CVE-2021-34793

Cisco Adaptive Security Appliance Software and Firepower Threat Defense Software Transparent Mode Denial of Service Vulnerability

HIGH
8.6
CVSS Score
Published: October 27, 2021 at 06:56 PM UTC
Updated: August 11, 2026 at 07:37 PM UTC
18 products affected across 1 vendorAdaptive Security ApplianceSecure Firewall Threat DefenseASA 5512-x Firmware

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Severity
HIGH8.6
Exploit prediction
EPSS 0.6%
SSVC
No known exploit

Description

A vulnerability in the TCP Normalizer of Cisco Adaptive Security Appliance (ASA) Software and Firepower Threat Defense (FTD) Software operating in transparent mode could allow an unauthenticated, remote attacker to poison MAC address tables, resulting in a denial of service (DoS) vulnerability. This vulnerability is due to incorrect handling of certain TCP segments when the affected device is operating in transparent mode. An attacker could exploit this vulnerability by sending a crafted TCP segment through an affected device. A successful exploit could allow the attacker to poison the MAC address tables in adjacent devices, resulting in network disruption.

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

v < 9.8.4.409.13.0 ≤ v < 9.14.3.99.15.0 ≤ v < 9.15.1.179.16.0 ≤ v < 9.16.2.39.9.0 ≤ v < 9.12.4.29
Affected range stops below 9.12.4.29, so it looks safe — our inference, not a vendor statement
Affected range stops below 9.14.3.9, so it looks safe — our inference, not a vendor statement
Affected range stops below 9.15.1.17, so it looks safe — our inference, not a vendor statement
Affected range stops below 9.16.2.3, so it looks safe — our inference, not a vendor statement
Affected range stops below 9.8.4.40, so it looks safe — our inference, not a vendor statement
cisco-asa-5505 (same device)
cisco-asa-5512-x (same device)
cisco-asa-5515-x (same device)
cisco-asa-5525-x (same device)
cisco-asa-5545-x (same device)
cisco-asa-5555-x (same device)
cisco-asa-5580 (same device)
cisco-asa-5585-x (same device)
v < 6.4.0.136.5.0 ≤ v < 6.6.56.7.0 ≤ v < 6.7.0.3

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

SSVC — Action Prioritization

SSVCNo known exploitAutomatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

8.6HIGHcisco
Attack Vector
NETWORK
Complexity
LOW
Privileges
NONE
User Interaction
NONE
Scope
CHANGED
Confidentiality
NONE
Integrity
NONE
Availability
HIGH
Vector?

Quick Info

StatusMODIFIED
CWE
CWE-924

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