SHA256 and Authenticode REDUX^2

Note: Microsoft has not confirmed this change yet; analysis below comes from looking at behavior of 14 signed installers.

In December of last year, I wrote about all of the different places hashes are used in code-signing. Then, in January I blogged that Windows 10 had stopped accepting SHA-1 certificates and certificate chains for Authenticode-signed binaries (unless a timestamp marked the binary as being signed before 1/1/2016).

I called out the fact that while SHA1 certificates were verboten, “SHA1 file digests are still allowed (heck, MD5 digests are still allowed!)”.

With Windows 10 build 14316, things have changed again. It appears that SHA-1 file digests are now forbidden too, at least in the download codepath of Edge and Internet Explorer. This interrupts the download of Firefox, Opera, Fiddler, and other programs:

Firefox signature rejected

Fiddler signature rejected

As with the earlier lockdown, if you examine the signature in Windows Explorer, it’ll tell you that everything is “OK.”

File Hash is SHA1 and Cert is SHA256, UI shows 'ok'

To resolve this, you should dual-sign your binaries using both SHA-1 and SHA-256. It appears that the same Windows 10 14316 build has a bug whereby a signature containing only a SHA256 file digest will cause the download in IE/Edge to hang at 99%.

-Eric

SHA256 and Authenticode REDUX^2

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