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JunosOperating system · Juniper

CVE-2021-0268

CRITICAL · 9.3 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 →
100/100
Remediation priority · Urgent
Remotely reachable No privileges

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 Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting') weakness in J-web of Juniper Networks Junos OS leads to buffer overflows, segment faults, or other impacts, which allows an attacker to modify the integrity of the device and exfiltration information from the device without authentication. The weakness can be exploited to facilitate cross-site scripting (XSS), cookie manipulation (modifying session cookies, stealing cookies) and more. This weakness can also be exploited by directing a user to a seemingly legitimate link from the affected site. The attacker requires no special access or permissions to the device to carry out such attacks. This issue affects: Juniper Networks Junos OS: 18.1 versions prior to 18.1R3-S11; 18.2 versions prior to 18.2R3-S5; 18.3 versions prior to 18.3R2-S4, 18.3R3-S3; 18.4 versions prior to 18.4R2-S5, 18.4R3-S3; 19.1 versions prior to 19.1R2-S2, 19.1R3-S2; 19.2 versions prior to 19.2R1-S5, 19.2R2; 19.3 versions prior to 19.3R3; 19.4 versions prior to 19.4R1-S3, 19.4R2, 19.4R3; 20.1 versions prior to 20.1R1-S2, 20.1R2. This issue does not affect Juniper Networks Junos OS versions prior to 18.1R1.

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

HTTP Response Splitting (CRLF injection) vulnerability in J-web component of Juniper Junos OS allows unauthenticated attackers to inject carriage return/line feed sequences into HTTP headers, enabling XSS attacks, cookie manipulation, and potentially causing buffer overflows or segment faults leading to device compromise.

MitigationApply Juniper-provided patches to all affected Junos OS versions (18.1R3-S11+, 18.2R3-S5+, 18.3R2-S4+/18.3R3-S3+, 18.4R2-S5+/18.4R3-S3+, 19.1R2-S2+/19.1R3-S2+, 19.2R1-S5+/19.2R2+, 19.3R3+, 19.4R1-S3+/19.4R2+/19.4R3+, 20.1R1-S2+/20.1R2+) or upgrade to a fixed release; disable J-web if not needed.

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:= 18.1= 18.2= 18.3= 18.4= 19.1= 19.2= 19.3= 19.4= 20.1

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
Required
Scope
Changed
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N

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 the Junos OS version
    Run the command 'show version' in the device CLI to obtain the installed Junos OS release version
    Affected if The version matches 18.1, 18.2, 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, or 20.1 (or a specific sub-version like 18.1R3-S10 or lower)
  2. Confirm J-web is enabled
    Run 'show configuration system services web-management' to check if the web-based management interface (J-web) is configured
    Affected if HTTP or HTTPS web-management is explicitly configured and enabled
  3. Check J-web accessibility
    Review the web-management configuration for interface bindings (such as 'http interface' or 'https interface' statements) to determine which network interfaces J-web listens on
    Affected if J-web is bound to any interface other than the dedicated management interface (e.g., fxp0) or is accessible from untrusted networks
  4. Verify HTTP service exposure
    Confirm that the device management IP or any interface IP is reachable from untrusted networks and J-web responds on HTTP/HTTPS ports
    Affected if The J-web service is exposed to network attackers rather than being restricted to trusted management networks

A device is affected if it runs a vulnerable Junos OS version (18.1-20.1 range) AND has J-web web-management enabled and accessible from a network where untrusted attackers can send HTTP requests.

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
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply Juniper-provided patches to all affected Junos OS versions (18.1R3-S11+, 18.2R3-S5+, 18.3R2-S4+/18.3R3-S3+, 18.4R2-S5+/18.4R3-S3+, 19.1R2-S2+/19.1R3-S2+, 19.2R1-S5+/19.2R2+, 19.3R3+, 19.4R1-S3+/19.4R2+/19.4R3+, 20.1R1-S2+/20.1R2+) or upgrade to a fixed release; disable J-web if not needed.

Recommended fix High confidence

Upgrade to 18.1R3-S11 (or later), 18.2R3-S5 (or later), 18.3R3-S3 (or later), or 18.4R3-S3 (or later) depending on your current branch

  1. Identify the current Junos OS version on the device using 'show version' command
  2. Determine which version branch (18.1, 18.2, 18.3, or 18.4) the device is currently running
  3. Download the appropriate fixed Junos OS software package from Juniper Networks (18.1R3-S11 for 18.1 branch, 18.2R3-S5 for 18.2 branch, 18.3R3-S3 for 18.3 branch, or 18.4R3-S3 for 18.4 branch)
  4. Upload the software package to the device using 'file copy' or similar method
  5. Schedule a maintenance window as upgrades require device reboot
  6. Perform the upgrade using 'request system software add <package-name>' command
  7. Reboot the device using 'request system reboot' command
  8. After reboot, verify the new version is installed using 'show version'
Caveat Standard Junos upgrade may cause brief service interruption; ensure compatibility with other network devices and review release notes for any feature changes

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

Fix this in Junos Scoped from the published advisory
  • Consultation8.0 h
  • Implementation16.0 h
  • Testing12.0 h
  • Review / QA8.0 h
44.0 hours of engineering $7,720
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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-2021-0268 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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