Enhancing RHEL Security: Understanding SHA-1 deprecation on RHEL 9

While Red Hat Enterprise Linux 9 (RHEL 9) deprecated SHA-1 for signing for security reasons, it is still used by many for signing packages. In this article, I’ll go over some typical problems users may face with Fedora SHA-1 status (including some possible workarounds), and how you can update your infrastructure to use a more secure SHA-256.

Infra mistakes → user struggles

While the default security policy might refuse SHA-1 all over the system, the problem we’re addressing today looks like this:

# dnf install package-name -y
Dependencies resolved.
=============================================================
 Package   Arch   Version   Repository                  Size
=============================================================
Installing:
 package-name x86_64 1-1.el9
                            repoName                    3.0 k

Transaction Summary
=============================================================
Install  1 Package

Total size: 3.0 k
Installed size: 21
Downloading Packages:
[SKIPPED] package-name-1-1.el9.x86_64.rpm: Already downloaded
warning: Signature not supported. Hash algorithm SHA1 not available.
Problem opening package package-name-1-1.el9.x86_64.rpm
The downloaded packages were saved in cache until the next successful transaction.
You can remove cached packages by executing 'dnf clean packages'.
Error: GPG check FAILED

Per the warning/error above, DNF on RHEL 9 refuses to install the package. DNF is told to check signatures (DNF configuration gpgcheck=1) but the package signature hash algorithm is SHA-1. RPM is forbidden to even check it on the system-level crypto layer.

Can this happen on RHEL 8 or older?

No, at least not by default. The packaging stack on RHEL 8 (and older) accepts SHA-1 signatures without complaints. Potential problems can occur during in-place upgrades from RHEL 8 to the RHEL 9 system. Please pay attention to the warnings issued by LEAPP.

Preparation for this crypto policy change started on RHEL 8. If you want to experiment on RHEL 8, you can bring the configuration from the future RHEL with:

# update-crypto-policies --set FUTURE

Why are packages signed by SHA-1 in 2022?

The official packages for RHEL and Fedora have been signed with SHA-256 for a very long time (all the currently supported distribution versions).

The problem is with third-party packages. SHA-1 is the default hash algorithm for the rpmsign utility on RHEL 7. Also, the default hash algorithm in OBS signd is still SHA-1. So third-party package providers might still unconsciously use SHA-1 signatures.

State of Fedora SHA-1 deprecation

In this case, RHEL has been moved forward earlier than Fedora. At the time of RHEL 9 release, the latest release of Fedora was version 36. At the time of writing this post, Fedora’s policy switch is planned for Fedora 39 (about +18 months after the RHEL 9 release). Still, if you want to experiment with such a setup on Fedora, there exists a special policy (Fedora 36+):

# update-crypto-policies --set TEST-FEDORA39
Setting system policy to TEST-FEDORA39
Note: System-wide crypto policies are applied on application start-up.
It is recommended to restart the system for the change of policies
to fully take place.

The expected failure can then be tested with:

# curl https://copr-be.cloud.fedoraproject.org/archive/2022-07-18-blogpost-sha1/test.repo > /etc/yum.repos.d/test.repo
# dnf install dummy-pkg
...
Importing GPG key 0xFCFBE669:
...
Key imported successfully
Import of key(s) didn't help, wrong key(s)?
Problem opening package dummy-pkg-20220622_1500-1.el7.x86_64.rpm. Failing package is: dummy-pkg-20220622_1500-1.el7.x86_64
 GPG Keys are configured as: https://copr-be.cloud.fedoraproject.org/archive/2022-07-18-blogpost-sha1/pubkey.gpg
The downloaded packages were saved in cache until the next successful transaction.
You can remove cached packages by executing 'dnf clean packages'.
Error: GPG check FAILED

How am I supposed to sign packages, then?

Please switch to SHA-256 (or SHA-512). With the rpmsign utility, one option is to move the signing host to a newer distro than RHEL 7.

One can also modify the %__gpg_sign_cmd macro (copy-pasted from the /usr/lib/rpm/macros file) so it contains the --digest-algo sha256 option. This can be done in the ~/.rpmmacros file:

%__gpg_sign_cmd                 %{__gpg} \
        gpg --batch --no-verbose --no-armor --passphrase-fd 3 \
        %{?_gpg_digest_algo:--digest-algo %{_gpg_digest_algo}} \
        --no-secmem-warning \
        --digest-algo sha256 \
        -u "%{_gpg_name}" -sbo %{__signature_filename} %{__plaintext_filename}

For more info take a look at signing KB article. With OBS signd, there’s sign -h sha256 option.

I still want to install SHA-1 signed packages!

This is discouraged. Even a signature from a years old RPM could be hacked recently by an attacker. If you really know what you are doing, there’s a possibility to use dnf --nogpgcheck option.

Alternatively you can also switch to the legacy crypto policy:

update-crypto-policies --set LEGACY

Or explicitly allow the SHA-1:

update-crypto-policies --set DEFAULT:SHA1

But please don’t forget to switch back, e.g.:

update-crypto-policies --set DEFAULT

Next steps

Now you know how to install SHA-1 signed packages (when really necessary and you understand the security consequences). But please don’t forget to report to your software provider that the SHA-1 problem exists!

From the other side, administrators, please fix your infrastructure. Make the SW distribution chain fluent again. Then forget about this post.