Yahoo Mail Bug Squashed By Cyber Security Flaw Bounty Hunter

Security researchers have uncovered a flaw in Yahoo Mail that would have enabled hackers to snoop on user’s emails or use the accounts to spread viruses to other people.

Yahoo’s HackerOne bug bounty program was responsible for surfacing flaw, which was spotted by Finnish white hat Jouko Pynnonen, who was awarded $10,000 (£7,947) for his efforts.

The email company moved quickly to squash the bug before it could cause havoc through the use XXS (cross-site scripting) attacks.

Yahoo Mail bug

Normally yahoo Mail filters messages in HTML format to spot any malicious code in order to block it before it passed through a user’s browser.

However, Pynnonen discovered that this barrier could be breached by with an email including a custom HTML link. Upon opening the email, the code would immediately activate JavaScript to render a share button in the email but with broken HTML pointing to a whitelisted site. This enabled the code to sneak past the Yahoo Mail filter.

“As long as the URL pointed to a white-listed website such as YouTube, it was not further sanity checked or encoded. The value was used as is for setting a div innerHTML to create the button,” explained Pynnonen.

Using this technique, more malicious code could be injected into the HTML and cause malware and viruses to be spread without the Yahoo Mail filter blocking them.

“I tried creating an email with “abusive” data-* attributes and bingo!, found a pathological case pretty quickly.” Added Pynnonen, noting the scope of the attack vector he discovered at the year anniversary of the Yahoo Mail bug that had allowed malicious JavaScript code to be stuck into carefully crafted emails.

What was particularly notable about the bug was that the embedded HTML link did not need to be clicked by a user for the JavaScript code to be executed. As such, this vector of cyber attack could catch out people who are more savvy to the danger of clicking on dodgy links in emails.

Luckily for Yahoo, this bug was spotted before it has had time to be rigorously exploited, as one major hack attack and data leak this year has already caused major problems for Yahoo.

Are you a security expert? Try our quiz!

Roland Moore-Colyer

As News Editor of Silicon UK, Roland keeps a keen eye on the daily tech news coverage for the site, while also focusing on stories around cyber security, public sector IT, innovation, AI, and gadgets.

Recent Posts

Vodafone Germany Confirms 2,000 Job Losses, Amid European Restructuring

More downsizing at Vodafone after German operation announces 2,000 jobs will be axed, as automation…

15 hours ago

AI Poses ‘Jobs Apocalypse’, Warns Report

IPPR report warns AI could remove almost 8 million jobs in the United Kingdom, with…

16 hours ago

Europe’s Longest Hyperloop Test Track Opens

European Hyperloop Center in the Netherlands seeks to advance futuristic transport technology, despite US setbacks

17 hours ago

NHS Scotland Confirms Clinical Data Published By Ransomware Gang

NHS Dumfries and Galloway condemns ransomware gang for publishing patients clinical data after cyberattack earlier…

18 hours ago

Fewer People Using Twitter After Musk Takeover – Report

Research data suggests fewer people are using Elon Musk's X, but platform insists 250 million…

21 hours ago

Julian Assange Wins Temporary Reprieve For US Extradition Appeal

US assurances required. Julian Assange handed a slender reprieve in fight against his extradition to…

23 hours ago