Summary

• Multiple vulnerabilities in the Application Level Gateway (ALG) for the Network Address Translation (NAT) feature of Cisco Adaptive Security Appliance (ASA) Software and Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to bypass the ALG and open unauthorized connections with a host located behind the ALG.

  For more information about these vulnerabilities, see the Details section of this advisory.

  Note: These vulnerabilities have been publicly discussed as NAT Slipstreaming. 

  Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability.

  This advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-natalg-bypass-cpKGqkng

  This advisory is part of the October 2021 release of the Cisco ASA, FTD, and FMC Security Advisory Bundled publication. For a complete list of the advisories and links to them, see Cisco Event Response: October 2021 Cisco ASA, FMC, and FTD Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  At the time of publication, this vulnerability affected Cisco ASA Software and Cisco FTD Software. 

  For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  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 Firepower Management Center (FMC) Software.

Details

• The vulnerabilities are not dependent on one another. Exploitation of one of the vulnerabilities is not required to exploit the other vulnerability. In addition, a software release that is affected by one of the vulnerabilities may not be affected by the other vulnerability.

  Details about the vulnerabilities are as follows:

  CVE-2021-34790: Cisco ASA Software and Cisco FTD Software Session Initiation Protocol ALG Bypass Vulnerability

  A vulnerability in the Session Initiation Protocol (SIP) ALG for the NAT feature of Cisco ASA Software and Cisco FTD Software could allow an unauthenticated, remote attacker to bypass the ALG and open unauthorized connections with a host located behind the ALG. The SIP ALG is enabled by default.

  This vulnerability is due to insufficient traffic validation for the SIP ALG. An attacker could exploit this vulnerability by sending crafted SIP traffic through the ALG to a host located behind the security device. A successful exploit could allow the attacker to open unauthorized connections to the remote host and undertake actions leveraging that host.

  Bug ID(s): CSCvw35444
  CVE ID: CVE-2021-34790
  Security Impact Rating (SIR): Medium
  CVSS Base Score: 4.7
  CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N

  CVE-2021-34791: Cisco ASA Software and Cisco FTD Software File Transfer Protocol ALG Bypass Vulnerability

  A vulnerability in the File Transfer Protocol (FTP) ALG for the NAT feature of Cisco ASA Software and Cisco FTD Software could allow an unauthenticated, remote attacker to bypass the ALG and open unauthorized connections with a host located behind the ALG. The FTP ALG is enabled by default.

  This vulnerability is due to insufficient traffic validation for the FTP ALG. An attacker could exploit this vulnerability by sending crafted FTP traffic through the ALG to a host located behind the security device. A successful exploit could allow the attacker to open unauthorized connections to the remote host and undertake actions leveraging that host.

  Bug ID(s): CSCvx50914
  CVE ID: CVE-2021-34791
  Security Impact Rating (SIR): Medium
  CVSS Base Score: 4.7
  CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:N

Workarounds

• There are no workarounds that address these vulnerabilities. However, there is a mitigation.

  Customers who do not need the SIP or FTP ALG may disable either or both ALGs to reduce the attack surface. This can be accomplished from the CLI for Cisco ASA Software or by using FlexConfig for Cisco FTD Software.

  While this mitigation 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.

  Fixed Releases

  At the time of publication, the release information in the following table(s) was accurate. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

  The left column lists Cisco software releases, and the right column indicates whether a release was affected by the vulnerability described in this advisory and which release included the fix for this vulnerability.

  ASA Software

  Cisco ASA Software Release	First Fixed Release for All the Vulnerabilities Described in this Advisory
  Earlier than 9.81	Migrate to a fixed release.
  9.8	9.8.4.40
  9.12	9.12.4.18
  9.13	Migrate to a fixed release.
  9.14	9.14.2.15
  9.15	9.15.1.15
  9.16	Not vulnerable.

  1. Cisco ASA Software releases 9.7 and earlier as well as releases 9.9, 9.10, and 9.13 have reached end of software maintenance. Customers are advised to migrate to a supported release that includes the fix for these vulnerabilities.

  FTD Software

  Cisco FTD Software Release	First Fixed Release for All the Vulnerabilities Described in this Advisory
  6.2.2 and earlier1	Migrate to a fixed release.
  6.2.3	Migrate to a fixed release.
  6.3.0	Migrate to a fixed release.
  6.4.0	6.4.0.12
  6.5.0	Migrate to a fixed release.
  6.6.0	6.6.5
  6.7.0	6.7.0.2
  7.0.0	Not vulnerable.

  1. Cisco FMC and FTD Software releases 6.2.2 and earlier, as well as releases 6.3.0 and 6.5.0, have reached end of software maintenance. Customers are advised to migrate to a supported release that includes the fix for these vulnerabilities.

  For instructions on upgrading your FTD device, see Cisco Firepower Management Center Upgrade Guide.

  The Cisco Product Security Incident Response Team (PSIRT) validates only the affected and fixed release information that is documented in this advisory.

Exploitation and Public Announcements

• The Cisco PSIRT is aware that proof-of-concept exploit code is available for the vulnerabilities described in this advisory.

  The Cisco PSIRT is not aware of any malicious use of the vulnerabilities that are described in this advisory.

Source

• Cisco was made aware of these vulnerabilities through public discussion of the NAT Slipstreaming attacks.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-natalg-bypass-cpKGqkng

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2021-OCT-27

  Show Less

---