CVE-2021-1220Network attack vector

Authenticated service interruption via insufficient HTTP error handling

Published Mar 24, 2021 · Updated Sep 17, 2024

Improper input validation in the Cisco IOS XE web UI allows remote authenticated users to interrupt web and VTY sessions via crafted HTTP packets. The web UI handles malformed HTTP input with insufficient error handling, becomes unresponsive, and consumes every available VTY line. Read-only privileges and an enabled HTTP Server feature are required; new remote sessions remain blocked until manual recovery, while console access remains available.

CVSS severity4.3
Medium
EPSS probability0.94%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper input validation in the Cisco IOS XE web UI allows remote authenticated users to interrupt web and VTY sessions via crafted HTTP packets. The web UI handles malformed HTTP input with insufficient error handling, becomes unresponsive, and consumes every available VTY line. Read-only privileges and an enabled HTTP Server feature are required; new remote sessions remain blocked until manual recovery, while console access remains available.

The record

CVE
CVE-2021-1220
Published
Mar 24, 2021
Updated
Sep 17, 2024
Vendor
ClamAV
Product
Cisco IOS XE Software
Classifications
CWE-20, T1499.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Mar 24, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 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 privilegesauthenticated

What is affected?

ClamAV · Cisco IOS XE Softwareversion=n/a

Published CVSS scores

4.3Cisco Systems, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
3.5NIST NVDAV:N/AC:M/Au:S/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 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-20Improper Input Validation
T1499.004Application or System Exploitation

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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