Cisco patches bug that could break its email security service with a single message

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Cisco has fixed a bug that could allow attackers to lock up its email security appliance with a single malicious email.

The bug, which has the ID CVE-2022-20653, affects Cisco's Email Security Appliance (ESA), an email security gateway product that detects and blocks email-borne malware, spam, and phishing attempts.

The problem lies in the ASyncOS operating system that the ESA uses, according to an advisory issued by the company this week.

The problem lies in the appliance's use of DNS-based Authentication of Named Entities (DANE) for security. DANE uses the more secure DNSSEC protocol to provide extra verification that a DNS record is legitimate. This makes it harder for malicious actors to spoof digital certificates or use man-in-the-middle attacks to misdirect DNS requests.

However, Cisco found that ASyncOS was unable to properly handle DNS name resolution, opening it up to exploit through malicious inputs.

In this case, the malicious input would be an email and, if crafted correctly, could freeze the appliance's management interface and stop it processing further emails until it had recovered.

Cisco has classified the vulnerability, which has a CVSS score of 7.5, as a denial of service (DoS) bug.

"Continued attacks could cause the device to become completely unavailable, resulting in a persistent DoS condition," Cisco warned.


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The DANE feature is not enabled by default, meaning that only those who have activated it will be affected. Those customers can install Cisco's software updates to fix the problem.

In the meantime, customers can also configure bounce messages from the ESA instead of from downstream dependent email servers to stop attackers exploiting the bug, the company said.

The ASyncOS software saw two other reported vulnerabilities last year. CVE-2021-1566 was a bug in its Cisco Advanced Malware protection for Endpoints integration, allowing the interception of remote traffic. The other, CVE-2021-1359, allowed attackers to gain root privileges.

Danny Bradbury

Danny Bradbury has been a print journalist specialising in technology since 1989 and a freelance writer since 1994. He has written for national publications on both sides of the Atlantic and has won awards for his investigative cybersecurity journalism work and his arts and culture writing. 

Danny writes about many different technology issues for audiences ranging from consumers through to software developers and CIOs. He also ghostwrites articles for many C-suite business executives in the technology sector and has worked as a presenter for multiple webinars and podcasts.