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Firepower Management Center Virtual ApplianceApplication · Cisco

CVE-2021-34781

HIGH · 7.5 CVSS v3.1 Published 2021-10-27
Fix available
A fix is available. Upgrade to 6.4.0.13 / 6.6.5 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A vulnerability in the SSH connection processing for multi-instance Cisco FTD deployments allows unauthenticated remote attackers to cause resource exhaustion and DoS. The root cause is improper error handling when SSH sessions fail to establish, combined with the ability to send a high rate of crafted SSH connections.

MitigationApply the Cisco software update/patch for CVE-2021-34781 when available. Implement rate limiting on SSH services and consider network-level filtering to reduce exposure to unauthenticated SSH connection attempts.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Firepower Management Center Virtual ApplianceApplication
Affected:= 6.3.0= 6.4.0= 6.4.0.13= 6.5.0= 6.6.0= 6.7.0.3= 6.7.1= 7.0.0
Secure Firewall Threat DefenseApplication
Affected:>= 6.3.0, < 6.4.0.13>= 6.5.0, < 6.6.5>= 6.7.0, < 6.7.0.3>= 7.0.0, < 7.0.1
Sourcefire Defense CenterApplication
Affected:= 6.3.0= 6.4.0= 6.4.0.13= 6.5.0= 6.6.0= 6.7.0.3= 6.7.1= 7.0.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify the Cisco product
    Determine if the system is Cisco Firepower Management Center (FMC) Virtual Appliance, Cisco Firepower Threat Defense (FTD), or Cisco Sourcefire Defense Center. Use 'show version' or check the system dashboard for product identification.
    Affected if The product is not one of FMC Virtual Appliance, FTD, or Sourcefire Defense Center (not affected)
  2. Check the installed version
    Run 'show version' on the device CLI or check the FMC web interface under System > Updates > Version. Compare the version string to the affected ranges: FMC/Sourcefire = 6.3.0, 6.4.0, 6.4.0.13, 6.5.0, 6.6.0, 6.7.0.3, 6.7.1, 7.0.0; FTD = 6.3.0 to <6.4.0.13, 6.5.0 to <6.6.5, 6.7.0 to <6.7.0.3, 7.0.0 to <7.0.1
    Affected if The installed version matches one of the affected versions listed
  3. Verify SSH service is enabled
    Run 'show ssh' or check the device configuration for SSH server status. The vulnerability requires SSH connection processing to be active.
    Affected if SSH server is enabled and accessible from network
  4. Confirm multi-instance deployment
    Check if the FTD is running in multi-instance mode (e.g., 'show instance' or via FMC multi-instance configuration). The vulnerability specifically affects multi-instance deployments.
    Affected if The deployment uses multi-instance configuration
  5. Look for resource exhaustion symptoms
    Monitor CPU and memory usage during periods of failed SSH connection attempts. Run 'show processes cpu' and 'show memory' during high SSH traffic. Check for unusual SSH connection failures in logs via 'show log' or the FMC event viewer.
    Affected if High CPU or memory usage correlating with SSH connection attempts, or excessive failed SSH sessions in logs

If the product is FMC Virtual Appliance, FTD, or Sourcefire Defense Center, the version is in the affected list, SSH is enabled, and it is a multi-instance deployment, the environment is likely affected by CVE-2021-34781.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 6.4.0.13 / 6.6.5 / 6.7.0.3 or later
Fixed in 6.4.0.136.6.56.7.0.3
Interim mitigation

Apply the Cisco software update/patch for CVE-2021-34781 when available. Implement rate limiting on SSH services and consider network-level filtering to reduce exposure to unauthenticated SSH connection attempts.

Recommended fix High confidence

Upgrade to the first fixed version in your branch: 6.4.0.13, 6.6.5, 6.7.0.3, or 7.0.1 (or later releases)

  1. Identify the currently installed FTD or FMC version using 'show version' command
  2. For Firepower Threat Defense (FTD): If running >= 6.3.0 and < 6.4.0.13, upgrade to 6.4.0.13 or later
  3. For Firepower Threat Defense (FTD): If running >= 6.5.0 and < 6.6.5, upgrade to 6.6.5 or later
  4. For Firepower Threat Defense (FTD): If running >= 6.7.0 and < 6.7.0.3, upgrade to 6.7.0.3 or later
  5. For Firepower Threat Defense (FTD): If running >= 7.0.0 and < 7.0.1, upgrade to 7.0.1 or later
  6. For Firepower Management Center Virtual Appliance: Upgrade from 6.3.0, 6.4.0, 6.4.0.13, or 6.5.0 to the latest 6.x release or 7.x
  7. For Sourcefire Defense Center: Upgrade from 6.3.0, 6.4.0, 6.4.0.13, or 6.5.0 to the latest 6.x release
  8. After upgrade, use 'show running-config' to verify the system is operational
Caveat Major version upgrades (e.g., 6.x to 7.x) may require compatibility checks with management software and could impact existing configurations

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Firepower Management Center Virtual Appliance Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-34781 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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