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AUSCERT External Security Bulletin Redistribution
Multiple Cisco Products Server Name Identification Data
28 September 2021
AusCERT Security Bulletin Summary
Product: 3000 Series Industrial Security Appliances (ISAs)
Publisher: Cisco Systems
Impact/Access: Unauthorised Access -- Remote/Unauthenticated
Access Confidential Data -- Remote/Unauthenticated
CVE Names: CVE-2021-34749
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
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
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:
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
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.
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
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
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.
| 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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