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JunosOperating system · Juniper

CVE-2021-31359

HIGH · 7.8 CVSS v3.1 Published 2021-10-19
Fix available
A fix is available. Upgrade to after 20.3 or later.
See remediation →
80/100
Remediation priority · High
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 local privilege escalation vulnerability in Juniper Networks Junos OS and Junos OS Evolved allows a local, low-privileged user to cause the Juniper DHCP daemon (jdhcpd) process to crash, resulting in a Denial of Service (DoS), or execute arbitrary commands as root. Continued processing of malicious input will repeatedly crash the system and sustain the Denial of Service (DoS) condition. Systems are only vulnerable if jdhcpd is running, which can be confirmed via the 'show system processes' command. For example: root@host# run show system processes extensive | match dhcp 26537 root -16 0 97568K 13692K RUN 0 0:01 3.71% jdhcpd This issue affects: Juniper Networks Junos OS: All versions, including the following supported releases: 15.1 versions prior to 15.1R7-S10; 17.4 versions prior to 17.4R3-S5; 18.3 versions prior to 18.3R3-S5; 18.4 versions prior to 18.4R3-S9; 19.1 versions prior to 19.1R3-S6; 19.2 versions prior to 19.2R1-S7, 19.2R3-S3; 19.3 versions prior to 19.3R2-S6, 19.3R3-S3; 19.4 versions prior to 19.4R3-S6; 20.1 versions prior to 20.1R2-S2, 20.1R3-S1; 20.2 versions prior to 20.2R3-S2; 20.3 versions prior to 20.3R3; 20.4 versions prior to 20.4R2-S1, 20.4R3; 21.1 versions prior to 21.1R1-S1, 21.1R2. Juniper Networks Junos OS Evolved: All versions prior to 20.4R2-S3-EVO; All versions of 21.1-EVO.

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

Local privilege escalation in Juniper Junos OS allows a low-privileged user to crash the jdhcpd daemon or execute arbitrary commands as root by sending malicious input to the DHCP service. The vulnerability is triggered when jdhcpd is running, which can be confirmed via 'show system processes extensive | match dhcp'. Repeated crashes sustain a DoS condition.

MitigationUpgrade Junos OS to the fixed versions listed in the advisory; if jdhcpd is not required in the environment, it may be disabled to reduce attack surface.

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= 17.4= 18.3= 18.4= 19.1= 19.2= 19.3= 19.4= 20.1= 20.2= 20.3= 20.4
Junos Os EvolvedOperating system
Affected:<= 20.3= 20.4= 21.1

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

CVSS:3.1/AV:L/AC:L/PR:L/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. Confirm jdhcpd daemon is running
    Execute 'show system processes extensive | match dhcp' on the Juniper device
    Affected if The jdhcpd process appears in the output, indicating the daemon is active
  2. Determine the installed Junos version
    Execute 'show version' to retrieve the exact Junos OS version installed on the device
    Affected if Version output shows any of these patterns: Junos 15.1, 17.4, 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, 20.1, 20.2, 20.3, 20.4 (standard Junos) or Junos OS Evolved versions 20.3 or earlier, 20.4, or 21.1
  3. Verify if DHCP service is enabled
    Review the device configuration for any DHCP-related service statements using 'show configuration | match dhcp' or check if DHCP relay/relay-agent features are configured under the [edit forwarding-options helpers] hierarchy
    Affected if DHCP features are configured and active, making the jdhcpd daemon operational and exploitable

The device is affected if jdhcpd is running and the installed Junos version matches one of the affected version ranges listed for this CVE.

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 a release after 20.3
Interim mitigation

Upgrade Junos OS to the fixed versions listed in the advisory; if jdhcpd is not required in the environment, it may be disabled to reduce attack surface.

Recommended fix High confidence

Upgrade to the fixed release for your version: 15.1R7-S10, 17.4R3-S5, 18.3R3-S5, 18.4R3-S9, 19.1R3-S6, 19.2R3-S3, 19.3R3-S3, 19.4R3-S6, 20.1R3-S1, 20.2R3-S2, 20.3R3, 20.4R3, or 21.1R2. For Evolved: 20.4R2-S3-EVO or later.

  1. 1. Verify jdhcpd is running by executing: run show system processes extensive | match dhcp
  2. 2. If jdhcpd is not required for operations, consider disabling it to reduce attack surface
  3. 3. Plan maintenance window for the upgrade, as it will require system reboot
  4. 4. Download the appropriate fixed Junos OS release for your hardware platform from the Juniper Networks support portal
  5. 5. Upload the Junos upgrade package to the device using scp or USB
  6. 6. Execute the upgrade using: request system software add <package-name> reboot
  7. 7. After reboot, verify the new Junos version is running: show version
  8. 8. Confirm jdhcpd process is running normally: run show system processes extensive | match dhcp
Caveat Junos OS upgrades may require additional license validation and compatibility checks for certain features; ensure configuration backup before upgrade.

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

Fix this in Junos Scoped from the published advisory
  • Consultation5.0 h
  • Implementation4.0 h
  • Testing6.0 h
  • Review / QA3.0 h
18.0 hours of engineering $3,160
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,056.

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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-2021-31359 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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