How to force older debian to forget about DST Root CA X3 Expiration and use ISRG Root X1 - SSL certificate problem: certificate has expired

Question

This relates to DST Root CA X3 Expiration (September 2021)
When searching online for a fix to apply on an older server (Debian 8 in my case) that does call to sites encrypted with letsencrypt with curl, they now seem to fail with the following message:

Example:

curl -fsSL https://deb.nodesource.com/setup_14.x | bash -

Fails silently, then trying it manually and removing the silent flag and bash pipe like this:

curl -L https://deb.nodesource.com/setup_14.x

curl: (60) SSL certificate problem: certificate has expired
More details here: http://curl.haxx.se/docs/sslcerts.html

curl performs SSL certificate verification by default, using a "bundle"
 of Certificate Authority (CA) public keys (CA certs). If the default
 bundle file isn't adequate, you can specify an alternate file
 using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
 the bundle, the certificate verification probably failed due to a
 problem with the certificate (it might be expired, or the name might
 not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
 the -k (or --insecure) option.

Trying the following commands doesn't solve the issue:

apt update
apt install -y ca-certificates openssl
update-ca-certificates

What can I do!? (answering my own question) ??

Answer 1

disclaimer; I'm no security expert (I know things, but you do you). Make sure you understand what you do before applying whatever fix shared here

Fix by upgrading your instances

Upgrade your instances. This problem won't happen on debian 9 or higher.

In the following example, I had this problem on a ruby:2.4.1 docker image which is based on Debian 8 (could be considered old). Upgrading to more recent docker image fixes this issue. Uprading to a more recent Debian version should also fix the issue.

I confirmed it does not happen when using ruby:2.7.0 docker image based on Debian 11 as shown here:

docker run --rm -it ruby:2.7.4 bash -c "cat /etc/issue"
Debian GNU/Linux 11 
 l

Fix for Debian 8 by commenting DST_Root_CA_X3.crt from /etc/ca-certificates.conf

Even if ISRG Root X1 is in place, if DST Root CA X3 is still present and in use, its verification seems to happen first so we can get rid of it by doing this:

1. install ca-certificates package
2. comment /mozilla/DST_Root_CA_X3.crt from /etc/ca-certificates.conf
3. make sure /usr/share/ca-certificates/mozilla/ISRG_Root_X1.crt is there (it should be)
4. update ca-certificates with update-ca-certificates

Example directly on your instance

cat /etc/issue
Debian GNU/Linux 8 
 l

sudo apt install -y ca-certificates
sudo sed -i '/^mozilla/DST_Root_CA_X3.crt$/ s/^/!/' /etc/ca-certificates.conf
sudo update-ca-certificates

Example Dockerfile:

FROM ruby:2.4.1 # uses debian 8

RUN apt update -qq 
    && apt install -y ca-certificates 
    && sed -i '/^mozilla/DST_Root_CA_X3.crt$/ s/^/!/' /etc/ca-certificates.conf 
    && update-ca-certificates 
    && rm -rf /var/lib/apt/lists/*

Fix using dpkg-reconfigure ca-certificates

As stated in the comments, you can also fix this interactively using the following command on the instance (requires ca-certificates package installed):

dpkg-reconfigure ca-certificates

Then disable mozilla/DST_Root_CA_X3.crt from the list.

Conclusion

If you'd like to learn more, you should read Scott Helme's post: Let's Encrypt's Root Certificate is expiring!

You can now curl letsencrypt sites safely with these safety glasses: ??