CVE-2021-1356Network attack vector

Authenticated denial of service via HTTP error handling

Published Mar 24, 2021 · Updated Nov 8, 2024

Denial of service in Cisco IOS XE Software allows remote authenticated users to exhaust vty lines by sending crafted HTTP packets. The web UI insufficiently handles malformed HTTP requests, becomes unresponsive, and leaves sessions consuming available vty lines. The HTTP Server feature must be enabled; read-only access suffices, new sessions fail, and manual recovery is required, 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

Denial of service in Cisco IOS XE Software allows remote authenticated users to exhaust vty lines by sending crafted HTTP packets. The web UI insufficiently handles malformed HTTP requests, becomes unresponsive, and leaves sessions consuming available vty lines. The HTTP Server feature must be enabled; read-only access suffices, new sessions fail, and manual recovery is required, while console access remains available.

The record

CVE
CVE-2021-1356
Published
Mar 24, 2021
Updated
Nov 8, 2024
Vendor
ClamAV
Product
Cisco IOS XE Software
Classifications
CWE-755, 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. 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 privilegesauthenticated

What is affected?

ClamAV · Cisco IOS XE Softwareversion=n/a

Published CVSS scores

4.3CiscoCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L/E:X/RL:X/RC:X
3.5NIST NVDAV:N/AC:M/Au:S/C:N/I:N/A:P
4.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

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-755Improper Handling of Exceptional Conditions
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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