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JunosOperating system · Juniper

CVE-2021-0261

HIGH · 7.5 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 →
84/100
Remediation priority · High
Remotely reachable 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
A vulnerability in the HTTP/HTTPS service used by J-Web, Web Authentication, Dynamic-VPN (DVPN), Firewall Authentication Pass-Through with Web-Redirect, and Captive Portal allows an unauthenticated attacker to cause an extended Denial of Service (DoS) for these services by sending a high number of specific requests. This issue affects: Juniper Networks Junos OS 12.3 versions prior to 12.3R12-S17 on EX Series; 12.3X48 versions prior to 12.3X48-D105 on SRX Series; 15.1 versions prior to 15.1R7-S8; 15.1X49 versions prior to 15.1X49-D230 on SRX Series; 16.1 versions prior to 16.1R7-S8; 17.4 versions prior to 17.4R2-S12, 17.4R3-S3; 18.1 versions prior to 18.1R3-S11; 18.2 versions prior to 18.2R3-S6; 18.3 versions prior to 18.3R2-S4, 18.3R3-S3; 18.4 versions prior to 18.4R2-S5, 18.4R3-S4; 19.1 versions prior to 19.1R2-S2, 19.1R3-S2; 19.2 versions prior to 19.2R1-S5, 19.2R3; 19.3 versions prior to 19.3R2-S4, 19.3R3; 19.4 versions prior to 19.4R1-S3, 19.4R2-S2, 19.4R3; 20.1 versions prior to 20.1R1-S3, 20.1R2; 20.2 versions prior to 20.2R1-S1, 20.2R2.

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

A vulnerability in the HTTP/HTTPS service used by multiple Junos OS web-based features (J-Web, Web Authentication, DVPN, Firewall Authentication Pass-Through with Web-Redirect, and Captive Portal) allows an unauthenticated remote attacker to cause an extended Denial of Service by sending a high volume of specific requests, rendering these services unavailable.

MitigationUpgrade Junos OS to the fixed versions specified in the advisory (e.g., 12.3R12-S17, 15.1R7-S8, 17.4R3-S3, 18.4R3-S4, 19.4R3, 20.2R2) for the applicable platform. Prioritize devices with exposed HTTP/HTTPS services to the internet or untrusted networks.

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:= 12.3= 12.3x48= 15.1x49= 15.1= 16.1= 17.3= 17.4= 18.1= 18.2= 18.3= 18.4= 19.1

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
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/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 installed Junos OS version
    Run the command 'show version' on the device CLI to obtain the exact Junos version string.
    Affected if The version matches any of these: 12.3, 12.3X48, 15.1X49, 15.1, 16.1, 17.3, 17.4, 18.1, 18.2, 18.3, 18.4, or 19.1 (or any intermediate release within these branches).
  2. Confirm HTTP service is enabled
    Run 'show configuration system services' or 'show configuration | display set | match http' to view the web management service configuration.
    Affected if HTTP or HTTPS service is explicitly configured under 'system services' (for example, 'set system services web-management http' or 'set system services web-management https').
  3. Verify J-Web or captive portal features are active
    Check the configuration for J-Web, DVPN, web-authentication, or captive-portal settings using 'show configuration' and searching for 'jweb', 'web-authentication', 'captive-portal', or 'dvpn'.
    Affected if Any of these features (J-Web, Web Authentication, DVPN, Firewall Authentication Pass-Through with Web-Redirect, or Captive Portal) are configured and enabled on the device.

A device is affected if it runs an affected Junos version (12.3 through 19.1 branches) AND has HTTP/HTTPS-based web services enabled, exposing J-Web or other vulnerable web authentication features to network access.

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

Upgrade Junos OS to the fixed versions specified in the advisory (e.g., 12.3R12-S17, 15.1R7-S8, 17.4R3-S3, 18.4R3-S4, 19.4R3, 20.2R2) for the applicable platform. Prioritize devices with exposed HTTP/HTTPS services to the internet or untrusted networks.

Fix this in Junos Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA2.0 h
20.0 hours of engineering $3,500
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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-2021-0261 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