CVE-2021-1616Network attack vector

Unauthenticated ALG bypass via crafted traffic

Published Sep 23, 2021 · Updated Nov 7, 2024

Insufficient data validation in Cisco IOS XE Software H.323 ALG allows remote attackers to bypass NAT controls via crafted traffic. The ALG accepts malformed traffic traversing NAT and fails to enforce the intended translation and filtering boundaries, allowing connections to devices behind the gateway. Exploitation is unauthenticated but requires the attacker to send traffic through a device configured for NAT with H.323 ALG enabled; successful exploitation enables unauthorized inbound connections without reported confidentiality or availability impact.

CVSS severity4.7
Medium
EPSS probability1.19%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Insufficient data validation in Cisco IOS XE Software H.323 ALG allows remote attackers to bypass NAT controls via crafted traffic. The ALG accepts malformed traffic traversing NAT and fails to enforce the intended translation and filtering boundaries, allowing connections to devices behind the gateway. Exploitation is unauthenticated but requires the attacker to send traffic through a device configured for NAT with H.323 ALG enabled; successful exploitation enables unauthorized inbound connections without reported confidentiality or availability impact.

The record

CVE
CVE-2021-1616
Published
Sep 23, 2021
Updated
Nov 7, 2024
Vendor
ClamAV
Product
Cisco IOS XE Software
Classifications
CWE-693, T1090
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?

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

4.7CiscoCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N
4.3NVDAV:N/AC:M/Au:N/C:N/I:P/A:N

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-693Protection Mechanism Failure
T1090Proxy

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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