CVE-2026-23968: Copier safe template has arbitrary filesystem read access via symlinks when _preserve_symlinks: false

Copier suggests that it’s safe to generate a project from a safe template, i.e. one that doesn’t use unsafe features like custom Jinja extensions which would require passing the --UNSAFE,--trust flag. As it turns out, a safe template can currently include arbitrary files/directories outside the local template clone location by using symlinks along with _preserve_symlinks: false (which is Copier’s default setting).

Imagine, e.g., a malicious template author who creates a template that reads SSH keys or other secrets from well-known locations and hopes for a user to push the generated project to a public location like github.com where the template author can extract the secrets.

Reproducible example:

• Illegally include a file in the generated project via symlink resolution:

echo "s3cr3t" > secret.txt

mkdir src/
pushd src/
ln -s ../secret.txt stolen-secret.txt
popd

uvx copier copy src/ dst/

cat dst/stolen-secret.txt