CVE-2026-21911

MAC learning interruption via management-interface flapping

Published Jan 15, 2026 · Updated Jan 15, 2026

Incorrect calculation in l2cpd in Juniper Networks Junos OS Evolved allows attackers to stop new MAC learning on LSI links. Repeated management-interface flaps trigger an IFBD epoch mismatch during forced MAC deletion, halt the GET_MAC processing path, and flood logs. Network-adjacent access sufficient to flap the management interface is required; the resulting log storm raises CPU use while new LSI MAC addresses remain unlearned.

CVSS severity7.1
High
EPSS probability0.24%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Incorrect calculation in l2cpd in Juniper Networks Junos OS Evolved allows attackers to stop new MAC learning on LSI links. Repeated management-interface flaps trigger an IFBD epoch mismatch during forced MAC deletion, halt the GET_MAC processing path, and flood logs. Network-adjacent access sufficient to flap the management interface is required; the resulting log storm raises CPU use while new LSI MAC addresses remain unlearned.

The record

CVE
CVE-2026-21911
Published
Jan 15, 2026
Updated
Jan 15, 2026
Vendor
Juniper Networks
Product
Junos OS Evolved
Classifications
CWE-682, T1499
Attack vector
adjacent
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 15, 2026CVE publishedPublication date reported by the CVE source.
  2. Jan 15, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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

  1. Affected versionversion=0 <21.4R3-S7-EVO
  2. Affected versionversion=22.2 <22.2R3-S4-EVO
  3. Affected versionversion=22.3 <22.3R3-S3-EVO
  4. Affected versionversion=22.4 <22.4R3-S2-EVO
  5. Affected versionversion=23.2 <23.2R2-S1-EVO
  6. Affected versionversion=23.4 <23.4R1-S2-EVO, 23.4R2-EVO

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesunauthenticated

What is affected?

Juniper Networks · Junos OS Evolvedversion=0 <21.4R3-S7-EVO; version=22.2 <22.2R3-S4-EVO; version=22.3 <22.3R3-S3-EVO; version=22.4 <22.4R3-S2-EVO; version=23.2 <23.2R2-S1-EVO; version=23.4 <23.4R1-S2-EVO, 23.4R2-EVO

Published CVSS scores

7.1Juniper NetworksCVSS:4.0/AV:A/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:L
6.5Juniper NetworksCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectoradjacent
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-682Incorrect Calculation
T1499Endpoint Denial of Service

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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