CVE-2021-31371
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 · uneditedJuniper Networks Junos OS uses the 128.0.0.0/2 subnet for internal communications between the RE and PFEs. It was discovered that packets utilizing these IP addresses may egress an QFX5000 Series switch, leaking configuration information such as heartbeats, kernel versions, etc. out to the Internet, leading to an information exposure vulnerability. This issue affects Juniper Networks Junos OS on QFX5110, QFX5120, QFX5200, QFX5210 Series, and QFX5100 with QFX 5e Series image installed: All versions prior to 17.3R3-S12; 18.1 versions prior to 18.1R3-S13; 18.3 versions prior to 18.3R3-S5; 19.1 versions prior to 19.1R3-S6; 19.2 versions prior to 19.2R1-S7, 19.2R3-S3; 19.3 versions prior to 19.3R2-S6, 19.3R3-S3; 19.4 versions prior to 19.4R1-S4, 19.4R3-S5; 20.1 versions prior to 20.1R2-S2, 20.1R3-S1; 20.2 versions prior to 20.2R3-S2; 20.3 versions prior to 20.3R3-S1; 20.4 versions prior to 20.4R2-S1, 20.4R3; 21.1 versions prior to 21.1R1-S1, 21.1R2;
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 confidenceJuniper QFX5000 Series switches use the 128.0.0.0/2 subnet for internal Routing Engine to Packet Forwarding Engine communications. Due to improper routing/switching configuration, these internal packets containing sensitive information (heartbeats, kernel versions) can egress the switch onto the public internet, exposing internal network architecture and system details.
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<= 17.2= 17.3= 18.1= 18.3= 19.1= 19.2= 19.3= 19.4= 20.1= 20.2= 20.3= 20.4CVSS 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
- Low
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
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.
-
Identify device model and Junos versionRun 'show version' and 'show chassis hardware' to confirm QFX5000 series and note the Junos versionAffected if Device is a QFX5000 series running any Junos version listed in affected versions (<=17.2, 17.3, 18.1, 18.3, 19.1-19.4, 20.1-20.4)
-
Check for egress firewall filter blocking 128.0.0.0/2Run 'show configuration firewall' or 'show configuration interfaces' to look for any firewall filter or ACL with destination prefix 128.0.0.0/2 or any more-specific prefix within that rangeAffected if No firewall filter or ACL exists that explicitly blocks or denies traffic destined to 128.0.0.0/2 on outbound interfaces
-
Verify routing table for 128.0.0.0/2 routesRun 'show route table inet.0 128.0.0.0/2' to see if there are any active routes for this prefix in the global routing tableAffected if Routes for 128.0.0.0/2 or subnets within it appear in the inet.0 routing table (indicating they could be reachable externally)
-
Capture outbound traffic to detect leakageUse 'monitor traffic interface <outbound-interface>' or packet capture on external-facing ports to look for packets with destination IP in 128.0.0.0/2 rangeAffected if Packets with destination addresses in 128.0.0.0/2 are observed leaving the switch toward external networks
If the device is a Juniper QFX5000 series running an affected Junos version and lacks egress filtering to block 128.0.0.0/2 traffic, internal RE-to-PFE communications may be leaking to the public internet.
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.
dbcve · scopedUpgrade Junos OS to a patched version as listed in the Juniper advisory, or implement egress filtering/ACLs to block 128.0.0.0/2 traffic from leaving the switch toward external networks.
Upgrade to one of the fixed releases: 17.3R3-S12, 18.1R3-S13, 18.3R3-S5, 19.1R3-S6, 19.2R1-S7, 19.2R3-S3, 19.3R2-S6, 19.3R3-S3, 19.4R1-S4, 19.4R3-S5, 20.1R2-S2, 20.1R3-S1, 20.2R3-S2, 20.3R3-S1, 20.4R2-S1, 20.4R3, 21.1R1-S1, or 21.1R2 depending on your current version branch
- 1. Identify the current Junos OS version on the affected QFX5000 Series switch using 'show version'
- 2. Based on the current version branch, identify the minimum fixed release from the reference: 17.3R3-S12, 18.1R3-S13, 18.3R3-S5, 19.1R3-S6, 19.2R1-S7/19.2R3-S3, 19.3R2-S6/19.3R3-S3, 19.4R1-S4/19.4R3-S5, 20.1R2-S2/20.1R3-S1, 20.2R3-S2, 20.3R3-S1, 20.4R2-S1/20.4R3, or 21.1R1-S1/21.1R2
- 3. Download the appropriate fixed Junos OS release from the Juniper Networks download portal
- 4. Upload the new Junos OS image to the switch using 'request system software add' or via USB
- 5. Reboot the switch to apply the new software version using 'request system reboot'
- 6. Verify the upgrade was successful and the new version is running using 'show version'
- 7. Confirm the fix is applied by verifying that packets to 128.0.0.0/2 subnet are no longer egressing the switch
Generated from the published advisory — verify against the referenced sources before acting.
- 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.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-31371 — 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-2021-31371 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
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.
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.
- 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