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Malicious Emails have the Potential to Bring Down Cisco Email Security Appliances

The vulnerability is caused by insufficient error handling in DNS name resolution.


Cisco notified customers this week that its Email Security Appliance (ESA) product is vulnerable to a high-severity denial of service (DoS) vulnerability that may be exploited using specially crafted emails. The CVE-2022-20653 vulnerability affects the DNS-based Authentication of Named Entities (DANE) email verification component of Cisco AsyncOS Software for ESA. It is remotely exploitable and does not require authentication. 

This vulnerability is caused by the software's insufficient error handling in DNS name resolution. An attacker could take advantage of this flaw by sending specially crafted email messages to a device that is vulnerable. A successful exploit could allow the attacker to make the device unavailable from management interfaces or to process additional email messages for a period of time until the device recovers, resulting in a denial of service (DoS) issue. Repeated attacks could render the gadget fully inoperable, resulting in a persistent DoS condition, said the company. 

This vulnerability affects Cisco ESA devices running a vulnerable version of Cisco AsyncOS Software with the DANE functionality enabled and downstream mail servers configured to deliver bounce messages. 

Customers can prevent exploitation of this vulnerability by configuring bounce messages from Cisco ESA rather than downstream reliant mail servers. While this workaround has been deployed and confirmed to be functional in a test environment, users should evaluate its relevance and efficacy in their own environment and under their own use conditions. Customers should be aware that any workaround or mitigation deployed may have a negative impact on network functioning or performance due to inherent customer deployment circumstances and limitations.

"Cisco has released free software updates that address the vulnerability described. Customers with service contracts that entitle them to regular software updates should obtain security fixes through their usual update channels. Customers may only install and expect support for software versions and feature sets for which they have purchased a license," the company said. 

Cisco has given credit to numerous persons who worked with the Dutch government's ICT services company DICTU for reporting the security flaw. According to the networking behemoth, there is no evidence of malicious exploitation. 

Cisco also issued two advisories this week, informing users of medium-severity issues impacting Cisco RCM for Cisco StarOS software (DoS vulnerability), as well as Cisco Prime Infrastructure and Cisco Evolved Programmable Network Manager (XSS vulnerability).
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