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GHSA-85v3-4m8g-hrh6

MediumCVSS 4.4 / 10
Published Apr 1, 2026·Last modified Apr 6, 2026
Affected Components(0)

No affected components available

Description

Summary

Copier's _subdirectory setting is documented as the subdirectory to use as the template root. However, the current implementation accepts parent-directory traversal such as .. and uses it directly when selecting the template root.

As a result, a template can escape its own directory and make Copier render files from the parent directory without --UNSAFE.

Details

The relevant code path is:

  1. the template defines _subdirectory
  2. Copier renders that string
  3. template_copy_root returns self.template.local_abspath / subdir
  4. Copier walks that directory as the template root

Relevant code:

The effective sink is:

subdir = self._render_string(self.template.subdirectory) or ""
return self.template.local_abspath / subdir

There is no check that the resulting path stays inside the template directory.

The documentation for _subdirectory describes it as:

Subdirectory to use as the template root when generating a project.

and explains it as a way to separate template metadata from template source code:

https://github.com/copier-org/copier/blob/7aa7021bd73797c982492bac3535515d4484fdb7/docs/configuring.md#L1582-L1646

That description fits values like template or poetry, but not ...

PoC

PoC 1: _subdirectory: .. escapes to the parent directory

mkdir -p root/template dst
echo 'loot' > root/loot.txt
printf '%s\n' '_subdirectory: ..' > root/template/copier.yml

copier copy --overwrite root/template dst
find dst -maxdepth 3 -type f | sort
cat dst/loot.txt

Expected output includes:

dst/loot.txt
dst/template/copier.yml
loot

This shows Copier is rendering from root/ rather than from root/template/.

Impact

If a user runs Copier on an untrusted template, that template can change the effective template root and make Copier render files from outside the intended template directory.

Practical impact:

  • template-root escape via ..
  • rendering of parent-directory files that were not meant to be part of the template
  • possible without --UNSAFE
Risk Scores
Base Score
4.4

The vulnerability requires local access to the device to be exploited. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. The attacker needs the user to perform some action, like clicking a link. The impact is confined to the system where the vulnerability exists. There is a low impact on the confidentiality of the information. There is a low impact on the integrity of the data.

Threat Intelligence
4.1

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
0.38%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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