The NixOS project has announced
a critical vulnerability in many versions of the Nix package
manager's daemon. The flaw was introduced as part of a fix for a
prior vulnerability in 2024. According to the advisory,
all default configurations of NixOS and systems building untrusted derivations are impacted.

A bug in the fix for CVE-2024-27297
allowed for arbitrary overwrites of files writable by the Nix process orchestrating the builds (typically the Nix daemon running as root in multi-user installations) by following symlinks during fixed-output
derivation output registration. This affects sandboxed Linux builds -
sandboxed macOS builds are unaffected. The location of the temporary
output used for the output copy was located inside the build chroot. A
symlink, pointing to an arbitrary location in the filesystem, could be
created by the derivation builder at that path. During output
registration, the Nix process (running in the host mount namespace)
would follow that symlink and overwrite the destination with the
derivation's output contents.

In multi-user installations, this allows all users able to submit
builds to the Nix daemon (allowed-users - defaulting to all users) to
gain root privileges by modifying sensitive files.

https://lwn.net/Articles/1066813/
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