Security readout for executives and security teams
Plain-English summary
Some Juniper QFX and PTX devices may show loopback input firewall filters in configuration even when those filters are not actually enforced. This can let traffic bypass intended control-plane filtering. The issue is operationally important where those devices protect management or infrastructure paths, but public sources do not show active exploitation.
Executive priority
Schedule remediation through normal network maintenance, with priority for core, data-center, and management-plane devices. This is not currently a public emergency based on the supplied evidence, but it can undermine a control relied on to limit sensitive infrastructure traffic.
Technical view
CVE-2021-0247 is a CWE-362 race condition in Junos OS firewall process dfwd. Input loopback firewall filters on QFX and PTX interfaces may fail to operate despite appearing in running configuration. CVSS is 5.1 medium, with high attack complexity, high privileges, user interaction, and changed scope.
Likely exposure
Exposure is limited to Juniper Networks Junos OS on QFX Series and PTX Series running the affected 14.1X53, 14.1, 15.1, 15.1X53, 16.x, 17.x, 18.x, 19.1, and 19.2 releases listed by Juniper. Environments relying on input loopback filters for control-plane protection have the highest concern.
Exploitation context
The bundle does not identify active exploitation, and CISA KEV status is false. Exploitation is not described as trivial: the CVSS vector indicates network reachability but high complexity, high privileges, and user interaction. Treat this primarily as a firewall enforcement failure requiring configuration and forwarding-plane validation.
Researcher notes
The key observable is enforcement divergence: filters appear configured, but PFE rules or counters may show they are not operating. The source bundle provides affected release ranges and detection direction, but does not provide exploit details, real-world exploitation evidence, or non-upgrade mitigations.
Mitigation direction
- Inventory QFX and PTX devices running Junos OS releases listed as affected.
- Upgrade affected devices to Juniper fixed releases or later, per JSA11140.
- Prioritize devices where loopback input filters protect management or routing-plane access.
- Review Juniper guidance before applying operational workarounds or release changes.
Validation and detection
- Compare running configuration with PFE-installed firewall rules on affected devices.
- Check firewall counters for expected packet increments on input loopback filters.
- Investigate filters that are configured but show missing or zero packet counts.
- Confirm post-upgrade that PFE rules and counters match expected enforcement.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
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ATT&CK lookup starting points
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CWE-362: Exact CWE lookup
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Open ATT&CK lookupCVE-2021-0247 mapping review
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Open ATT&CK lookup- Severity
- Medium
- CVSS
- 5.1 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:C/C:L/I:L/A:L
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:C/C:L/I:L/A:L13.7Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
5.1MediumVector: CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:C/C:L/I:L/A:L
Source materials
- CVE List V5 sourceCVE List V5
- https://kb.juniper.net/JSA11140CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.