CVE-2026-34726: Copier Path Traversal Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-85v3-4m8g-hrh6",
  "modified" : "2026-04-06T17:32:29Z",
  "published" : "2026-04-01T22:28:49Z",
  "aliases" : [ "CVE-2026-34726" ],
  "summary" : "Copier `_subdirectory` allows template root escape via parent-directory traversal",
  "details" : "### Summary\n\nCopier'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.\n\nAs a result, a template can escape its own directory and make Copier render files from the parent directory without `--UNSAFE`.\n\n### Details\n\nThe relevant code path is:\n\n1. the template defines `_subdirectory`\n2. Copier renders that string\n3. `template_copy_root` returns `self.template.local_abspath / subdir`\n4. Copier walks that directory as the template root\n\nRelevant code:\n\n- <https://github.com/copier-org/copier/blob/7aa7021bd73797c982492bac3535515d4484fdb7/copier/_main.py#L1056-L1062>\n- <https://github.com/copier-org/copier/blob/7aa7021bd73797c982492bac3535515d4484fdb7/copier/_template.py#L504-L513>\n\nThe effective sink is:\n\n```python\nsubdir = self._render_string(self.template.subdirectory) or \"\"\nreturn self.template.local_abspath / subdir\n```\n\nThere is no check that the resulting path stays inside the template directory.\n\nThe documentation for `_subdirectory` describes it as:\n\n> Subdirectory to use as the template root when generating a project.\n\nand explains it as a way to separate template metadata from template source code:\n\n<https://github.com/copier-org/copier/blob/7aa7021bd73797c982492bac3535515d4484fdb7/docs/configuring.md#L1582-L1646>\n\nThat description fits values like `template` or `poetry`, but not `..`.\n\n### PoC\n\n#### PoC 1: `_subdirectory: ..` escapes to the parent directory\n\n```sh\nmkdir -p root/template dst\necho 'loot' > root/loot.txt\nprintf '%s\\n' '_subdirectory: ..' > root/template/copier.yml\n\ncopier copy --overwrite root/template dst\nfind dst -maxdepth 3 -type f | sort\ncat dst/loot.txt\n```\n\nExpected output includes:\n\n```text\ndst/loot.txt\ndst/template/copier.yml\nloot\n```\n\nThis shows Copier is rendering from `root/` rather than from `root/template/`.\n\n### Impact\n\nIf 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.\n\nPractical impact:\n\n- template-root escape via `..`\n- rendering of parent-directory files that were not meant to be part of the template\n- possible without `--UNSAFE`",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "copier"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "9.14.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/copier-org/copier/security/advisories/GHSA-85v3-4m8g-hrh6"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-34726"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/copier-org/copier/commit/cb80a3ffc9c3787de3ed837e04ca29a0ff8ca3df"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/copier-org/copier"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/copier-org/copier/releases/tag/v9.14.1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-01T22:28:49Z",
    "nvd_published_at" : "2026-04-02T19:21:32Z"
  }
}