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 3.4.10.071) 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!

Or

Sign up to gain exclusive access to email subscriptions, event invitations, competitions, giveaways, and much more.

Membership is free, and your security and privacy remain protected. View our privacy policy before signing up.

Error: Please check your email address.
Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Matthew Broersma

Techworld.com
Show Comments

Brand Post

Most Popular Reviews

Latest Articles

Resources

PCW Evaluation Team

Luke Hill

MSI GT75 TITAN

I need power and lots of it. As a Front End Web developer anything less just won’t cut it which is why the MSI GT75 is an outstanding laptop for me. It’s a sleek and futuristic looking, high quality, beast that has a touch of sci-fi flare about it.

Emily Tyson

MSI GE63 Raider

If you’re looking to invest in your next work horse laptop for work or home use, you can’t go wrong with the MSI GE63.

Laura Johnston

MSI GS65 Stealth Thin

If you can afford the price tag, it is well worth the money. It out performs any other laptop I have tried for gaming, and the transportable design and incredible display also make it ideal for work.

Andrew Teoh

Brother MFC-L9570CDW Multifunction Printer

Touch screen visibility and operation was great and easy to navigate. Each menu and sub-menu was in an understandable order and category

Louise Coady

Brother MFC-L9570CDW Multifunction Printer

The printer was convenient, produced clear and vibrant images and was very easy to use

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.

Featured Content

Product Launch Showcase

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?