See release notes at https://github.com/JoinMarket-Org/joinmarket-clientserver/blob/master/docs/release-notes/release-notes-0.4.2.md
This fixes a bug in message processing for Makers (yield-generator runners) that can cause a crash (but is not a coin loss/security risk), so is to be considered an immediately required update.
As noted in the above link, you will need to follow the 0.4.0 upgrade process if you are upgrading from pre-0.4.0.
Additional distribution note: I (@AdamISZ ) have updated my pubkey used for signing code, it is to be found here. It's been signed with my old key, and can be found on all my existing social media/online accounts. It's upgraded in security settings and the old one was expiring.
Additional note on installation (this is also on the release notes); if you still have problems installing due to libsodium, @fivepiece notes the following:
A known issue exists in install.sh in which the following error will be returned :
gpg: can't open `libsodium-1.0.13.tar.gz.sig' gpg: verify signatures failed: file open error Libsodium was not built. Exiting.
This issue might affect users who have installed Joinmarket previously using the --no-gpg-validation flag, and are now re-installing without the use of the flag (as is the default install method). To work around this issue, remove the libsodium-1.0.13.tar.gz file from the ./deps/cache/ directory and re-run install.sh.