What We Know About Cross-Signing for Down-Level Windows Versions As of Today (2 March 2021)

@“Peter_Viscarola_(OSR)” said:

  • The catch: I can’t find a way to purchase a relevant code-signing cert from any of these CAs (AddTrust and UTN-USERFirst-Object are both part of Sectigo now). If you can, let us know!

Hi Peter

Purchasing from Entrust is only possible from the following link :

https://www.entrust.com/digital-security/certificate-solutions/products/digital-signing/code-signing-certificates

Which then redirects you to filling up a form so they can contact you. I guess you can specify the specific CA in the request form.

Sectigo is also the following :

https://sectigo.com/ssl-certificates-tls/code-signing

But i couldn’t find anything for code signing in GoDaddy or Starfield, i guess the best way is to contact them.

We contacted Entrust and as you know got a vague answer, but we suggest other folks to also contact Entrust and the other 4 companies so maybe we can get a clear answer.

@“Peter_Viscarola_(OSR)” said:

  • One solution that we know works on Windows 7: Attestation Sign your driver package and binary for Windows 10. The resulting driver package will install on Windows 7 (with KB4474419, which enabled SHA-256 signing installed). Note that this works as long as you DO NOT sign the driver binary with your own signature before you submit it to MSFT for Attestation Signing.

Well now, there’s something I’ll want to go test. What I recall is that the attestation signing process generated a .CAT containing only the platform IDs selected during submission, which are all Windows 10 variants. And that attested installation set was rejected as a signed driver during installation on the Windows 7 platform or any other platforms, because there was effectively “no .CAT file” (valid for those platforms).

But indeed we are signing the binaries prior to attested submission, specifically because our intention is to run on both Windows 10 and pre-Windows 10. Once the dual-signed binaries are received back from Microsoft, we are able to generate a second .INF and .CAT, and sign that .CAT ourselves. In order to have a working “pre-Windows 10” .INF to install with on such platforms, while using the Microsoft-generated .CAT for installing Windows 10 platforms. i.e. Same binaries, two different .INFs and .CATs.

If Microsoft is now including a Windows 7-compatible platform ID in the .CAT created for attested signing – if you didn’t sign the driver binaries ahead of time – indeed that opens up an additional useful option. An option that Microsoft could withdraw at any moment, of course, but how many aspects of this aren’t exactly like that anyway.

If Microsoft is now including a Windows 7-compatible platform ID in the .CAT created for attested signing

There’s no chance that’s happening, so I don’t know how it works. I only know that it does work, and this has been verified and repro’ed by multiple folks.

I suspect the CAT isn’t used on Win7, however, because on Win7 I can edit the INF file on the target install system, and the driver still installs without error.

Ugly, huh?

Peter

Purchasing from Entrust is only possible from the following link

I have an enquiry in to Entrust as of this morning to purchase one of their certs. I’ll post back what I hear.

Peter

@“Peter_Viscarola_(OSR)” said:
I suspect the CAT isn’t used on Win7, however, because on Win7 I can edit the INF file on the target install system, and the driver still installs without error.

That certainly would seem unusual, since neither actual WHQL signed packages nor cross-signed packages have worked that way on Windows 7 platform previously. Meaning you do get the angry red “installation set has been tampered with” warning when modifying the .INF away from the content it was signed with, which confirms both the .CAT and the contained hash for the .INF file were being verified.

I modified our signing process to no longer pre-sign the binaries with our company code signing certificate, such that only the Attested signing signatures would be present on the binaries. For what it’s worth, neither the “.CAT isn’t used on Win7” assertion, nor the “Attested package installs without error if you didn’t pre-sign the binaries” assertion held true in testing here.

Your expectation regarding the OS attribute list in the .CAT was correct, and by selecting all non-ARM64 platforms during attested signing, the .CAT we received only contains the Windows 10 platforms. Which was the same as what we receive when we do pre-sign the binaries:

_v100,_v100_X64,_v100_RS1,_v100_X64_RS1,_v100_RS2,_v100_X64_RS2,_v100_RS3,_v100_X64_RS3,_v100_RS4,_v100_X64_RS4,_v100_RS5,_v100_X64_RS5,_v100_19H1,_v100_X64_19H1,_v100_Vb,_v100_X64_Vb

But on a Windows 7 Professional x64 machine up to date with SHA-2 support, with this Microsoft Attestation-only signed package I get the “Windows can’t verify the publisher of this driver software” as the interactive angry red publisher verification message. The setupapi.dev.log confirms during _VERIFY_FILE_SIGNATURE that its attempt to verify the .INF file is what failed:

Verifying file against specific (valid) catalog failed! (0xe0000244)
“Error 0xe0000244: This software was tested for compliance with Windows Logon requirements on a different version of Windows, and may not be compatible with this version” as the reason the publisher verification failure was presented.

I’m not a hardware driver, though. This is for a NetClient-class component being installed through INetCfg/NetSetup. So maybe that’s enough to explain the split in the observations. But “signing of driver package for hardware drivers is ignored” still seems like a highly unusual proposition to accept. Maybe knowing what setupapi.dev.log revealed during these multiple successful verifications of the behavior might work toward explaining it, too.

Windows 7 does not actually require a CAT file. That didn’t start until Windows 8.

@Tim_Roberts said:
Windows 7 does not actually require a CAT file. That didn’t start until Windows 8.

This list knows better than I do, so I’ll assume for now this is also a “when installing hardware drivers” scenario that I don’t get to experience with my component. What I can say is that during my component’s .INF-based driver installation on Windows 7 x64, the setupapi.dev.log is clearly citing both the .INF file being executed and the .CAT file it’s using in it’s attempt to verify the signed status of this .INF. And this verification fails when the .CAT file is not valid for the Windows 7 platform.

That certainly sounds like a good and fortunate scenario for hardware drivers though, if the same doesn’t apply to hardware drivers. Allowing them to employ Attested signing to work around the inability to cross-sign, and then rely on the absence of .CAT-based verification when that same valid-for-Windows-10 .INF is invoked on Windows 7 platform.

Just got notified of this lovely little addition to the page on cross-signing:

Will I be able to continue signing drivers if my certificate chains to a cross-cert that expires after 2021?
No, using any code signing certificate to sign a kernel-mode driver without a WHQL signature after July 1st, 2021 is a violation of Microsoft Trusted Root Program (TRP) policy. Certificates in violation of Microsoft TRP policies will be revoked.

So, yeah… there’s that.

Peter

@Alan_Adams

on a Windows 7 Professional x64 machine up to date with SHA-2 support, with this Microsoft Attestation-only signed package I get the “Windows can’t verify the publisher of this driver software” as the interactive angry red publisher verification message.

Yes… you get the angry red pop-up. BUT… in our case, the driver installed successfully, even after that.

Looking at setupapi.dev.log, we see:

sig:                          Verifying file against specific (valid) catalog failed! (0xe0000244)

! sig: Error 0xe0000244: The software was tested for compliance with Windows Logo requirements on a different version of Windows, and may not be compatible with this version.
sig: {_VERIFY_FILE_SIGNATURE exit(0xe0000244)} 14:44:36.474
!!! sto: An unexpected error occurred while validating driver package. Assuming that driver package is unsigned. Catalog = Nothing_KMDF.cat, Error = 0xE0000244
! sto: Driver package is considered unsigned, but user wants to install driver package anyway.

Now… on Windows 8.1, trying to install the same PnP driver, the process fails with the usual error:

Peter

@CaptainFlint

Because you are in the process of getting an Entrust cert, I wanted to be sure you saw the post above about MSFT threatening to revoke the cert of anybody who cross-signs after 1 July 2020.

Peter

@“Peter_Viscarola_(OSR)” said:
Because you are in the process of getting an Entrust cert, I wanted to be sure you saw the post above about MSFT threatening to revoke the cert of anybody who cross-signs after 1 July 2020.

Yes, thank you for your concern, I appreciate it! I have already noticed that remark earlier, and it is indeed highly unsettling. But we need a new EV certificate in any case; our current one is close to expiring. It doesn’t matter if we get the new certificate from Entrust or any other CA, as long as MS recognizes it for the Dashboard company account. Therefore, we are continuing the process of purchasing. If we’re unable to also use the certificate for cross-signing, like we did before — well, there’s nothing we can do about it…

In that case, I’d go with the cheapest supported EV cert, whixh should be
ANY?

it is indeed highly unsettling

That is nicely said. Yes, exactly.

If we’re unable to also use the certificate for cross-signing, like we did before — well, there’s nothing we can do about it…

Sad, but true.

Well… MSFT new policy saved OSR US$1K (the cost of a 3-year Entrust OV code signing cert). So, that’s something I guess.

Peter

@“Peter_Viscarola_(OSR)” said:
Just got notified of this lovely little addition to the page on cross-signing:

Will I be able to continue signing drivers if my certificate chains to a cross-cert that expires after 2021?
No, using any code signing certificate to sign a kernel-mode driver without a WHQL signature after July 1st, 2021 is a violation of Microsoft Trusted Root Program (TRP) policy. Certificates in violation of Microsoft TRP policies will be revoked.

So, yeah… there’s that.

Peter

Well there’s that i guess, not a good news.

So what’s next? Has Microsoft made its final decision and not backing down?

@“Peter_Viscarola_(OSR)” said:
Just got notified of this lovely little addition to the page on cross-signing:

Will I be able to continue signing drivers if my certificate chains to a cross-cert that expires after 2021?
No, using any code signing certificate to sign a kernel-mode driver without a WHQL signature after July 1st, 2021 is a violation of Microsoft Trusted Root Program (TRP) policy. Certificates in violation of Microsoft TRP policies will be revoked.

So, yeah… there’s that.

Peter

So lets say we buy a 3 year cert from Entrust, then if the cert expires in 3 years, how can Microsoft even know when did we cross sign it? considering its not expired yet, we don’t even need to timestamp the certificate, therefore we can cross sign it locally without network connection, and load it successfully. Am i missing something here?

The cross-certificates are issued BY MICROSOFT. They can certainly interfere in the trust chain. I don’t think they could make any currently signed packages go invalid, but they can certainly prevent any new packages from being signed.

They can.
If the excerpt from the license is true (that cross signing after
April 1st is against the license), they can revoke your cert, thus
making it invalid for all machines that update to thee new CRL.

If you do it locally, I doubt anything will happen, but if the driver
goes into the wild, they can catch it easily.
Whether they wish to revoke certs that were not abused to make havoc
is a different thing.

But they are making us leave Windows :frowning:

The cross-certificates are issued BY MICROSOFT. They can certainly
interfere in the trust chain. I don’t think they could make any currently
signed packages go invalid, but they can certainly prevent any new packages
from being signed.


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@Dejan_Maksimovic said:
If you do it locally, I doubt anything will happen, but if the driver
goes into the wild, they can catch it easily.
Whether they wish to revoke certs that were not abused to make havoc
is a different thing.

@Tim_Roberts said:
The cross-certificates are issued BY MICROSOFT. They can certainly interfere in the trust chain. I don’t think they could make any currently signed packages go invalid, but they can certainly prevent any new packages from being signed.

But how can they even know when i signed the driver if i do it locally without even using a timestamp?!! i can literally revert the date of my signing machine to 2020/early-2021, sign it locally without internet connection, and then its done. how on earth can they stop this?

And we are not even using our drivers in the wild, we want this in our local company for less than 40 computers, ranging from windows 7 to 10. and we HAVE TO support old windows 7 machines and cannot upgrade them.

But our main question is, if we purchase a 3 year code signing EV certificate from Entrust right now, does this mean that we can cross sign our drivers locally without using a timestamp server for 3 years? again i have to emphasize that we want this for our company’s computers, its not a commercial driver… (and NO, we do not want to use test signing mode…)

I don’t think anyone knows the answer to that.

@henrik_meida said:
But our main question is, if we purchase a 3 year code signing EV certificate from Entrust right now, does this mean that we can cross sign our drivers locally without using a timestamp server for 3 years? again i have to emphasize that we want this for our company’s computers, its not a commercial driver… (and NO, we do not want to use test signing mode…)

Not using timestamps means, that in 3 years all the drivers’ signatures will become invalid with the expiration of the main certificate. Because timestamping prevents exactly that: it identifies, that you signed the driver at the moment in time, when all the involved certificates were valid.

Technically, if MS wants to push it real hard, they can simply revoke the non-expired cross-certificates, and signing will become… not really impossible, but problematic at best. And, at a guess, just to use drivers signed with a revoked cross-certificate, you’d have to prevent all your machines from updating the CRLs (which in itself is a bit dangerous: certificates may be revoked due to them being used maliciously).

As Tim said, nobody knows for sure, we can only guess what will happen after July 1. I personally am inclined to think, that MS will not go to such drastic measures, and that internal-only use should be safe, even with timestamping (I don’t think timestamp CAs report to Microsoft; even more, I’m not even sure CAs themselves know what certificate you are using; most probably, they receive just a hash of the file). But, on the other hand, I would never have believed, that one day MS would blow up the whole cross-certificate system, and yet here we are…