CVE-2021-0269
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 · uneditedThe improper handling of client-side parameters in J-Web of Juniper Networks Junos OS allows an attacker to perform a number of different malicious actions against a target device when a user is authenticated to J-Web. An attacker may be able to supersede existing parameters, including hardcoded parameters within the HTTP/S session, access and exploit variables, bypass web application firewall rules or input validation mechanisms, and otherwise alter and modify J-Web's normal behavior. An attacker may be able to transition victims to malicious web services, or exfiltrate sensitive information from otherwise secure web forms. This issue affects: Juniper Networks Junos OS: All versions prior to 17.4R3-S3; 18.1 versions prior to 18.1R3-S12; 18.2 versions prior to 18.2R3-S6; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R3-S6; 19.1 versions prior to 19.1R3-S4; 19.2 versions prior to 19.2R3-S1; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R2-S2, 19.4R3; 20.1 versions prior to 20.1R2; 20.2 versions prior to 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 confidenceJ-Web in Juniper Networks Junos OS improperly handles client-side parameters, allowing an authenticated attacker to supersede existing parameters (including hardcoded HTTP/S session parameters), access/exploit variables, bypass web application firewall rules and input validation, modify application behavior, redirect victims to malicious web services, and exfiltrate sensitive information from web forms.
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= 17.4= 18.1= 18.2= 18.3= 18.4= 19.1= 19.2= 19.3= 19.4= 20.1= 20.2CVSS 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
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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 checksWork through these to decide whether this CVE applies to you.
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Verify J-Web is enabledRun 'show system services web-management' or check configuration for 'system services web-management'Affected if J-Web is enabled and accessible - the vulnerability only applies when J-Web is active
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Check installed Junos OS versionRun 'show version' or 'show system information' to obtain the exact Junos version installedAffected if The installed version is 17.4, 18.1, 18.2, 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, 20.1, or 20.2 (any variant within these release trains)
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Confirm J-Web interface accessibilityVerify the J-Web HTTP/HTTPS service is listening and reachable by checking 'show system services web-management http' or 'show system services web-management https'Affected if J-Web is reachable over the network to untrusted users
User is affected if J-Web is enabled AND the Junos version falls within 17.4 through 20.2 AND the J-Web interface is network-accessible to the attacker.
Generated from the published advisory. Verify against your own configuration.
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 · scopedUpgrade to a Junos OS version that includes the fix (17.4R3-S3, 18.1R3-S12, 18.2R3-S6, 18.3R3-S4, 18.4R3-S6, 19.1R3-S4, 19.2R3-S1, 19.3R3-S1, 19.4R2-S2, 19.4R3, 20.1R2, 20.2R2 or later). If immediate patching is not possible, disable J-Web or restrict administrative access to trusted IPs only.
18.3R3-S4 or later (or the equivalent fixed release for your version branch)
- Identify the currently installed Junos OS version using 'show version' command
- Determine the appropriate upgrade path based on current version branch (17.4, 18.1, 18.2, 18.3, etc.)
- Download the appropriate fixed release from the Juniper Networks support portal: 17.4R3-S3 or later, 18.1R3-S12 or later, 18.2R3-S6 or later, 18.3R3-S4 or later, 18.4R3-S6 or later, 19.1R3-S4 or later, 19.2R3-S1 or later, 19.3R3-S1 or later, 19.4R2-S2/19.4R3 or later, 20.1R2 or later, 20.2R2 or later
- Upload the Junos upgrade package to the device using 'request system software add' command
- Reboot the device after upgrade completes using 'request system reboot' command
- Verify the new version is running using 'show version'
- If J-Web is not required, consider disabling it via 'set system services web-management disable' as an additional mitigation
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation3.0 h
- Testing4.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-0269 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-0269 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
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