CVE-2026-34726

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:

• https://github.com/copier-org/copier/blob/7aa7021bd73797c982492bac3535515d4484fdb7/copier/_main.py#L1056-L1062
• https://github.com/copier-org/copier/blob/7aa7021bd73797c982492bac3535515d4484fdb7/copier/_template.py#L504-L513

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/.

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

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-34726 has a CVSS score of 4.4 (Medium). The vector is requires local access, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (9.14.1); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-34726? CVE-2026-34726 is a medium-severity path traversal vulnerability in copier (pip), affecting versions < 9.14.1. It is fixed in 9.14.1. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-34726? CVE-2026-34726 has a CVSS score of 4.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of copier are affected by CVE-2026-34726? copier (pip) versions < 9.14.1 is affected.
4. Is there a fix for CVE-2026-34726? Yes. CVE-2026-34726 is fixed in 9.14.1. Upgrade to this version or later.
5. Is CVE-2026-34726 exploitable, and should I be worried? Whether CVE-2026-34726 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-34726 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-34726? Upgrade copier to 9.14.1 or later.