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             AUSCERT External Security Bulletin Redistribution

          Multiple Cisco Products Server Name Identification Data
                        Exfiltration Vulnerability
                             28 September 2021


        AusCERT Security Bulletin Summary

Product:           3000 Series Industrial Security Appliances (ISAs)
                   FTD Software
                   WSA Software
Publisher:         Cisco Systems
Impact/Access:     Unauthorised Access      -- Remote/Unauthenticated
                   Access Confidential Data -- Remote/Unauthenticated
Resolution:        Patch/Upgrade
CVE Names:         CVE-2021-34749  

Original Bulletin: 

Revision History:  September 28 2021: Vendor updated affected products and mitagation advice
                   August    19 2021: Initial Release

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Multiple Cisco Products Server Name Identification Data Exfiltration

Priority:        Medium
Advisory ID:     cisco-sa-sni-data-exfil-mFgzXqLN
First Published: 2021 August 18 16:00 GMT
Last Updated:    2021 September 27 16:31 GMT
Version 1.1:     Interim
Workarounds:     No workarounds available
Cisco Bug IDs:   CSCvy50873 CSCvy64824 CSCvy76771 CSCvz34379
CVE Names:       CVE-2021-34749
CWEs:            CWE-200


  o A vulnerability in the web filtering features of multiple Cisco products
    could allow an unauthenticated, remote attacker to bypass web reputation
    filters and threat detection mechanisms on an affected device and
    exfiltrate data from a compromised host to a blocked external server.

    This vulnerability is due to inadequate inspection of the Server Name
    Identification (SNI) header in the SSL/TLS handshake. An attacker could
    exploit this vulnerability by using data from the TLS client hello packet
    to communicate with a blocked external server. A successful exploit could
    be used to exfiltrate data from a protected network. The attacker must
    compromise a host on the network to exfiltrate the sensitive data.

    The following Snort rule can be used to detect possible exploitation of
    this vulnerability: Snort SID 58062.

    There are no workarounds that address this vulnerability.

    This advisory is available at the following link:

Affected Products

  o Vulnerable Products

    At the time of publication, this vulnerability affected all open source
    Snort project releases earlier than Release 2.9.18. For more information
    about open source Snort, see the Snort website .
    At the time of publication, this vulnerability affected the following Cisco
    products if they were configured with the SSL/TLS decryption option and
    were also using web reputation or URL filtering features:

       3000 Series Industrial Security Appliances (ISAs)
       4000 Series Integrated Services Routers (ISRs) (except 4321 ISRs)
       Catalyst 8000V Edge Software
       Catalyst 8200 Series Edge Platforms
       Catalyst 8300 Series Edge Platforms
       Cloud Services Router 1000V Series (CSR 1000V)
       Firepower Threat Defense (FTD) Software, if the SSL/TLS decryption
        option is enabled
       Integrated Services Virtual Router (ISRv)
       Web Security Appliance (WSA), both physical and virtual devices,
        deployed in transparent mode

    For information about which Cisco software releases were vulnerable at the
    time of publication, 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.

    Additional Considerations for FTD

    For an FTD device to be vulnerable, at least one SSL decryption policy must
    be enabled. As a prerequisite, an SSL decryption policy must be associated
    with a running access control policy.

    Determine Whether an SSL Decryption Policy Is Enabled on Cisco FTD Software

    There are two methods for determining whether an SSL decryption policy is

    Option 1: Use the CLI

    Use the show ssl-policy-config CLI command to verify whether an SSL
    decryption policy is enabled on a device. The following example shows the
    output of the show ssl-policy-config command on a device that does not have
    an SSL policy configured and is not vulnerable:

        > show ssl-policy-config
        SSL policy not yet applied.

    Any other output returned by the show ssl-policy-config command indicates
    that an SSL policy is configured and the device is vulnerable.

    For more information about the show ssl-policy-config command, see the
    Cisco Firepower Threat Defense Command Reference .

    Option 2: Use the GUI

    To determine whether an SSL decryption policy is enabled on a device, check
    the appropriate policy:

       For devices managed by Firepower Management Center (FMC) :
        Policies > Access Control > SSL
       For devices managed by Firepower Device Manager (FDM) :
        Policies > SSL Decryption

    Additional Considerations for WSA

    For a WSA device to be vulnerable, the HTTPS Proxy feature must be enabled
    and at least one decryption policy must be configured. The HTTPS Proxy
    feature is disabled by default.

    To determine whether the HTTPS Proxy feature is enabled for a WSA,
    administrators can log in to the web interface of the WSA and navigate to
    Security Services > HTTPS Proxy . The value in the HTTPS Proxy field
    indicates whether the feature is enabled or disabled.

    Products Confirmed Not Vulnerable

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

    The following products either already inspect the SNI header, do not
    perform SSL/TLS inspection, or do not use any web reputation or URL
    filtering feature to detect malicious web domains that could be used as
    recipients for data exfiltration. Therefore, Cisco has confirmed that these
    products are not affected by this vulnerability:

       1000 Series ISRs
       4321 ISRs
       Adaptive Security Appliance (ASA) Software
       Catalyst 8500 Series Edge Platforms
       Firepower Management Center (FMC) Software
       Meraki security appliances, all models
       Open source Snort 3


  o Using SNIcat or a similar tool, a remote attacker could exploit this
    vulnerability by sending sensitive data to an arbitrary server, hiding it
    in the SNI header of the TLS client hello packet. This vulnerability does
    not provide a means for an attacker to obtain the data in the first place;
    the attacker must have already compromised a host in a protected network to
    use for collecting and exfiltrating the confidential information.

    This advisory describes a filter bypass technique that could be used to
    circumvent protections that are based on web reputation filters, URL
    filtering, and threat detection. There is not a simple and deterministic
    way to identify every instance of data exfiltration because attackers may
    obfuscate the leaked data and use arbitrary, nonmalicious domains as
    recipients. However, Cisco is currently working to develop a solution that
    would extend the web reputation, URL filtering, or threat inspection
    features to the SNI header. This fix will mitigate this attack in cases
    where destination servers have a poor reputation or are already explicitly
    blocked by the administrators. This advisory will be updated once this
    solution becomes available.

    In the meantime, to detect and mitigate any attack performed with the
    SNIcat tool, Cisco has released a Snort rule with the SID 58062. To ensure
    full protection, the action for the rule should be set to Block .


  o For potential workarounds on a specific Cisco product, refer to the
    Description section in the bug ID(s) at the top of this advisory.

Fixed Software

  o 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

    Fixed Releases

    For information about fixed software releases , see the Details section in
    the bug ID(s) at the top of this advisory.

Exploitation and Public Announcements

  o The Cisco Product Security Incident Response Team (PSIRT) is aware that
    proof-of-concept exploit code is available for the vulnerability described
    in this advisory.

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


  o Cisco would like to thank Morten Marstrander and Alvaro Gutierrez from
    mnemonic, along with Matteo Malvica, for discovering and reporting this

Cisco Security Vulnerability Policy

  o To learn about Cisco security vulnerability disclosure policies and
    publications, see the Security Vulnerability Policy . This document also
    contains instructions for obtaining fixed software and receiving security
    vulnerability information from Cisco.


  o https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-sni-data-exfil-mFgzXqLN

Revision History

  o +---------+----------------------------+----------+---------+-------------+
    | Version |        Description         | Section  | Status  |    Date     |
    |         | Provided additional        |          |         |             |
    |         | information about the      | Multiple |         |             |
    | 1.1     | vulnerability, affected    | sections | Interim | 2021-SEP-27 |
    |         | products, and possible     |          |         |             |
    |         | mitigations.               |          |         |             |
    | 1.0     | Initial public release.    | -        | Interim | 2021-AUG-18 |

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