CVE-2021-0233
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 · uneditedA vulnerability in Juniper Networks Junos OS ACX500 Series, ACX4000 Series, may allow an attacker to cause a Denial of Service (DoS) by sending a high rate of specific packets to the device, resulting in a Forwarding Engine Board (FFEB) crash. Continued receipt of these packets will sustain the Denial of Service (DoS) condition. This issue affects Juniper Networks Junos OS on ACX500 Series, ACX4000 Series: 17.4 versions prior to 17.4R3-S2.
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 confidenceA vulnerability in Juniper Networks Junos OS on ACX500 and ACX4000 Series routers allows a remote attacker to cause a Denial of Service by sending a high rate of specifically crafted packets. This triggers a Forwarding Engine Board (FFEB) crash, and continued receipt of these packets sustains the DoS condition.
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= 17.4CVSS 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 checksWork through these to decide whether this CVE applies to you.
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Identify router modelRun 'show chassis hardware' or check the physical device label to confirm it is an ACX500 or ACX4000 Series routerAffected if Device is not an ACX500 or ACX4000 series - this CVE only applies to those specific models
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Check Junos OS versionRun 'show version' or 'show version detail' to obtain the installed Junos OS versionAffected if Version is exactly 17.4 (17.4R1 or base 17.4 release) - versions before 17.4 or 17.4R3-S2 and later are not affected
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Verify network exposureReview interface configurations and firewall policies to determine if the device accepts packets from untrusted network sources on exposed interfacesAffected if Device has unfiltered inbound access from external/untrusted networks - exploitation requires receiving crafted packets
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Check for FEB crash logsRun 'show log messages' or 'show system crash' and look for Forwarding Engine Board crash entries or repeated Feb trap messagesAffected if Recent FEB crashes or Feb-related errors are present in logs, especially if correlated with high packet rate traffic events
A user is affected if they have an ACX500 or ACX4000 router running Junos OS version 17.4 (base release) that is exposed to untrusted network traffic, as exploitation requires receiving the specifically crafted packets.
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.
From vendor dataUpgrade Junos OS to version 17.4R3-S2 or later. Implement rate-limiting or packet filtering at network edges to block malicious traffic before reaching affected devices.
- Consultation4.0 h
- Implementation8.0 h
- Testing6.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,600.
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Free · runs locallyCheck whether your project pulls in CVE-2021-0233 — 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-2021-0233 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
- No weaponised exploit code, or anything meant to cause harm
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