dbcve.org
JunosOperating system · Juniper

CVE-2021-0257

MEDIUM · 6.5 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 →
70/100
Remediation priority · Elevated
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
On Juniper Networks MX Series and EX9200 Series platforms with Trio-based MPCs (Modular Port Concentrators) where Integrated Routing and Bridging (IRB) interfaces are configured and mapped to a VPLS instance or a Bridge-Domain, certain Layer 2 network events at Customer Edge (CE) devices may cause memory leaks in the MPC of Provider Edge (PE) devices which can cause an out of memory condition and MPC restart. When this issue occurs, there will be temporary traffic interruption until the MPC is restored. An administrator can use the following CLI command to monitor the status of memory usage level of the MPC: user@device> show system resource-monitor fpc FPC Resource Usage Summary Free Heap Mem Watermark : 20 % Free NH Mem Watermark : 20 % Free Filter Mem Watermark : 20 % * - Watermark reached Slot # % Heap Free RTT Average RTT 1 87 PFE # % ENCAP mem Free % NH mem Free % FW mem Free 0 NA 88 99 1 NA 89 99 When the issue is occurring, the value of “% NH mem Free” will go down until the MPC restarts. This issue affects MX Series and EX9200 Series with Trio-based PFEs (Packet Forwarding Engines), including MX-MPC1-3D, MX-MPC1E-3D, MX-MPC2-3D, MX-MPC2E-3D, MPC-3D-16XGE, and CHAS-MXxx Series MPCs. No other products or platforms are affected by this issue. This issue affects Juniper Networks Junos OS on MX Series, EX9200 Series: 17.3 versions prior to 17.3R3-S10; 17.4 versions prior to 17.4R3-S3; 18.2 versions prior to 18.2R3-S7; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R3-S6; 19.2 versions prior to 19.2R3-S2; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R2-S2, 19.4R3; 20.2 versions prior to 20.2R1-S3, 20.2R2; 20.3 versions prior to 20.3R1-S1,, 20.3R2. This issue does not affect Juniper Networks Junos OS: 17.3 versions prior to 17.3R3-S8; 17.4 versions prior to 17.4R3-S2; 18.1; 18.2 versions prior to 18.2R3-S4; 18.3 versions prior to 18.3R3-S2; 18.4 versions prior to 18.4R3-S1; 19.1; 19.2 versions prior to 19.2R2; 19.3 versions prior to 19.3R3; 19.4 versions prior to 19.4R2.

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

Memory leak in Juniper MX Series and EX9200 Series with Trio-based MPCs when IRB interfaces are mapped to VPLS instances or Bridge-Domains. Certain Layer 2 network events at CE devices cause memory to leak in the MPC of PE devices, leading to out-of-memory condition and MPC restart. Monitor via 'show system resource-monitor fpc' - '% NH mem Free' will decrease until restart.

MitigationUpgrade Junos OS to a patched version (17.3R3-S10, 17.4R3-S3, 18.2R3-S7, 18.3R3-S4, 18.4R3-S6, 19.2R3-S2, 19.3R3-S1, 19.4R2-S2, 20.2R1-S3, 20.2R2, 20.3R1-S1, or 20.3R2 depending on current branch). Monitor memory levels using provided CLI command.

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= 18.2= 18.3= 18.4= 19.2= 19.3= 19.4= 20.2= 20.3

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
Adjacent
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:A/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 'show version' to confirm the device is a Juniper MX Series router or EX9200 switch
    Affected if Device is NOT an MX Series or EX9200 (different platforms are not affected)
  2. Confirm Trio-based MPC
    Run 'show chassis hardware detail' to list MPC (Modular Port Concentrator) line cards. Trio is Juniper's chipset for MPCs. Check hardware inventory for MPC types
    Affected if Device does NOT have Trio-based MPCs installed (the flaw only affects Trio-based MPCs)
  3. Verify affected Junos version
    Run 'show version' to obtain the installed Junos OS version. Compare against affected versions: 17.3, 17.4, 18.2, 18.3, 18.4, 19.2, 19.3, 19.4, 20.2, or 20.3
    Affected if Installed version does NOT match one of these exact versions (the flaw only exists in these specific versions)
  4. Confirm IRB interface configuration
    Run 'show configuration interfaces irb' and 'show configuration routing-instances' or 'show configuration bridge-domains' to check if IRB interfaces are mapped to VPLS instances or Bridge-Domains
    Affected if No IRB interfaces are configured with VPLS or Bridge-Domain associations (the leak only triggers when IRB is mapped to these)
  5. Monitor FPC memory for depletion
    Run 'show system resource-monitor fpc' repeatedly over time. Observe the '% NH mem Free' value - a decreasing trend indicates the memory leak
    Affected if '% NH mem Free' percentage is steadily decreasing toward zero (confirms active exploitation of this vulnerability)

You are affected if you have an MX Series or EX9200 with Trio-based MPCs, run one of the specific affected Junos versions, use IRB interfaces mapped to VPLS or Bridge-Domains, and observe declining NH memory on the FPC.

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 a patched version (17.3R3-S10, 17.4R3-S3, 18.2R3-S7, 18.3R3-S4, 18.4R3-S6, 19.2R3-S2, 19.3R3-S1, 19.4R2-S2, 20.2R1-S3, 20.2R2, 20.3R1-S1, or 20.3R2 depending on current branch). Monitor memory levels using provided CLI command.

Fix this in Junos Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing4.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,080
Get help mitigating

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,328.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2021-0257 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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-2021-0257 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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