CVE-2021-1625Network attack vector

Traffic injection via uninspected return flows

Published Sep 23, 2021 · Updated Nov 7, 2024

Improper access control in Cisco IOS XE Software allows remote attackers to inject UDP traffic through a Zone-Based Policy Firewall. The firewall fails to inspect ICMP and UDP responder-to-initiator flows, allowing IOS XE session state to expire while the flow remains active in UTD. Exploitation requires Zone-Based Policy Firewall with UTD or AppQoE; consequences are injected UDP traffic, broken flows, or inaccurate HSL figures.

CVSS severity5.8
Medium
EPSS probability0.91%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper access control in Cisco IOS XE Software allows remote attackers to inject UDP traffic through a Zone-Based Policy Firewall. The firewall fails to inspect ICMP and UDP responder-to-initiator flows, allowing IOS XE session state to expire while the flow remains active in UTD. Exploitation requires Zone-Based Policy Firewall with UTD or AppQoE; consequences are injected UDP traffic, broken flows, or inaccurate HSL figures.

The record

CVE
CVE-2021-1625
Published
Sep 23, 2021
Updated
Nov 7, 2024
Vendor
ClamAV
Product
Cisco IOS XE Software
Classifications
CWE-284, T1562.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 23, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 7, 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?

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  1. Affected versionversion=n/a

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

ClamAV · Cisco IOS XE 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

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 10, 2026Sep 16, 2026
Latest reporting daySep 16, 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
T1562.004Impair Defenses: Disable or Modify System Firewall

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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