CVE-2021-1394Network attack vector

Web interface outage via crafted TCP packet flood

Published Mar 24, 2021 · Updated Nov 8, 2024

Denial of service in Cisco IOS XE Software on NCS 520 Routers allows remote attackers to interrupt the web management interface. The ingress traffic manager incorrectly processes a large volume of crafted IPv4 TCP packets addressed to the router, making the web interface unavailable. Exploitation requires the HTTP or HTTPS server to be enabled; transit traffic and traffic sent to the Management Ethernet interface remain unaffected.

CVSS severity5.3
Medium
EPSS probability1.27%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Cisco IOS XE Software on NCS 520 Routers allows remote attackers to interrupt the web management interface. The ingress traffic manager incorrectly processes a large volume of crafted IPv4 TCP packets addressed to the router, making the web interface unavailable. Exploitation requires the HTTP or HTTPS server to be enabled; transit traffic and traffic sent to the Management Ethernet interface remain unaffected.

The record

CVE
CVE-2021-1394
Published
Mar 24, 2021
Updated
Nov 8, 2024
Vendor
ClamAV
Product
Cisco IOS XE Software
Classifications
CWE-399, T1499.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 24, 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 XE Softwareversion=n/a

Published CVSS scores

5.3Cisco Systems, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L/E:X/RL:X/RC:X
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:N/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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-399Resource Management Errors
T1499.002Service Exhaustion Flood

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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