Google, Microsoft and Yahoo fix serious email weakness

Use of weak DKIM signing keys could allow spoofed email messages to look legitimate, US-CERT warned

Google, Microsoft and Yahoo have remedied a cryptographic weakness in their email systems that could allow an attacker to create a spoofed message that passes a mathematical security verification.

The weakness affects DKIM, or DomainKeys Identified Mail, a security system used by major email senders. DKIM wraps a cryptographic signature around an email that verifies the domain name through which the message was sent, which helps more easily filter out spoofed messages from legitimate ones.

The problem lies with signing keys that are less than 1,024 bits, which can be factored due to increasing computer power. US-CERT said in an advisory issued Wednesday that signing keys less than 1,024 bits are weak, and that keys up to RSA-768 bits have been factored.

The issue came to light after Florida-based mathematician Zachary Harris was sent an email from a Google recruiter that used only a 512-bit key, according to a report published Wednesday by Wired magazine.

Thinking it might be some clever test by Google, he factored the key, then used it to send a spoofed message from Sergey Brin to Larry Page, Google's founders.

It wasn't a test but in fact a serious problem, one in which emails that could be bogus would be trusted. According to the DKIM standard, email messages that have keys shorter that 1,024 bits are not necessarily rejected.

Harris found the problem wasn't limited to Google, but also Microsoft and Yahoo, all of whom appeared to have fixed the issue as of two days ago, according to US-CERT. Harris told Wired he found either 512-bit or 768-bit keys in use at PayPal, Yahoo, Amazon, eBay, Apple, Dell, LinkedIn, Twitter, SBCGlobal, US Bank, HP, Match.com and HSBC.

Weak signing keys are a boon for cybercriminals. They selectively target people with emails containing malicious links in an attempt to exploit a computer's software and install malware, a style of attack known as spear phishing. If an email contains the correct DKIM signature, it's more likely to end up in a recipient's inbox.

US-CERT also warned of another problem. The DKIM specification allows a sender to flag that it is testing DKIM in messages. Some recipients will "accept DKIM messages in testing mode when the messages should be treated as if they were not DKIM signed," US-CERT said.

Send news tips and comments to jeremy_kirk@idg.com. Follow me on Twitter: @jeremy_kirk

Join the Good Gear Guide newsletter!

Error: Please check your email address.
Rocket to Success - Your 10 Tips for Smarter ERP System Selection

Tags securityU.S. Computer Emergency Readiness Teamencryption

Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Jeremy Kirk

IDG News Service
Show Comments

Most Popular Reviews

Latest Articles

Resources

PCW Evaluation Team

Matthew Stivala

HP OfficeJet 250 Mobile Printer

The HP OfficeJet 250 Mobile Printer is a great device that fits perfectly into my fast paced and mobile lifestyle. My first impression of the printer itself was how incredibly compact and sleek the device was.

Armand Abogado

HP OfficeJet 250 Mobile Printer

Wireless printing from my iPhone was also a handy feature, the whole experience was quick and seamless with no setup requirements - accessed through the default iOS printing menu options.

Azadeh Williams

HP OfficeJet Pro 8730

A smarter way to print for busy small business owners, combining speedy printing with scanning and copying, making it easier to produce high quality documents and images at a touch of a button.

Andrew Grant

HP OfficeJet Pro 8730

I've had a multifunction printer in the office going on 10 years now. It was a neat bit of kit back in the day -- print, copy, scan, fax -- when printing over WiFi felt a bit like magic. It’s seen better days though and an upgrade’s well overdue. This HP OfficeJet Pro 8730 looks like it ticks all the same boxes: print, copy, scan, and fax. (Really? Does anyone fax anything any more? I guess it's good to know the facility’s there, just in case.) Printing over WiFi is more-or- less standard these days.

Ed Dawson

HP OfficeJet Pro 8730

As a freelance writer who is always on the go, I like my technology to be both efficient and effective so I can do my job well. The HP OfficeJet Pro 8730 Inkjet Printer ticks all the boxes in terms of form factor, performance and user interface.

Michael Hargreaves

Windows 10 for Business / Dell XPS 13

I’d happily recommend this touchscreen laptop and Windows 10 as a great way to get serious work done at a desk or on the road.

Featured Content

Latest Jobs

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?