A security researcher has discovered a critical security bug in multiple F5 firewalls and load balancers that causes HTTPS encrypted connections to leak sensitive data. The bug affects almost 1,000 popular websites and website owners are advised to check for the vulnerability urgently.

The security flaw, known as Ticklebleed, was discovered by Cloudflare cryptography engineer Filippo Valsorda. Some packets – a unit of data routed between an origin and destination on the internet – sent by a Cloudflare customer using the Railgun web optimisation tool caused an error that confused Cloudflare's Railgun TLS stack.

When Valsorda and his colleagues investigated, they realised that if you were to deliberately craft some packets and send them to a vulnerable website, they would enable a hacker to obtain small chunks of information (31 bytes) residing in the memory of connected web servers.

Of course, 31 bytes sounds like only a tiny amount of data, and the information leaked could be completely innocent and inconsequential. There is always the risk that SSL session IDs and cryptographic keys, used to secure encrypted HTTPS connections, could be leaked. These keep your financial transactions on websites safe, for example during online shopping.

Valsorda contacted F5 and would not identify the websites affected by the vulnerability, but he says it affects at least 949 of the top one million websites on the internet, and he is concerned that the flaw could expose the same types of information as the serious Heartbleed OpenSSL cryptographic library bug, discovered in 2014.

Ars Technica used a publicly available tool to analyse Valsorda's vulnerability disclosure and discovered that the security bug is definitely affecting popular websites including Ancestry UK, Ancestry US, cloud-based payroll firm Paychex, South American online auctions site Mercado Libre, Argentinian national newspaper Clarín and the global internet advertising platform AppNexus, although the latter says its site has now been fixed.

Check to see if your site is affected as soon as possible

F5 has issued a security advisory with a comprehensive table listing all the specific BIG-IP SSL virtual server products that are vulnerable to this attack. Users with affected products are advised to eliminate the vulnerability by upgrading to a version of the virtual server that is known not to be vulnerable.

It is also a good idea to disable the Session Ticket option on all affected Client SSL profiles, which is what Cloudflare did. You can check whether your website is affected by using this tool.

"After realising the security implications of this issue we compartmentalised it inside the company, made sure our Support team would advise our customer to simply disable Session Tickets, and sought to contact F5," Valsorda wrote in a blog post.

"After a couple misdirected emails that were met by requests for Serial Numbers, we got in contact with the F5 SIRT, exchanged PGP keys, and provided a report and a PoC. It's unclear what data might be exfiltrated via this vulnerability, but Heartbleed and the Cloudflare Heartbleed Challenge taught us not to make assumptions of safety with uninitialised memory."