CVE-2026-34726 is a medium-severity (CVSS 4.4) Path Traversal vulnerability in copier. A fix is available for copier — see the affected versions and patch details below.
Copier `_subdirectory` allows template root escape via parent-directory traversal
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-34726.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
copierReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
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:
- the template defines
_subdirectory - Copier renders that string
template_copy_rootreturnsself.template.local_abspath / subdir- 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:
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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | copier | all versions | 9.14.1pip install --upgrade 'copier==9.14.1' |
Affected Products
copiercopier-orgDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for copier, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update copier to 9.14.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34726 is resolved across your whole dependency graph.
Workarounds
Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.
Frequently Asked Questions
Is CVE-2026-34726 in your dependencies?
Find it across PyPI, including transitive dependencies.