CVE-2021-34793 | Cisco Adaptive Security Appliance Software and Firepower Threat Defense Software Transparent Mode Denial of Service Vulnerability

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.

Published: 2021-10-27 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-34793 is rated Moderate Risk (41.6/100): CVSS High severity, with low exploitation likelihood (EPSS 0.08%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-34793

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2025-03-17 0.16% 0.08% -0.08%
2 2024-07-12 0.12% 0.16% +0.04%
3 2023-03-07 — 0.12% —

Full EPSS history (7 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-34793

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
8.6 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 4.0 [email protected]
8.6 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 4.0 [email protected]
5.0 2.0 MEDIUM
AV:N/AC:L/Au:N/C:N/I:N/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:N)
No integrity impact.
Availability impact (A:P)
Partial availability impact.
 10.0 2.9 [email protected]

Weakness enumeration for CVE-2021-34793

Affected software / configurations for CVE-2021-34793

Vendor Product Version Raw CPE
cisco adaptive_security_appliance < 9.8.4.40 cpe:2.3:a:cisco:adaptive_security_appliance:*:*:*:*:*:*:*:*
cisco firepower_threat_defense < 6.4.0.13 cpe:2.3:a:cisco:firepower_threat_defense:*:*:*:*:*:*:*:*
cisco firepower_threat_defense >= 6.5.0, < 6.6.5 cpe:2.3:a:cisco:firepower_threat_defense:*:*:*:*:*:*:*:*
cisco firepower_threat_defense >= 6.7.0, < 6.7.0.3 cpe:2.3:a:cisco:firepower_threat_defense:*:*:*:*:*:*:*:*
cisco adaptive_security_appliance_software >= 9.9.0, < 9.12.4.29 cpe:2.3:o:cisco:adaptive_security_appliance_software:*:*:*:*:*:*:*:*
cisco adaptive_security_appliance_software >= 9.13.0, < 9.14.3.9 cpe:2.3:o:cisco:adaptive_security_appliance_software:*:*:*:*:*:*:*:*
cisco adaptive_security_appliance_software >= 9.15.0, < 9.15.1.17 cpe:2.3:o:cisco:adaptive_security_appliance_software:*:*:*:*:*:*:*:*
cisco adaptive_security_appliance_software >= 9.16.0, < 9.16.2.3 cpe:2.3:o:cisco:adaptive_security_appliance_software:*:*:*:*:*:*:*:*
cisco asa_5512-x_firmware 009.008\(004.025\) cpe:2.3:o:cisco:asa_5512-x_firmware:009.008\(004.025\):*:*:*:*:*:*:*
cisco asa_5505_firmware 009.008\(004.025\) cpe:2.3:o:cisco:asa_5505_firmware:009.008\(004.025\):*:*:*:*:*:*:*
cisco asa_5515-x_firmware 009.008\(004.025\) cpe:2.3:o:cisco:asa_5515-x_firmware:009.008\(004.025\):*:*:*:*:*:*:*
cisco asa_5525-x_firmware 009.008\(004.025\) cpe:2.3:o:cisco:asa_5525-x_firmware:009.008\(004.025\):*:*:*:*:*:*:*
cisco asa_5545-x_firmware 009.008\(004.025\) cpe:2.3:o:cisco:asa_5545-x_firmware:009.008\(004.025\):*:*:*:*:*:*:*
cisco asa_5555-x_firmware 009.008\(004.025\) cpe:2.3:o:cisco:asa_5555-x_firmware:009.008\(004.025\):*:*:*:*:*:*:*
cisco asa_5580_firmware 009.008\(004.025\) cpe:2.3:o:cisco:asa_5580_firmware:009.008\(004.025\):*:*:*:*:*:*:*
cisco asa_5585-x_firmware 009.008\(004.025\) cpe:2.3:o:cisco:asa_5585-x_firmware:009.008\(004.025\):*:*:*:*:*:*:*

References for CVE-2021-34793

cvelogic Threat Intelligence