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JunosOperating system · Juniper

CVE-2021-31372

HIGH · 8.8 CVSS v3.1 Published 2021-10-19
Fix available
A fix is available. Upgrade to after 18.2 or later.
See remediation →
94/100
Remediation priority · Urgent
Remotely reachable 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
An Improper Input Validation vulnerability in J-Web of Juniper Networks Junos OS allows a locally authenticated J-Web attacker to escalate their privileges to root over the target device. This issue affects: Juniper Networks Junos OS All 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 19.2R3-S3; 19.3 versions prior to 19.3R3-S3; 19.4 versions prior to 19.4R3-S5; 20.1 versions prior to 20.1R3-S1; 20.2 versions prior to 20.2R3-S2; 20.3 versions prior to 20.3R3-S1; 20.4 versions prior to 20.4R3; 21.1 versions prior to 21.1R2, 21.1R3; 21.2 versions prior to 21.2R1-S1, 21.2R2;

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

CVE-2021-31372 is an improper input validation vulnerability in J-Web (the web-based management interface) of Juniper Networks Junos OS. A locally authenticated attacker with valid J-Web credentials can exploit this vulnerability to escalate their privileges from their current user level to root (full administrative) privileges on the target device. This is a local privilege escalation flaw, not a remote code execution or authentication bypass issue.

MitigationUpgrade Junos OS to one of the fixed versions listed in the Juniper advisory (18.3R3-S5 and later, 19.4R3-S5 and later, 20.1R3-S1 and later, 20.2R3-S2 and later, 20.3R3-S1 and later, 20.4R3 and later, 21.1R2/21.1R3 and later, or 21.2R1-S1/21.2R2 and later). If upgrading is not immediately feasible, restrict J-Web access to trusted management networks only or disable J-Web and use CLI-based management.

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:<= 18.2= 18.3= 18.4= 19.1= 19.2= 19.3= 19.4= 20.1= 20.2= 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
Network
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/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 J-Web is enabled
    Check the system configuration for J-Web (httpd) service activation. This is typically done via 'show system services' or reviewing the configuration file for web-management or J-Web related settings.
    Affected if J-Web is actively enabled and accessible on the device. The vulnerability does not apply if J-Web is disabled.
  2. Identify the Junos OS version
    Run 'show version' or 'show system information' to obtain the installed Junos OS version number.
    Affected if The installed version matches one of the affected ranges: any version <= 18.2, or exactly 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, 20.1, 20.2, 20.3, 20.4, or 21.1.
  3. Verify local user accounts exist
    Review local user accounts configured on the device using 'show system login' or by checking the [system login] configuration hierarchy.
    Affected if There is at least one local user account with J-Web access credentials. The attacker needs valid J-Web credentials to exploit this privilege escalation flaw.
  4. Check for recent authentication events
    Review system logs or authentication logs for J-Web login activity using 'show log messages' or equivalent log inspection.
    Affected if Successful J-Web authentications have occurred, indicating the attack surface is active.

You are affected if J-Web is enabled, your Junos OS version is in the affected list, and you have local user accounts with J-Web access credentials.

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 18.2
Interim mitigation

Upgrade Junos OS to one of the fixed versions listed in the Juniper advisory (18.3R3-S5 and later, 19.4R3-S5 and later, 20.1R3-S1 and later, 20.2R3-S2 and later, 20.3R3-S1 and later, 20.4R3 and later, 21.1R2/21.1R3 and later, or 21.2R1-S1/21.2R2 and later). If upgrading is not immediately feasible, restrict J-Web access to trusted management networks only or disable J-Web and use CLI-based management.

Recommended fix High confidence

18.3R3-S5 or later (for 18.x branch); 19.1R3-S6 or later (for 19.1); or migrate to a currently supported release like 21.2R2+

  1. 1. Identify the current Junos OS version running on the device using 'show version'
  2. 2. Determine which version branch your current release belongs to (18.2 or earlier, 18.3, 18.4, or 19.1)
  3. 3. Download the appropriate fixed release for your version branch from the Juniper Networks support portal
  4. 4. For Junos 18.2 or earlier: upgrade to 18.3R3-S5 or later
  5. 5. For Junos 18.3: upgrade to 18.3R3-S5 or later
  6. 6. For Junos 18.4: upgrade to 18.4R3-S9 or later
  7. 7. For Junos 19.1: upgrade to 19.1R3-S6 or later
  8. 8. Transfer the upgrade package to the device and verify integrity using MD5/SHA checksums provided by Juniper
Caveat Review Juniper release notes for potential compatibility issues; some older hardware platforms may have limited support for newer Junos releases; ensure configuration compatibility before upgrading

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

Fix this in Junos Scoped from the published advisory
  • Consultation6.0 h
  • Implementation4.0 h
  • Testing6.0 h
  • Review / QA3.0 h
19.0 hours of engineering $3,360
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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-2021-31372 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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