CVE-2021-34793Network attack vector

Transparent-mode DoS via TCP segment mishandling

Published Oct 27, 2021 · Updated Nov 7, 2024

Cisco ASA Software transparent-mode TCP Normalizer vulnerability allows remote attackers to disrupt networks via crafted TCP segments. The normalizer incorrectly handles certain TCP segments and can poison MAC address tables in adjacent devices when transparent firewall mode is enabled. No authentication is required, but traffic must traverse an affected transparent-mode device; the bounded impact is network disruption and denial of service.

CVSS severity8.6
High
EPSS probability0.58%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cisco ASA Software transparent-mode TCP Normalizer vulnerability allows remote attackers to disrupt networks via crafted TCP segments. The normalizer incorrectly handles certain TCP segments and can poison MAC address tables in adjacent devices when transparent firewall mode is enabled. No authentication is required, but traffic must traverse an affected transparent-mode device; the bounded impact is network disruption and denial of service.

The record

CVE
CVE-2021-34793
Published
Oct 27, 2021
Updated
Nov 7, 2024
Vendor
ClamAV
Product
Cisco Adaptive Security Appliance (ASA) Software
Classifications
CWE-924, T1498
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 27, 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?

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  1. Affected versionversion=n/a

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

ClamAV · Cisco Adaptive Security Appliance (ASA) Softwareversion=n/a

Published CVSS scores

8.6Cisco PSIRTCVSS:3.1/AV:N/AC:L/PR:N/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 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-924Improper Enforcement of Message Integrity During Transmission
T1498Network Denial of Service

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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