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JunosOperating system · Juniper

CVE-2021-0264

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 processing of traffic matching a firewall filter containing a syslog action in Juniper Networks Junos OS on MX Series with MPC10/MPC11 cards installed, PTX10003 and PTX10008 Series devices, will cause the line card to crash and restart, creating a Denial of Service (DoS). Continued receipt and processing of packets matching the firewall filter can create a sustained Denial of Service (DoS) condition. When traffic hits the firewall filter, configured on lo0 or any physical interface on the line card, containing a term with a syslog action (e.g. 'term <name> then syslog'), the affected line card will crash and restart, impacting traffic processing through the ports of the line card. This issue only affects MX Series routers with MPC10 or MPC11 line cards, and PTX10003 or PTX10008 Series packet transport routers. No other platforms or models of line cards are affected by this issue. Note: This issue has also been identified and described in technical service bulletin TSB17931 (login required). This issue affects: Juniper Networks Junos OS on MX Series: 19.3 versions prior to 19.3R3-S2; 19.4 versions prior to 19.4R3-S2; 20.1 versions prior to 20.1R3; 20.2 versions prior to 20.2R2-S2, 20.2R3; 20.3 versions prior to 20.3R3; 20.4 versions prior to 20.4R2. Juniper Networks Junos OS Evolved on PTX10003, PTX10008: All versions prior to 20.4R2-EVO. This issue does not affect Juniper Networks Junos OS versions prior to 19.3R1.

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 Junos OS on MX Series (with MPC10/MPC11 cards) and PTX10003/PTX10008 devices causes a line card crash and restart when traffic matches a firewall filter containing a syslog action (e.g., 'term <name> then syslog'), creating a sustained Denial of Service condition.

MitigationRemove or disable the syslog action from affected firewall filters, or upgrade to the fixed Junos OS versions (19.3R3-S2, 19.4R3-S2, 20.1R3, 20.2R2-S2/20.2R3, 20.3R3, 20.4R2 or later for MX Series; 20.4R2-EVO or later for PTX10003/PTX10008).

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:= 19.3= 19.4= 20.1= 20.2= 20.3= 20.4
Junos Os EvolvedOperating system
Affected:= 18.3= 19.1= 19.2= 19.3= 20.1= 20.2= 20.3= 20.4

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. Identify device model
    Run 'show chassis hardware' or check the device model. Confirm it is an MX Series router with MPC10 or MPC11 line cards, or a PTX10003 or PTX10008 device.
    Affected if Device is not an MX Series with MPC10/MPC11 cards, or not a PTX10003/PTX10008
  2. Check Junos OS version
    Run 'show version' or 'show version detail' to obtain the installed Junos OS version.
    Affected if The installed version is 19.3, 19.4, 20.1, 20.2, 20.3, or 20.4 for Junos; or 18.3, 19.1, 19.2, 19.3, 20.1, 20.2, 20.3, or 20.4 for Junos OS Evolved
  3. Identify firewall filters with syslog action
    Run 'show configuration firewall' and search for any filter terms containing the 'syslog' action (e.g., 'then syslog').
    Affected if Any firewall filter on the device contains a syslog action term

A user is affected if they run a vulnerable Junos version on MX Series with MPC10/MPC11 or PTX10003/PTX10008, AND have a firewall filter configured with a syslog action that could match incoming traffic.

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

Remove or disable the syslog action from affected firewall filters, or upgrade to the fixed Junos OS versions (19.3R3-S2, 19.4R3-S2, 20.1R3, 20.2R2-S2/20.2R3, 20.3R3, 20.4R2 or later for MX Series; 20.4R2-EVO or later for PTX10003/PTX10008).

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-2021-0264 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