Update 2019-08-01: Fixed packages are now available. QSB #050 has been updated accordingly.
We have just published Qubes Security Bulletin (QSB) #050: Reinstalling a TemplateVM does not reset the private volume. The text of this QSB is reproduced below. This QSB and its accompanying signatures will always be available in the Qubes Security Pack (qubes-secpack).
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---===[ Qubes Security Bulletin #50 ]===--- 2019-08-01 Reinstalling a TemplateVM does not reset the private volume History ======== 2019-08-01: Added list of fixed packages and patching instructions 2019-07-24: Initial version Description ============ In Qubes OS, we have the ability to reinstall a TemplateVM by running `qubes-dom0-update --action=reinstall qubes-template-...` in dom0.  This is supposed to reset the corresponding TemplateVM to the state of the published package, i.e., no local changes should remain. One uncommon reason to perform such a reinstallation is that you suspect that a TemplateVM may be compromised. In such cases, it is very important that no local changes persist in order to ensure that the TemplateVM is no longer compromised. Due to a regression in R4.0 , however, reinstalling a TemplateVM using qubes-dom0-update does not completely reset all local changes to that TemplateVM. Although the tool itself and our documentation claim that the private volume of the TemplateVM is reset during reinstallation, the private volume does not actually get reset. This could allow a TemplateVM to remain compromised across a reinstallation of that TemplateVM using qubes-dom0-update. Patching ========= The specific packages that resolve the problems discussed in this bulletin are as follows: For Qubes 4.0: - qubes-core-admin-client, python3-qubesadmin version 4.0.26 The packages are to be installed in dom0 via the Qubes VM Manager or via the qubes-dom0-update command as follows: For updates from the stable repository (not immediately available): $ sudo qubes-dom0-update For updates from the security-testing repository: $ sudo qubes-dom0-update --enablerepo=qubes-dom0-security-testing These packages will migrate from the security-testing repository to the current (stable) repository over the next two weeks after being tested by the community. Workaround =========== Independently of patching (see above), the following workaround is available: Rather than using the qubes-dom0-update method of reinstalling a TemplateVM, you can instead manually remove the TemplateVM, then install it again. Detailed instructions for this manual method are documented here: https://www.qubes-os.org/doc/reinstall-template/#manual-method Credits ======== Thank you to Andrey Bienkowski <firstname.lastname@example.org> for discovering and reporting this issue. References ===========  https://www.qubes-os.org/doc/reinstall-template/  https://github.com/QubesOS/qubes-core-admin-linux/commit/552fd062ea2bb6c2d05faa1e64e172503cacbdbf#diff-6b87ee5cdb9e63b703415a14e5a505cdL192 -- The Qubes Security Team https://www.qubes-os.org/security/