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JunosOperating system · Juniper

CVE-2021-0207

HIGH · 7.5 CVSS v3.1 Published 2021-01-15
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
An improper interpretation conflict of certain data between certain software components within the Juniper Networks Junos OS devices does not allow certain traffic to pass through the device upon receipt from an ingress interface filtering certain specific types of traffic which is then being redirected to an egress interface on a different VLAN. This causes a Denial of Service (DoS) to those clients sending these particular types of traffic. Such traffic being sent by a client may appear genuine, but is non-standard in nature and should be considered as potentially malicious, and can be targeted to the device, or destined through it for the issue to occur. This issues affects IPv4 and IPv6 traffic. An indicator of compromise may be found by checking log files. You may find that traffic on the input interface has 100% of traffic flowing into the device, yet the egress interface shows 0 pps leaving the device. For example: [show interfaces "interface" statistics detail] Output between two interfaces would reveal something similar to: Ingress, first interface: -------------------- Interface Link Input packets (pps) Output packets (pps) et-0/0/0 Up 9999999999 (9999) 1 (0) -------------------- Egress, second interface: -------------------- Interface Link Input packets (pps) Output packets (pps) et-0/0/1 Up 0 (0) 9999999999 (0) -------------------- Dropped packets will not show up in DDoS monitoring/protection counters as issue is not caused by anti-DDoS protection mechanisms. This issue affects: Juniper Networks Junos OS: 17.3 versions prior to 17.3R3-S7 on NFX250, QFX5K Series, EX4600; 17.4 versions prior to 17.4R2-S11, 17.4R3-S3 on NFX250, QFX5K Series, EX4600; 18.1 versions prior to 18.1R3-S9 on NFX250, QFX5K Series, EX2300 Series, EX3400 Series, EX4600; 18.2 versions prior to 18.2R3-S3 on NFX250, QFX5K Series, EX2300 Series, EX3400 Series, EX4300 Multigigabit, EX4600; 18.3 versions prior to 18.3R3-S1 on NFX250, QFX5K Series, EX2300 Series, EX3400 Series, EX4300 Multigigabit, EX4600 Series; 18.4 versions prior to 18.4R1-S5, 18.4R2-S3, 18.4R3 on NFX250, QFX5K Series, EX2300 Series, EX3400 Series, EX4300 Multigigabit, EX4600 Series; 19.1 versions prior to 19.1R1-S5, 19.1R2-S1, 19.1R3 on NFX250, QFX5K Series, EX2300 Series, EX3400 Series, EX4300 Multigigabit, EX4600 Series; 19.2 versions prior to 19.2R1-S5, 19.2R2 on NFX250, QFX5K Series, EX2300 Series, EX3400 Series, EX4300 Multigigabit, EX4600 Series; 19.3 versions prior to 19.3R2-S3, 19.3R3 on NFX250, QFX5K Series, EX2300 Series, EX3400 Series, EX4300 Multigigabit, EX4600 Series; 19.4 versions prior to 19.4R1-S2, 19.4R2 on NFX250, NFX350, QFX5K Series, EX2300 Series, EX3400 Series, EX4300 Multigigabit, EX4600 Series. This issue does not affect Junos OS releases prior to 17.2R2.

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 data interpretation conflict in Juniper Junos OS causes certain IPv4/IPv6 traffic to be dropped when forwarded from a filtered ingress interface to an egress interface on a different VLAN. The traffic enters the device but fails to exit, creating a DoS condition where ingress shows 100% traffic arrival but egress shows 0 pps output. The issue stems from improper handling of non-standard but seemingly legitimate traffic patterns.

MitigationUpgrade Junos OS to the fixed versions (17.3R3-S7, 17.4R2-S11/17.4R3-S3, 18.1R3-S9, 18.2R3-S3, 18.3R3-S1, 18.4R1-S5/18.4R2-S3/18.4R3, 19.1R1-S5/19.1R2-S1/19.1R3, 19.2R1-S5/19.2R2, 19.3R2-S3/19.3R3, or 19.4R1-S2/19.4R2) for the affected platforms (NFX250, QFX5K, EX2300, EX3400, EX4300, EX4600 Series).

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:= 17.3= 17.4= 18.1= 18.2= 18.3= 18.4= 19.1= 19.2= 19.3= 19.4

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. Check Junos OS version
    Run `show version` or `show version detail` to retrieve the installed Junos version
    Affected if Version matches 17.3, 17.4, 18.1, 18.2, 18.3, 18.4, 19.1, 19.2, 19.3, or 19.4 (any variant within these releases)
  2. Identify hardware platform
    Run `show chassis hardware` or `show system information` to identify the device model
    Affected if Platform is NFX250, QFX5K, EX2300, EX3400, EX4300, or EX4600 series
  3. Verify firewall filter is applied
    Run `show firewall` and review filter configurations applied to ingress interfaces using `show configuration firewall`
    Affected if A firewall filter is active on the ingress interface processing the traffic
  4. Confirm inter-VLAN forwarding scenario
    Run `show vlans` and `show interfaces` to verify traffic is being forwarded from one VLAN to a different VLAN across distinct interfaces
    Affected if Traffic flows from a filtered ingress interface to an egress interface on a different VLAN
  5. Monitor for traffic drop condition
    Run `show interface statistics` on ingress and egress interfaces simultaneously; compare ingress packet-per-second counters to egress output
    Affected if Ingress shows active traffic arrival while egress shows zero or significantly reduced output packets

Device is affected if running a vulnerable Junos version on an affected platform with active firewall filters and inter-VLAN traffic forwarding, resulting in ingress traffic not appearing at egress.

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 (17.3R3-S7, 17.4R2-S11/17.4R3-S3, 18.1R3-S9, 18.2R3-S3, 18.3R3-S1, 18.4R1-S5/18.4R2-S3/18.4R3, 19.1R1-S5/19.1R2-S1/19.1R3, 19.2R1-S5/19.2R2, 19.3R2-S3/19.3R3, or 19.4R1-S2/19.4R2) for the affected platforms (NFX250, QFX5K, EX2300, EX3400, EX4300, EX4600 Series).

Fix this in Junos Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing3.0 h
  • Review / QA2.0 h
14.0 hours of engineering $2,490
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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-2021-0207 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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