CVE-2021-34781 | Cisco Firepower Threat Defense Software SSH Connections Denial of Service Vulnerability

A vulnerability in the processing of SSH connections for multi-instance deployments of Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on the affected device. This vulnerability is due to a lack of proper error handling when an SSH session fails to be established. An attacker could exploit this vulnerability by sending a high rate of crafted SSH connections to the instance. A successful exploit could allow the attacker to cause resource exhaustion, which causes a DoS condition on the affected device. The device must be manually reloaded to recover.

Published: 2021-10-27 Last update: 2026-06-16 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-34781 is rated Moderate Risk (59/100): CVSS High severity, with medium exploitation likelihood (EPSS 1.35%). 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-2021-34781

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 2026-06-15 0.43% 1.35% +0.92%
2 2025-03-30 0.58% 0.43% -0.15%
3 2025-03-29 — 0.58% —

Full EPSS history (11 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-34781

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]
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/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:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
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 3.6 [email protected]
7.1 2.0 HIGH
AV:N/AC:M/Au:N/C:N/I:N/A:C Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:M)
Exploitation needs some favorable conditions, but not exceptional ones.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:N)
No integrity impact.
Availability impact (A:C)
Complete availability impact.
 8.6 6.9 [email protected]

Weakness enumeration for CVE-2021-34781

Affected software / configurations for CVE-2021-34781

Vendor Product Version Raw CPE
cisco firepower_management_center_virtual_appliance 6.3.0 cpe:2.3:a:cisco:firepower_management_center_virtual_appliance:6.3.0:*:*:*:*:*:*:*
cisco firepower_management_center_virtual_appliance 6.4.0 cpe:2.3:a:cisco:firepower_management_center_virtual_appliance:6.4.0:*:*:*:*:*:*:*
cisco firepower_management_center_virtual_appliance 6.4.0.13 cpe:2.3:a:cisco:firepower_management_center_virtual_appliance:6.4.0.13:*:*:*:*:*:*:*
cisco firepower_management_center_virtual_appliance 6.5.0 cpe:2.3:a:cisco:firepower_management_center_virtual_appliance:6.5.0:*:*:*:*:*:*:*
cisco firepower_management_center_virtual_appliance 6.6.0 cpe:2.3:a:cisco:firepower_management_center_virtual_appliance:6.6.0:*:*:*:*:*:*:*
cisco firepower_management_center_virtual_appliance 6.7.0.3 cpe:2.3:a:cisco:firepower_management_center_virtual_appliance:6.7.0.3:*:*:*:*:*:*:*
cisco firepower_management_center_virtual_appliance 6.7.1 cpe:2.3:a:cisco:firepower_management_center_virtual_appliance:6.7.1:*:*:*:*:*:*:*
cisco firepower_management_center_virtual_appliance 7.0.0 cpe:2.3:a:cisco:firepower_management_center_virtual_appliance:7.0.0:*:*:*:*:*:*:*
cisco firepower_threat_defense >= 6.3.0, < 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 firepower_threat_defense >= 7.0.0, < 7.0.1 cpe:2.3:a:cisco:firepower_threat_defense:*:*:*:*:*:*:*:*
cisco sourcefire_defense_center 6.3.0 cpe:2.3:a:cisco:sourcefire_defense_center:6.3.0:*:*:*:*:*:*:*
cisco sourcefire_defense_center 6.4.0 cpe:2.3:a:cisco:sourcefire_defense_center:6.4.0:*:*:*:*:*:*:*
cisco sourcefire_defense_center 6.4.0.13 cpe:2.3:a:cisco:sourcefire_defense_center:6.4.0.13:*:*:*:*:*:*:*
cisco sourcefire_defense_center 6.5.0 cpe:2.3:a:cisco:sourcefire_defense_center:6.5.0:*:*:*:*:*:*:*
cisco sourcefire_defense_center 6.6.0 cpe:2.3:a:cisco:sourcefire_defense_center:6.6.0:*:*:*:*:*:*:*
cisco sourcefire_defense_center 6.7.0.3 cpe:2.3:a:cisco:sourcefire_defense_center:6.7.0.3:*:*:*:*:*:*:*
cisco sourcefire_defense_center 6.7.1 cpe:2.3:a:cisco:sourcefire_defense_center:6.7.1:*:*:*:*:*:*:*
cisco sourcefire_defense_center 7.0.0 cpe:2.3:a:cisco:sourcefire_defense_center:7.0.0:*:*:*:*:*:*:*

References for CVE-2021-34781

cvelogic Threat Intelligence