Security researcher: MIME flaw bypasses virus scanners

Some of the most popular anti-virus scanners on the market are open to exploitation according to a security researcher.

Hendrik Weimer, author of Quantenblog, said he had found a way of encoding viruses so they go undetected by most of the scanners he tested.

The exploit involves MIME encoding, which was the subject of serious concern two years ago. Weimer said the trick could be significantly more dangerous than a vulnerability that lets attackers bypass a single virus scanner. "Much rarer are discoveries of new attack classes that are able to blindfold not one but many virus scanners. Here is one," he said in a blog post.

Multipurpose Internet Mail Extensions (MIME) is used to encode emails so that they can be handled by Simple Mail Transport Protocol (SMTP), which is designed to handle text only. Base64 encoding, an encoding method that falls under the MIME standard, uses an alphabet of 64 characters, each representing a previously defined value.

MIME decoders are supposed to ignore characters that aren't part of the defined Base64 alphabet, but Weimer found that such characters inserted in an encoded message could allow dangerous content to bypass a virus filter. "Some virus scanners will happily pass viruses once they come in an unusual but still RFC-compliant encoding," he wrote. "This is even more astonishing given such attacks have already been discussed before."

He had even better success when some levels of multipart/mixed content were wrapped around the malicious attachment. "Then, only one of the six tested virus scanners was able to detect the EICAR file," he wrote. Researchers use EICAR files to test virus scanners. Vulnerable scanners included BitDefender Mail Protection for SMB 2.0, ClamAV 0.88.7, F-Prot Anti-virus for Linux x86 Mail Servers 4.6.61 and Kaspersky Anti-Virus for Linux Mail Server 5.5.10, he said.

F-Secure Anti-Virus for Linux Gateways 4.65 was less vulnerable, as was F-Prot in some situations, while Avast! for Linux/Unix Servers 2.0.0 and Barracuda Spam Firewall (Firmware version weren't vulnerable.

Weimer said a workaround was to use a separate daemon to perform correct MIME decoding, though not all antivirus products provide the command-line scanner necessary for this.

In 2004, information security consultancy Corsaire identified eight ambiguities in MIME that could allow dangerous content to slip past detection systems. The vulnerabilities were widespread and required many rounds of patching.

Join the newsletter!

Error: Please check your email address.
Rocket to Success - Your 10 Tips for Smarter ERP System Selection
Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Matthew Broersma
Show Comments

Cool Tech

SanDisk MicroSDXC™ for Nintendo® Switch™

Learn more >

Breitling Superocean Heritage Chronographe 44

Learn more >

Toys for Boys

Family Friendly

Panasonic 4K UHD Blu-Ray Player and Full HD Recorder with Netflix - UBT1GL-K

Learn more >

Stocking Stuffer

Razer DeathAdder Expert Ergonomic Gaming Mouse

Learn more >

Christmas Gift Guide

Click for more ›

Most Popular Reviews

Latest Articles


PCW Evaluation Team

Edwina Hargreaves

WD My Cloud Home

I would recommend this device for families and small businesses who want one safe place to store all their important digital content and a way to easily share it with friends, family, business partners, or customers.

Walid Mikhael

Brother QL-820NWB Professional Label Printer

It’s easy to set up, it’s compact and quiet when printing and to top if off, the print quality is excellent. This is hands down the best printer I’ve used for printing labels.

Ben Ramsden

Sharp PN-40TC1 Huddle Board

Brainstorming, innovation, problem solving, and negotiation have all become much more productive and valuable if people can easily collaborate in real time with minimal friction.

Sarah Ieroianni

Brother QL-820NWB Professional Label Printer

The print quality also does not disappoint, it’s clear, bold, doesn’t smudge and the text is perfectly sized.

Ratchada Dunn

Sharp PN-40TC1 Huddle Board

The Huddle Board’s built in program; Sharp Touch Viewing software allows us to easily manipulate and edit our documents (jpegs and PDFs) all at the same time on the dashboard.

George Khoury

Sharp PN-40TC1 Huddle Board

The biggest perks for me would be that it comes with easy to use and comprehensive programs that make the collaboration process a whole lot more intuitive and organic

Featured Content

Product Launch Showcase

Latest Jobs

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?