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Junos Os EvolvedOperating system · Juniper

CVE-2021-31356

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 command injection vulnerability in command processing on Juniper Networks Junos OS Evolved allows an attacker with authenticated CLI access to be able to bypass configured access protections to execute arbitrary shell commands within the context of the current user. The vulnerability allows an attacker to bypass command authorization restrictions assigned to their specific user account and execute commands that are available to the privilege level for which the user is assigned. For example, a user that is in the super-user login class, but restricted to executing specific CLI commands could exploit the vulnerability to execute any other command available to an unrestricted admin user. This vulnerability does not increase the privilege level of the user, but rather bypasses any CLI command restrictions by allowing full access to the shell. This issue affects Juniper Networks Junos OS Evolved: All versions prior to 20.4R3-S1-EVO; All versions of 21.1-EVO and 21.2-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

A command injection vulnerability in Junos OS Evolved's command processing allows authenticated CLI users to bypass configured command authorization restrictions and execute arbitrary shell commands within their current user context. While the vulnerability does not escalate privileges, it allows users with restricted CLI command access to gain full shell access, effectively circumventing assigned command restrictions.

MitigationUpgrade Junos OS Evolved to version 20.4R3-S1-EVO or later, or a fixed version beyond 21.2-EVO. In the meantime, limit CLI access to trusted personnel and monitor for unusual shell access patterns.

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
Junos Os EvolvedOperating system
Affected:<= 20.3= 20.4= 21.1= 21.2

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
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. Identify Junos OS Evolved version
    Execute 'show version' or 'show version invoke-on all-routing-engines' to display the installed Junos OS Evolved version
    Affected if Version matches 20.3 or earlier, 20.4, 21.1, or 21.2 (fixed versions are > 21.2 or >= 20.4R3-S1-EVO)
  2. Confirm CLI command authorization is in use
    Review the configuration with 'show configuration system login' and look for authorization controls defined under user or class settings
    Affected if Command authorization restrictions are configured for user accounts

Environment is affected if running a vulnerable Junos OS Evolved version (20.3 and earlier, 20.4, 21.1, or 21.2) and CLI command authorization restrictions are in place that users could bypass to access the shell.

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 Evolved to version 20.4R3-S1-EVO or later, or a fixed version beyond 21.2-EVO. In the meantime, limit CLI access to trusted personnel and monitor for unusual shell access patterns.

Recommended fix High confidence

20.4R3-S1-EVO or later for 20.4 branch; for 21.1-EVO and 21.2-EVO users, upgrade to the next stable release beyond 21.2 that includes the fix

  1. 1. Identify the current Junos OS Evolved version by running 'show version' in CLI
  2. 2. Determine the target fixed release: 20.4R3-S1-EVO or later for the 20.4 branch, or the next stable release beyond 21.2-EVO
  3. 3. Schedule a maintenance window for the upgrade
  4. 4. Back up the current configuration using 'request system configuration backup' or 'save' command
  5. 5. Download the appropriate Junos OS Evolved software image from the Juniper Networks support portal
  6. 6. Transfer the software image to the device and verify integrity using checksums provided by Juniper
  7. 7. Perform the upgrade using 'request system software add <package-name>' command
  8. 8. Reboot the device using 'request system reboot'
Caveat Review Juniper release notes for any compatibility warnings; some older hardware platforms may have limited upgrade paths; ensure configuration compatibility before upgrading

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

Fix this in Junos Os Evolved Scoped from the published advisory
  • Consultation4.0 h
  • Implementation3.0 h
  • Testing4.0 h
  • Review / QA2.0 h
13.0 hours of engineering $2,300
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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-2021-31356 in production — separate from our analysis above.

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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
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