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JunosOperating system · Juniper

CVE-2021-0238

MEDIUM · 5.5 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 →
57/100
Remediation priority · Elevated
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
When a MX Series is configured as a Broadband Network Gateway (BNG) based on Layer 2 Tunneling Protocol (L2TP), executing certain CLI command may cause the system to run out of disk space, excessive disk usage may cause other complications. An administrator can use the following CLI command to monitor the available disk space: user@device> show system storage Filesystem Size Used Avail Capacity Mounted on /dev/gpt/junos 19G 18G 147M 99% /.mount <<<<< running out of space tmpfs 21G 16K 21G 0% /.mount/tmp tmpfs 5.3G 1.7M 5.3G 0% /.mount/mfs This issue affects Juniper Networks Junos OS on MX Series: 17.3R1 and later versions prior to 17.4R3-S5, 18.1 versions prior to 18.1R3-S13, 18.2 versions prior to 18.2R3-S7; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R3-S7; 19.1 versions prior to 19.1R3-S4; 19.2 versions prior to 19.2R1-S6, 19.2R3-S2; 19.3 versions prior to 19.3R3-S2; 19.4 versions prior to 19.4R2-S4, 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 20.4R1-S1, 20.4R2; This issue does not affect Juniper Networks Junos OS versions prior to 17.3R1.

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

When an MX Series router configured as a Broadband Network Gateway (BNG) using Layer 2 Tunneling Protocol (L2TP) executes certain CLI commands, it causes excessive disk space consumption that can lead to denial of service. The system runs out of disk space, potentially affecting other system operations.

MitigationApply the appropriate Junos OS software update from Juniper Networks that addresses this vulnerability. Administrators should monitor disk space using 'show system storage' and avoid the triggering CLI commands until patched.

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= 18.1= 18.2= 18.3= 19.1= 19.2= 19.3= 19.4= 20.1= 20.2= 20.3= 20.4

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

CVSS:3.1/AV:L/AC:L/PR:L/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 model
    Run 'show chassis hardware' or 'show version' to confirm the device is an MX Series router
    Affected if Device is NOT an MX Series router - not affected by this CVE on MX platforms only
  2. Verify Junos version
    Run 'show version' and compare the installed Junos version against the affected list: 17.3, 18.1, 18.2, 18.3, 19.1, 19.2, 19.3, 19.4, 20.1, 20.2, 20.3, 20.4
    Affected if Version matches one of the affected versions listed in the CVE
  3. Confirm BNG configuration
    Run 'show configuration | match bng' or 'show configuration protocols bng' to check if Broadband Network Gateway is configured
    Affected if BNG is not configured - the vulnerability requires BNG to be enabled
  4. Verify L2TP is active
    Run 'show l2tp tunnel' or 'show configuration protocols l2tp' to check if Layer 2 Tunneling Protocol is configured and active
    Affected if L2TP is not configured - the vulnerability requires L2TP to be in use
  5. Check disk space consumption
    Run 'show system storage' to examine current disk space usage; look for unusual or excessive consumption
    Affected if Disk space is abnormally high or full, especially if other system operations are impacted

The environment is affected if: (1) it is an MX Series router, (2) running an affected Junos version (17.3 through 20.4), (3) configured as a BNG with L2TP enabled, AND (4) excessive disk consumption is observed after executing certain CLI commands.

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 appropriate Junos OS software update from Juniper Networks that addresses this vulnerability. Administrators should monitor disk space using 'show system storage' and avoid the triggering CLI commands until patched.

Fix this in Junos Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing4.0 h
  • Review / QA2.0 h
18.0 hours of engineering $3,200
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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-2021-0238 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
  • No spam, self-promotion, credentials, or personal data