Summary

• A vulnerability in the deterministic random bit generator (DRBG), also known as pseudorandom number generator (PRNG), in Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software for Cisco ASA 5506-X, ASA 5508-X, and ASA 5516-X Firewalls could allow an unauthenticated, remote attacker to cause a cryptographic collision, enabling the attacker to discover the private key of an affected device.

  This vulnerability is due to insufficient entropy in the DRBG for the affected hardware platforms when generating cryptographic keys. An attacker could exploit this vulnerability by generating a large number of cryptographic keys on an affected device and looking for collisions with target devices. A successful exploit could allow the attacker to impersonate an affected target device or to decrypt traffic secured by an affected key that is sent to or from an affected target device.

  Cisco has released software updates that address this vulnerability. There are workarounds that address this vulnerability for some affected configurations.

  This advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asa5500x-entropy-6v9bHVYP

Affected Products

• Vulnerable Products

  At the time of publication, this vulnerability affected the following Cisco products if they were running a Cisco ASA Software release earlier than Release 9.12.1¹ or a Cisco FTD Software release earlier than Release 6.4.0¹ and had features that use ECDSA or RSA keys enabled:

  • ASA 5506-X Security Appliances
  • ASA 5506H-X Security Appliances
  • ASA 5506W-X Security Appliances
  • ASA 5508-X Security Appliances
  • ASA 5516-X Security Appliances

  1. Enhancements to the entropy source in the DRBG that were put in place as result of following the robust Cisco secure development lifecycle (SDL) process prevent this issue from happening in Cisco ASA Software Releases 9.12.1 and later and Cisco FTD Software Releases 6.4.0 and later.

  Note: Devices that are running a fixed release of Cisco ASA Software or Cisco FTD Software may also be affected by this vulnerability if they are configured with an ECDSA or RSA key pair that has been generated while running an affected release. If there is doubt, Cisco recommends regenerating all ECDSA and RSA key pairs that are configured on the device.

  For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.

  ASA Software

  In the following table, the left column lists the most common Cisco ASA features that are potentially vulnerable. The right column indicates the basic configuration for the feature from the show running-config CLI command, if it can be determined.

  Cisco ASA Feature	Possible Vulnerable Configuration
  Adaptive Security Device Manager (ASDM)1	http server enable http
  AnyConnect SSL VPN	webvpn enable
  Cisco Security Manager1	http server enable http
  Clientless SSL VPN (WebVPN)	webvpn enable
  IKEv1 VPN (Remote Access and LAN-to-LAN) using Certificate-based Authentication	crypto ikev1 enable crypto ikev1 policy authentication rsa-sig tunnel-group ipsec-attributes trust-point
  IKEv2 VPN (Remote Access and LAN-to-LAN) using Certificate-based Authentication	crypto ikev2 enable tunnel-group ipsec-attributes ikev2 remote-authentication certificate ikev2 local-authentication certificate
  Local Certificate Authority (CA)	crypto ca server no shutdown
  Mobile Device Manager (MDM) Proxy	mdm-proxy enable
  Mobile User Security (MUS)	webvpn mus password mus server enable port mus
  Proxy Bypass	webvpn proxy-bypass
  REST API1	rest-api image disk0:/ rest-api agent
  SSH Access2,3	ssh

  1. ASDM, CSM, and REST API services are accessible only from an IP address in the configured http command range.
  2. SSH service is accessible only from an IP address in the configured ssh command range.
  3. There is no workaround that addresses this vulnerability for SSH access.

  FTD Software

  In the following table, the left column lists the most common Cisco FTD features that are potentially vulnerable. The right column indicates the basic configuration for the feature from the show running-config CLI command, if it can be determined.

  Cisco FTD Feature	Possible Vulnerable Configuration
  AnyConnect SSL VPN1,2	webvpn enable
  Clientless SSL VPN (WebVPN)2	webvpn enable
  HTTP Service enabled3,4	http server enable http
  IKEv1 VPN (Remote Access and LAN-to-LAN) using Certificate-based Authentication1,2	crypto ikev1 enable crypto ikev1 policy authentication rsa-sig tunnel-group ipsec-attributes trust-point
  IKEv2 VPN (Remote Access and LAN-to-LAN) using Certificate-based Authentication1,2	crypto ikev2 enable tunnel-group ipsec-attributes ikev2 remote-authentication certificate ikev2 local-authentication certificate
  SSH Service5,6	ssh

  1. Remote Access VPN features are enabled by choosing Devices > VPN > Remote Access in the Cisco FMC or Device > Remote Access VPN in Cisco Firepower Device Manager (FDM).
  2. The Clientless SSL VPN feature is not officially supported but can be enabled through FlexConfig.
  3. The HTTP feature is enabled by choosing Firepower Threat Defense Platform Settings > HTTP in the Cisco Firepower Management Console (FMC).
  4. HTTP service is accessible only from an IP address in the configured http command range.
  5. SSH is accessible only from an IP address in the configured ssh command range.
  6. There is no workaround that addresses this vulnerability for SSH access.

  
  

  Products Confirmed Not Vulnerable

  Only products listed in the Vulnerable Products section of this advisory are known to be affected by this vulnerability.

  Cisco has confirmed that this vulnerability does not affect Cisco ASA Software or FTD Software running on the following platforms:

  • 3000 Series Industrial Security Appliances (ISAs)
  • ASA 5525-X, 5545-X, 5555-X, and 5585-X Security Appliances
  • ASA Services Module for Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Routers
  • Adaptive Security Virtual Appliances (ASAv)
  • Firepower 2100 Series
  • Firepower 4100 Series
  • Firepower 9300 Series
  • Firepower Threat Defense Virtual (FTDv/NGFWv)
  • Secure Firewall 3100 Series

Workarounds

• There is a workaround that addresses this vulnerability for cryptographic keys that are used with SSL/TLS features only. Cryptographic keys for use with SSH cannot be imported to the device, so there is no workaround for SSH.

  To avoid the use of potentially weak cryptographic keys, administrators can generate a key pair and a corresponding certificate on a trusted device outside of the Cisco ASA or Cisco FTD device and then import the base 64 encoded PKCS #12 file that contains the keys and certificate(s) to the Cisco ASA or Cisco FTD device using the crypto ca import pkcs12 command in global configuration mode.

  For more details, including steps for how to accomplish this task using the Cisco Adaptive Security Device Manager (ASDM), see ASA 8.x: Renew and Install the SSL Certificate with ASDM.

  While this workaround has been deployed and was proven successful in a test environment, customers should determine the applicability and effectiveness in their own environment and under their own use conditions. Customers should be aware that any workaround or mitigation that is implemented may negatively impact the functionality or performance of their network based on intrinsic customer deployment scenarios and limitations. Customers should not deploy any workarounds or mitigations before first evaluating the applicability to their own environment and any impact to such environment.

Fixed Software

• When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories page, to determine exposure and a complete upgrade solution.

  In all cases, customers should ensure that the devices to be upgraded contain sufficient memory and confirm that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, customers are advised to contact the Cisco Technical Assistance Center (TAC) or their contracted maintenance providers.

  Cisco ASA, FMC, and FTD Software

  To help customers determine their exposure to vulnerabilities in Cisco ASA, FMC, and FTD Software, Cisco provides the Cisco Software Checker. This tool identifies any Cisco security advisories that impact a specific software release and the earliest release that fixes the vulnerabilities that are described in each advisory (“First Fixed”). If applicable, the tool also returns the earliest release that fixes all the vulnerabilities that are described in all the advisories that the Software Checker identifies (“Combined First Fixed”).

  To use the tool, go to the Cisco Software Checker page and follow the instructions. Alternatively, use the following form to search for vulnerabilities that affect a specific software release. To use the form, follow these steps:

  1. Choose which advisories the tool will search-all advisories, only advisories with a Critical or High Security Impact Rating (SIR), or only this advisory.
  2. Choose the appropriate software.
  3. Choose the appropriate platform.
  4. Enter a release number-for example, 9.16.2.11 for Cisco ASA Software or 6.6.7 for Cisco FTD Software.
  5. Click Check.

  Only this advisory All Critical and High advisories All advisories Cisco ASA Software Cisco FMC Software Cisco FTD Software Any Platform 3000 Series Industrial Security Appliances (ISA) ASA 5500-X Series Firewalls ASA Service Module Adaptive Security Virtual Appliance (ASAv) Firepower 1000 Series Firepower 2100 Series Firepower 4100 Series Firepower 9000 Series Secure Firewall 3100 Series

  Note: After upgrading to a fixed release, administrators must regenerate all ECDSA and RSA key pairs that were generated on an affected device while running a vulnerable release.

Exploitation and Public Announcements

• The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.

Source

• Cisco would like to thank ANSSI/CERT-FR, the French national and governmental CSIRT, for reporting this vulnerability.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asa5500x-entropy-6v9bHVYP

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	-	Final	2023-MAR-22

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