CVE-2021-1389Network attack vector

Unauthenticated ACL bypass via crafted IPv6 packets

Published Feb 4, 2021 · Updated Nov 8, 2024

Improper IPv6 traffic processing in Cisco IOS XR Software allows remote attackers to bypass interface ACLs with crafted transit packets. Affected devices fail to classify certain IPv6 extension-header traffic correctly before applying a configured IPv6 ACL. The device must be a listed NCS hardware platform with an IPv6 ACL, and successful packets can reach resources that the ACL should protect.

CVSS severity5.8
Medium
EPSS probability1.22%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper IPv6 traffic processing in Cisco IOS XR Software allows remote attackers to bypass interface ACLs with crafted transit packets. Affected devices fail to classify certain IPv6 extension-header traffic correctly before applying a configured IPv6 ACL. The device must be a listed NCS hardware platform with an IPv6 ACL, and successful packets can reach resources that the ACL should protect.

The record

CVE
CVE-2021-1389
Published
Feb 4, 2021
Updated
Nov 8, 2024
Vendor
ClamAV
Product
Cisco IOS XR Software
Classifications
CWE-284, T1599
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 4, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 8, 2024Record 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=n/a

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

ClamAV · Cisco IOS XR Softwareversion=n/a

Published CVSS scores

5.8Cisco Systems, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N/E:X/RL:X/RC:X
6.4NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:N
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-284Improper Access Control
T1599Network Boundary Bridging

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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