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Secure Firewall Threat DefenseApplication · Cisco

CVE-2023-20042

HIGH · 8.6 CVSS v3.1 Published 2023-11-01
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
95/100
Remediation priority · Urgent
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 AnyConnect SSL VPN feature of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability is due to an implementation error within the SSL/TLS session handling process that can prevent the release of a session handler under specific conditions. An attacker could exploit this vulnerability by sending crafted SSL/TLS traffic to an affected device, increasing the probability of session handler leaks. A successful exploit could allow the attacker to eventually deplete the available session handler pool, preventing new sessions from being established and causing a DoS condition.

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 AnyConnect SSL VPN feature of Cisco ASA and FTD software allows an unauthenticated remote attacker to cause denial of service by sending crafted SSL/TLS traffic that prevents proper release of session handlers, eventually depleting the session handler pool and preventing new VPN connections.

MitigationApply Cisco's security patches for this vulnerability to affected ASA and FTD devices; consider implementing rate limiting or restricting VPN access as interim mitigation while patching.

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
Secure Firewall Threat DefenseApplication
Affected:= 7.0.0= 7.0.0.1= 7.0.1= 7.0.1.1= 7.0.2= 7.0.2.1= 7.0.3= 7.0.4= 7.0.5= 7.1.0= 7.1.0.1= 7.1.0.2
Adaptive Security Appliance SoftwareOperating system
Affected:= 9.16.1= 9.16.1.28= 9.16.2= 9.16.2.3= 9.16.2.7= 9.16.2.11= 9.16.2.13= 9.16.2.14= 9.16.3= 9.16.3.3= 9.16.3.14= 9.16.3.15

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
Changed
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/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 device product and software version
    Log into the ASA or FTD device and run the 'show version' command. Look for the software version number in the output.
    Affected if The version matches one of the affected ranges: ASA 9.16.1 through 9.16.3.15, or FTD 7.0.0 through 7.1.0.2
  2. Confirm the device is running ASA or FTD software
    The 'show version' output will display either 'Cisco Adaptive Security Appliance Software' or 'Cisco Firepower Threat Defense' as the software type.
    Affected if The device runs ASA or FTD software matching the affected versions from step 1
  3. Verify AnyConnect SSL VPN is enabled
    Run 'show run webvpn' or 'show running-config webvpn' to check if AnyConnect VPN is configured and active.
    Affected if AnyConnect SSL VPN is enabled and accepting connections (the vulnerability exists in this feature)
  4. Check if the VPN is externally accessible
    Review the interface configuration with 'show run interface' and verify the outside/outside-facing interface has webvpn enabled. Check if port 443 is open to external traffic.
    Affected if The AnyConnect VPN listener is bound to an externally accessible interface, allowing remote attackers to send the crafted SSL/TLS traffic
  5. Monitor for session handler depletion symptoms
    Run 'show vpn-sessiondb' or 'show webvpn session' to observe the current session count. Compare against the session handler pool limit. Look for any session entries that appear stuck or not properly released.
    Affected if Session count is approaching or has reached the maximum pool limit, or sessions remain in a hung state after disconnection attempts

The environment is affected if the device runs ASA 9.16.x or FTD 7.0.x-7.1.x with AnyConnect SSL VPN enabled and accessible, allowing remote attackers to send crafted SSL/TLS traffic that depletes session handlers.

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
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply Cisco's security patches for this vulnerability to affected ASA and FTD devices; consider implementing rate limiting or restricting VPN access as interim mitigation while patching.

Recommended fix Moderate confidence

Upgrade to fixed 7.0.x release for FTD or fixed 9.16.x release for ASA as specified in Cisco advisory for CVE-2023-20042

  1. 1. Identify the exact Cisco ASA or FTD software version currently installed using 'show version' command
  2. 2. Navigate to Cisco Security Advisories at sec.cloudapps.cisco.com and search for CVE-2023-20042 to obtain the official fixed release numbers
  3. 3. For Firepower Threat Defense: Upgrade to the fixed 7.0.x release as specified in the Cisco advisory (7.0.2 or later if listed)
  4. 4. For ASA Software: Upgrade to the fixed 9.16.x release as specified in the Cisco advisory (9.16.3 or later if listed)
  5. 5. Before upgrading, backup the current device configuration
  6. 6. Upload the new software image to the device flash storage
  7. 7. Reload the new image using 'reload' command or through the management interface
  8. 8. After upgrade, verify the new version is running with 'show version'
Caveat Standard upgrade risks apply - test in lab environment first, ensure configuration compatibility, and plan maintenance window as upgrade causes brief service interruption

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

Fix this in Secure Firewall Threat Defense Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,800
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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-2023-20042 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
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  • No spam, self-promotion, credentials, or personal data