CVE-2021-1620Network attack vector

Authenticated VPN login denial via unreleased IP addresses

Published Sep 23, 2021 · Updated Nov 7, 2024

Resource exhaustion in Cisco IOS with IKEv2 AutoReconnect enabled allows remote authenticated users to prevent VPN logins. The IKEv2 AutoReconnect code fails to release a local-pool IP address when a non-AnyConnect client encounters certain connection failures. The feature is disabled by default; repeated connection attempts can drain the pool and leave no addresses for subsequent VPN sessions.

CVSS severity7.7
High
EPSS probability1.07%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Resource exhaustion in Cisco IOS with IKEv2 AutoReconnect enabled allows remote authenticated users to prevent VPN logins. The IKEv2 AutoReconnect code fails to release a local-pool IP address when a non-AnyConnect client encounters certain connection failures. The feature is disabled by default; repeated connection attempts can drain the pool and leave no addresses for subsequent VPN sessions.

The record

CVE
CVE-2021-1620
Published
Sep 23, 2021
Updated
Nov 7, 2024
Vendor
ClamAV
Product
Cisco IOS
Classifications
CWE-563, T1499.002
Attack vector
network
Privileges
authenticated

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?

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 IOSversion=n/a

Published CVSS scores

7.7CiscoCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H

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.

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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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-563Assignment to Variable without Use
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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