Cisco Secure Email Gateway Filters Bypassed Due to Malware Scanner Issue

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An anonymous researcher has disclosed several methods that can be used to bypass some of the filters in Cisco’s Secure Email Gateway appliance and deliver malware using specially crafted emails.

In a November 14 post on the Full Disclosure mailing list, the researcher said they had been in contact with the vendor, but claimed they did not receive a satisfactory response within a given timeframe.

“As the attack complexity is low and exploits have already been published by a third party there must be no further delay in making the threads publicly known,” the researcher said.

The bypass methods rely on “error tolerance and different MIME decoding capabilities of email clients”. According to the researcher, an attacker could deliver a piece of malware to organizations protected by Cisco Secure Email Gateway through three different methods that work against Outlook, Thunderbird, Mutt, Vivaldi and other email clients.

Cisco has published a bug report in response to these findings, clarifying that some filtering features can be bypassed by a remote, unauthenticated attacker due to an issue with Sophos and McAfee malware scanning engines.

The networking giant said the problem affects its Secure Email Gateway product, formerly known as Cisco Email Security Appliance (ESA), when running with a default configuration.

“The issue is due to improper identification of potentially malicious emails or attachments. An attacker could exploit this issue by sending a malicious email with malformed Content-Type headers (MIME Type) through an affected device. An exploit could allow the attacker to bypass default anti-malware filtering features based on the affected scanning engines and successfully deliver malicious messages to the end clients,” Cisco explained.

The company told SecurityWeek that this is not a vulnerability in the Secure Email Gateway product itself.

“We have recommended a workaround to mitigate the issue in most scenarios,” a Cisco spokesperson said. “Cisco’s anti-malware providers have also committed to provide a fix that will be pushed directly to customers. No actions are needed on the part of the customer. Our top priority is the satisfaction and support of our customers and partners, and we are committed to communicating openly and transparently with our stakeholders.”

SecurityWeek has also reached out to Sophos and McAfee for comment. While McAfee has not responded, Sophos said it’s aware of the issue and it has already been working on hardening the integration of its scanning engine with Cisco’s product. The cybersecurity firm noted that, in the meantime, Cisco has provided a workaround for users.

The researcher who disclosed the bypass methods pointed to an open source toolkit designed for generating emails that test security products against such attacks. The researcher suggested that the workaround proposed by Cisco does address the methods exploited by the open source tool, but it does not prevent all of them.

Security bypass methods involving Multipurpose Internet Mail Extensions (MIME) headers have been known for many years and have been found in the products of several vendors.

Related: Microsoft Patches MotW Zero-Day Exploited for Malware Delivery

Related: Outlook Security Feature Bypass Allowed Sending Malicious Links

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