Tell HN: H&R Block tax software installs a TLS backdoor

157 points

3 months ago

Just a PSA for folks here in the US because tax season is coming up and some of you may be using H&R Block Business 2025. I discovered that the software installs a root CA named "WK ATX ServerHost 2024" (expiry 2049) into your local machine trusted root certificate store. They also helpfully include the private key to this certificate in a DLL file. This certificate does not identify itself as "H&R Block" anywhere and does not get uninstalled when you uninstall the software.

I've been able to successfully use this root CA + mitmproxy to manipulate TLS traffic on a brand new virtual machine on the same network with a DNS spoofing attack. Demo: https://www.youtube.com/watch?v=5paxvYkz1QE

To test if your machine is vulnerable visit this page: https://hrbackdoor.yifanlu.com and if you do not get any warning or error message from your browser then you have the backdoor installed. If your browser does complain, you can choose to visit the page anyways for more details on the vulnerability.

Is it negligence or a "real" back door? It's impossible to tell and since the private key is out there, anyone can use it so the point is moot. There is no legitimate reason why they need to install a wildcard root CA under a different name. When I contacted them about it their statement includes "similar findings have been identified through internal security assessments" meaning they know about this issue but have not fixed it. I would not trust H&R Block software at this point.

If you didn't get bit by this, congratulations. See this post as a reminder to audit your trusted root CA store.

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