Networks Junos OS devices configured as DHCPv6 local server or DHCPv6 Relay Agent, the JDHCPD daemon crashes when processing malformed DHCPv6 packets, causing automatic daemon restart. Sustained receipt of malformed packets creates a continuous DoS condition. DHCPv4 is unaffected.

MitigationApply the vendor-provided Junos OS patches for affected versions (17.3R3-S12, 17.4R3-S5, 18.1R3-S13, 18.2R3-S8, 18.3R3-S5, 18.4R1-S8/18.4R3-S7, 19.1R3-S5, 19.2R3-S2, 19.3R3-S2, 19.4R3-S2, 20.1R3, 20.2R2-S3/20.2R3, 20.3R2, 20.4R2 and later). Consider network-level filtering of suspicious DHCPv6 traffic as interim mitigation.

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:= 17.3= 17.4= 18.1= 18.2= 18.3= 18.4= 19.1= 19.2= 19.3= 19.4= 20.1= 20.2

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
Adjacent
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:A/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 Junos OS version
    Run 'show version' command on the Juniper device to obtain the installed Junos OS version
    Affected if The installed version matches 17.3, 17.4, 18.1, 18.2, 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, 20.1, or 20.2
  2. Verify DHCPv6 local server configuration
    Run 'show configuration system services dhcpv6' or 'show configuration protocols dhcpv6' to check if the device is configured as a DHCPv6 local server
    Affected if DHCPv6 local server configuration is present on the device
  3. Verify DHCPv6 Relay Agent configuration
    Run 'show configuration protocols dhcpv6-relay' to check if the device is configured as a DHCPv6 Relay Agent
    Affected if DHCPv6 Relay Agent configuration is present on the device
  4. Confirm JDHCPD daemon status
    Run 'show system processes extensive | match jdhcpd' or check daemon logs for recent crashes
    Affected if JDHCPD daemon has recently restarted or crashed, and DHCPv6 services are enabled

A device is affected if it runs a vulnerable Junos OS version (17.3-20.2) AND is configured as either a DHCPv6 local server or DHCPv6 Relay Agent, making it susceptible to the JDHCPD daemon crash when processing malformed DHCPv6 packets.

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.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the vendor-provided Junos OS patches for affected versions (17.3R3-S12, 17.4R3-S5, 18.1R3-S13, 18.2R3-S8, 18.3R3-S5, 18.4R1-S8/18.4R3-S7, 19.1R3-S5, 19.2R3-S2, 19.3R3-S2, 19.4R3-S2, 20.1R3, 20.2R2-S3/20.2R3, 20.3R2, 20.4R2 and later). Consider network-level filtering of suspicious DHCPv6 traffic as interim mitigation.

Fix this in Junos Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing3.0 h
  • Review / QA2.0 h
14.0 hours of engineering $2,490
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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-2021-0240 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
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