Any Emacs package that you install (whether from a package repository such as ELPA or MELPA, or via package-vc-install) has full read/write/delete access to your data on your computer that is accessible by the logged-in user, and (assuming no particular security protections are in place) full network access to exfiltrate information. The 2024 xz utils backdoor incident illustrated the amount of effort that malicious actors make to compromise our computers. Operating systems are introducing sandboxing mechanisms for software installed from the internet to protect against these kinds of attacks, but these protections are not effective for user-extensible software like Emacs. For this reason, it is worthwhile spending some time reading the package’s source code to check for suspicious behaviours, introduced by a malicious package developer or one whose computer has been compromised by an attacker.

If you install Emacs packages via your operating system’s package distribution mechanism, the package maintainers will — in the best case — play a gatekeeper role and check the source code on behalf of users. Please note however that widely used package archives like MELPA are not equipped to undertake any security checks before package updates are distributed to users. The Elpaca package manager provides special support for reviewing diffs to a package before installing them, which is definitely a good practice. Running packages in a software container until you’ve been able to review them from a software security perspective, is a partial response to some of these concerns.