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Application Delivery Controller FirmwareOperating system · Citrix

CVE-2021-22955

HIGH · 7.5 CVSS v3.1 Published 2021-12-07
Fix available
A fix is available. Upgrade to 11.1-65.23 / 12.1-63.22 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 unauthenticated denial of service vulnerability exists in Citrix ADC <13.0-83.27, <12.1-63.22 and 11.1-65.23 when configured as a VPN (Gateway) or AAA virtual server could allow an attacker to cause a temporary disruption of the Management GUI, Nitro API, and RPC communication.

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

An unauthenticated remote attacker can cause denial of service on Citrix ADC devices configured as a VPN (Gateway) or AAA virtual server, disrupting the Management GUI, Nitro API, and RPC communication. The vulnerability affects versions prior to 13.0-83.27, 12.1-63.22, and 11.1-65.23.

MitigationApply the vendor-supplied patches (13.0-83.27, 12.1-63.22, or 11.1-65.23) or upgrade to a supported version. Until patched, consider restricting network access to management interfaces and VPN/AAA virtual servers from untrusted networks.

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
Application Delivery Controller FirmwareOperating system
Affected:<= 11.1-65.23>= 12.1, < 12.1-63.22>= 13.0, < 13.0-83.27
GatewayApplication
Affected:< 11.1-65.23>= 12.1, < 12.1-63.22>= 13.0, < 13.0-83.27

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. Check Citrix ADC or Gateway firmware version
    Log into the management GUI and navigate to System > Information > Version, or run 'show version' via CLI. Alternatively, query the Nitro API endpoint /nitro/v1/config/ns version information.
    Affected if The installed version is 11.1-65.23 or earlier, OR between 12.1 and 12.1-63.22, OR between 13.0 and 13.0-83.27.
  2. Verify VPN (Gateway) virtual server configuration
    Run 'show vpn vserver' via CLI, or check the configuration under Traffic Management > Virtual Servers in the GUI. Look for any virtual servers of type VPN or Gateway.
    Affected if Any VPN or Gateway virtual server is configured and enabled.
  3. Verify AAA virtual server configuration
    Run 'show aaa vserver' via CLI, or check the configuration under AAA > Virtual Servers in the GUI.
    Affected if Any AAA virtual server is configured and enabled.
  4. Assess network exposure of management and VPN interfaces
    Review access control lists, firewall rules, or NSIP/VIP listener bindings. Check if the management GUI (ports 80/443), Nitro API, or VPN virtual servers are reachable from untrusted networks.
    Affected if Management interface or VPN/AAA virtual servers are exposed to untrusted networks without proper access restrictions.

If the device runs an affected firmware version AND has a VPN/Gateway or AAA virtual server configured, and these interfaces are network-accessible, the environment is vulnerable to unauthenticated remote DoS.

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 11.1-65.23 / 12.1-63.22 / 13.0-83.27 or later
Fixed in 11.1-65.2312.1-63.2213.0-83.27
Interim mitigation

Apply the vendor-supplied patches (13.0-83.27, 12.1-63.22, or 11.1-65.23) or upgrade to a supported version. Until patched, consider restricting network access to management interfaces and VPN/AAA virtual servers from untrusted networks.

Recommended fix High confidence

13.0-83.27 (or latest 13.0.x), 12.1-63.22 (or latest 12.1.x), or 11.1-65.23 (or latest 11.1.x) depending on current release train

  1. 1. Identify the currently installed Citrix ADC or Gateway version using the web interface (System > Diagnostics > View Version) or via Nitro API.
  2. 2. Determine which release train (13.0, 12.1, or 11.1) your device is running.
  3. 3. For release train 13.0: Upgrade to version 13.0-83.27 or later.
  4. 4. For release train 12.1: Upgrade to version 12.1-63.22 or later.
  5. 5. For release train 11.1: Upgrade to version 11.1-65.23 or later.
  6. 6. Schedule the upgrade during a maintenance window as the ADC/Gateway will require a reboot.
  7. 7. After upgrade, verify the new version is running and test VPN/AAA virtual server functionality.
Caveat Upgrading across major release trains may require compatibility testing; ensure custom configurations and scripts are validated after upgrade

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

Fix this in Application Delivery Controller Firmware Scoped from the published advisory
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing3.0 h
  • Review / QA2.0 h
14.0 hours of engineering $2,490
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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-2021-22955 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data