Bug exposes OpenSSH servers to brute-force password guessing attacks

The keyboard-interactive authentication setting could allow for thousands of password retries, a researcher found

Digital key

Digital key

A bug in OpenSSH, the most popular software for secure remote access to UNIX-based systems, could allow attackers to bypass authentication retry restrictions and execute many password guesses.

A security researcher who uses the online alias Kingcope disclosed the issue on his blog last week, but he only requested a public vulnerability ID to be assigned Tuesday.

By default, OpenSSH servers allow six authentication retries before closing a connection and the OpenSSH client allows three incorrect password entries, Kingcope said.

However, OpenSSH servers with keyboard-interactive authentication enabled, which is the default setting on many systems, including FreeBSD ones, can be tricked to allow many authentication retries over a single connection, according to the researcher.

"With this vulnerability an attacker is able to request as many password prompts limited by the 'login grace time' setting, that is set to two minutes by default," Kincope said.

Depending on the server and the connection, two minutes could allow for thousands of retries, which could be enough to guess common or weak passwords using dictionary-based attacks.

According to a discussion on Reddit, setting PasswordAuthentication to 'no' in the OpenSSH configuration and using public-key authentication does not prevent this attack, because keyboard-interactive authentication is a different subsystem that also relies on passwords.

Therefore, users should set ChallengeResponseAuthentication and KbdInteractiveAuthentication to 'no' in their configurations.

Join the PC World newsletter!

Error: Please check your email address.

Tags redditsecurityAccess control and authenticationExploits / vulnerabilities

Our Back to Business guide highlights the best products for you to boost your productivity at home, on the road, at the office, or in the classroom.

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

Lucian Constantin

IDG News Service
Show Comments

Essentials

Microsoft L5V-00027 Sculpt Ergonomic Keyboard Desktop

Learn more >

Lexar® JumpDrive® S57 USB 3.0 flash drive

Learn more >

Mobile

Lexar® JumpDrive® S45 USB 3.0 flash drive 

Learn more >

Exec

Audio-Technica ATH-ANC70 Noise Cancelling Headphones

Learn more >

HD Pan/Tilt Wi-Fi Camera with Night Vision NC450

Learn more >

Lexar® JumpDrive® C20c USB Type-C flash drive 

Learn more >

Lexar® Professional 1800x microSDHC™/microSDXC™ UHS-II cards 

Learn more >

Budget

Back To Business Guide

Click for more ›

Most Popular Reviews

Latest News Articles

Resources

PCW Evaluation Team

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.

Aysha Strobbe

Windows 10 / HP Spectre x360

Ultimately, I think the Windows 10 environment is excellent for me as it caters for so many different uses. The inclusion of the Xbox app is also great for when you need some downtime too!

Mark Escubio

Windows 10 / Lenovo Yoga 910

For me, the Xbox Play Anywhere is a great new feature as it allows you to play your current Xbox games with higher resolutions and better graphics without forking out extra cash for another copy. Although available titles are still scarce, but I’m sure it will grow in time.

Featured Content

Latest Jobs

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?