CVE-2021-0247Network attack vector

Firewall bypass via dfwd race condition

Published Apr 22, 2021 · Updated Sep 16, 2024

A race condition in the dfwd process in Juniper Networks Junos OS allows attackers to bypass input loopback firewall filters. Concurrent handling can leave a configured filter absent from the Packet Forwarding Engine, so matching traffic does not increment its counter. Reachability requires high privileges, network access, user interaction, and winning the race; successful exploitation evades the affected traffic controls.

CVSS severity5.5
Medium
EPSS probability0.56%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

A race condition in the dfwd process in Juniper Networks Junos OS allows attackers to bypass input loopback firewall filters. Concurrent handling can leave a configured filter absent from the Packet Forwarding Engine, so matching traffic does not increment its counter. Reachability requires high privileges, network access, user interaction, and winning the race; successful exploitation evades the affected traffic controls.

The record

CVE
CVE-2021-0247
Published
Apr 22, 2021
Updated
Sep 16, 2024
Vendor
Juniper Networks
Product
Junos OS
Classifications
CWE-362, T1562.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Apr 22, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 16, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=14.1R1 <14.1*
  2. Affected versionversion=14.1X53 <14.1X53-D53
  3. Affected versionversion=15.1 <15.1R7-S6
  4. Affected versionversion=15.1X53 <15.1X53-D593
  5. Affected versionversion=16.1 <16.1R7-S7
  6. Affected versionversion=16.2 <16.2R2-S11, 16.2R3
  7. Affected versionversion=17.1 <17.1R2-S11, 17.1R3-S2
  8. Affected versionversion=17.2 <17.2R1-S9, 17.2R3-S3
  9. Affected versionversion=17.3 <17.3R2-S5, 17.3R3-S7
  10. Affected versionversion=17.4 <17.4R2-S9, 17.4R3
  11. Affected versionversion=18.1 <18.1R3-S9
  12. Affected versionversion=18.2 <18.2R2-S6, 18.2R3-S3
  13. Affected versionversion=18.3 <18.3R1-S7, 18.3R2-S3, 18.3R3-S1
  14. Affected versionversion=18.4 <18.4R1-S5, 18.4R2-S3, 18.4R3
  15. Affected versionversion=19.1 <19.1R1-S4, 19.1R2-S1, 19.1R3
  16. Affected versionversion=19.2 <19.2R1-S3, 19.2R2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Juniper Networks · Junos OSversion=14.1R1 <14.1*; version=14.1X53 <14.1X53-D53; version=15.1 <15.1R7-S6; version=15.1X53 <15.1X53-D593; version=16.1 <16.1R7-S7; version=16.2 <16.2R2-S11, 16.2R3; version=17.1 <17.1R2-S11, 17.1R3-S2; version=17.2 <17.2R1-S9, 17.2R3-S3; version=17.3 <17.3R2-S5, 17.3R3-S7; version=17.4 <17.4R2-S9, 17.4R3; version=18.1 <18.1R3-S9; version=18.2 <18.2R2-S6, 18.2R3-S3; version=18.3 <18.3R1-S7, 18.3R2-S3, 18.3R3-S1; version=18.4 <18.4R1-S5, 18.4R2-S3, 18.4R3; version=19.1 <19.1R1-S4, 19.1R2-S1, 19.1R3; version=19.2 <19.2R1-S3, 19.2R2

Published CVSS scores

5.1Juniper NetworksCVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:C/C:L/I:L/A:L
5.5NVDCVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:C/C:L/I:L/A:L
6.8NVDAV:N/AC:M/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-362Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
T1562.004Impair Defenses: Disable or Modify System Firewall

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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