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JunosOperating system · Juniper

CVE-2021-0205

MEDIUM · 5.8 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 →
67/100
Remediation priority · Elevated
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
When the "Intrusion Detection Service" (IDS) feature is configured on Juniper Networks MX series with a dynamic firewall filter using IPv6 source or destination prefix, it may incorrectly match the prefix as /32, causing the filter to block unexpected traffic. This issue affects only IPv6 prefixes when used as source and destination. This issue affects MX Series devices using MS-MPC, MS-MIC or MS-SPC3 service cards with IDS service configured. This issue affects: Juniper Networks Junos OS 17.3 versions prior to 17.3R3-S10 on MX Series; 17.4 versions prior to 17.4R3-S3 on MX Series; 18.1 versions prior to 18.1R3-S11 on MX Series; 18.2 versions prior to 18.2R3-S6 on MX Series; 18.3 versions prior to 18.3R3-S4 on MX Series; 18.4 versions prior to 18.4R3-S6 on MX Series; 19.1 versions prior to 19.1R2-S2, 19.1R3-S3 on MX Series; 19.2 versions prior to 19.2R3-S1 on MX Series; 19.3 versions prior to 19.3R2-S5, 19.3R3-S1 on MX Series; 19.4 versions prior to 19.4R3 on MX Series; 20.1 versions prior to 20.1R2 on MX Series; 20.2 versions prior to 20.2R2 on MX Series;

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

On Juniper MX series devices, the Intrusion Detection Service (IDS) incorrectly handles IPv6 prefix matching in dynamic firewall filters. When IPv6 source or destination prefixes are used, the system misinterprets them as /32 (single host) instead of their actual prefix length, causing the filter to block legitimate traffic that falls outside the intended scope. This only affects MX Series routers with MS-MPC, MS-MIC, or MS-SPC3 service cards running affected Junos OS versions.

MitigationUpgrade Junos OS to a version that addresses this vulnerability (17.3R3-S10, 17.4R3-S3, 18.1R3-S11, 18.2R3-S6, 18.3R3-S4, 18.4R3-S6, 19.1R3-S3, 19.2R3-S1, 19.3R3-S1, 19.4R3, 20.1R2, or 20.2R2 or later). As a workaround, avoid using IPv6 prefixes in IDS dynamic filters until the patch is applied.

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= 20.1= 20.2

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
Changed
Confidentiality
None
Integrity
None
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L

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. Confirm device is an MX Series router
    Run 'show chassis hardware' or 'show system information' to identify the router model
    Affected if Device is not an MX Series router (not affected)
  2. Verify presence of service cards
    Run 'show chassis hardware' and look for MS-MPC, MS-MIC, or MS-SPC3 cards in the hardware inventory
    Affected if No MS-MPC, MS-MIC, or MS-SPC3 service cards present (not affected)
  3. Check Junos OS version
    Run 'show version' and compare the installed version against the affected list: 17.3, 17.4, 18.1, 18.2, 18.3, 18.4, 19.1, 19.2, 19.3, 19.4, 20.1, 20.2
    Affected if Version is one of the listed affected versions
  4. Verify IDS is configured
    Run 'show configuration | display set | match ids' or 'show configuration services ids' to check if Intrusion Detection Service is enabled
    Affected if IDS service is not configured (not affected)
  5. Check for IPv6 prefixes in dynamic firewall filters
    Run 'show configuration firewall | display set' and look for dynamic filter terms containing 'ipv6-prefix' or IPv6 prefix addresses in source-prefix-list or destination-prefix-list under IDS-related filter configurations
    Affected if IPv6 source or destination prefixes are used in IDS dynamic firewall filters

You are affected if you have an MX Series router with an MS-MPC/MS-MIC/MS-SPC3 card, running an affected Junos version, with IDS enabled and IPv6 prefixes configured in dynamic firewall filters.

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 version that addresses this vulnerability (17.3R3-S10, 17.4R3-S3, 18.1R3-S11, 18.2R3-S6, 18.3R3-S4, 18.4R3-S6, 19.1R3-S3, 19.2R3-S1, 19.3R3-S1, 19.4R3, 20.1R2, or 20.2R2 or later). As a workaround, avoid using IPv6 prefixes in IDS dynamic filters until the patch is applied.

Fix this in Junos Scoped from the published advisory
  • Consultation4.0 h
  • Implementation2.0 h
  • Testing8.0 h
  • Review / QA4.0 h
18.0 hours of engineering $3,080
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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-2021-0205 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