CVE-2026-28684: python-dotenv Symlink Following Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-mf9w-mj56-hr94",
  "modified" : "2026-04-21T14:38:57Z",
  "published" : "2026-04-21T14:38:57Z",
  "aliases" : [ "CVE-2026-28684" ],
  "summary" : "python-dotenv: Symlink following in set_key allows arbitrary file overwrite via cross-device rename fallback",
  "details" : "### Summary\n\n`set_key()` and `unset_key()` in python-dotenv follow symbolic links when rewriting `.env` files, allowing a local attacker to overwrite arbitrary files via a crafted symlink when a cross-device rename fallback is triggered.\n\n\n### Details\n\nThe `rewrite()` context manager in `dotenv/main.py` is used by both `set_key()` and `unset_key()` to safely modify `.env` files. It works by writing to a temporary file (created in the system's default temp directory, typically `/tmp`) and then using `shutil.move()` to replace the original file.\n\nWhen the `.env` path is a symbolic link and the temp directory resides on a different filesystem than the target (a common configuration on Linux systems using tmpfs for `/tmp`), the following sequence occurs:\n\n1. `shutil.move()` first attempts `os.rename()`, which fails with an `OSError` because atomic renames cannot cross device boundaries.\n2. On failure, `shutil.move()` falls back to `shutil.copy2()` followed by `os.unlink()`.\n3. `shutil.copy2()` calls `shutil.copyfile()` with `follow_symlinks=True` by default.\n4. This causes the content to be written to the **symlink target** rather than replacing the symlink itself.\n\nAn attacker who has write access to the directory containing a `.env` file can pre-place a symlink pointing to any file that the application process has write access to. When the application (or a privileged process such as a deploy script, Docker entrypoint, or CI pipeline) calls `set_key()` or `unset_key()`, the symlink target is overwritten with the new `.env` content.\n\nThis vulnerability does not require a race condition and is fully deterministic once the preconditions are met.\n\n### Impact\nThe primary impacts are to **integrity** and **availability**:\n\n- **File overwrite / destruction (DoS):** An attacker can cause an application or privileged process to corrupt or destroy configuration files, database configs, or other sensitive files it would not normally have access to modify.\n- **Integrity violation:** The target file's original content is replaced with `.env`-formatted content controlled by the attacker.\n- **Potential privilege escalation:** In scenarios where a privileged process (running as root or a service account) calls `set_key()`, the attacker can leverage this to write to files beyond their own access level.\n\nThe scope of impact depends on the application using python-dotenv and the privileges under which it runs.\n\n\n### Proof of Concept\n\nThe following script demonstrates the vulnerability. It requires `/tmp` and the user's home directory to reside on different devices (common on systemd-based Linux systems with tmpfs).\n\n```python\nimport os\nimport sys\nimport tempfile\nfrom dotenv import set_key\n\n# Pre-condition: /tmp must be on a different device than the target directory.\ntmp_dev = os.stat(\"/tmp\").st_dev\nhome_dev = os.stat(os.path.expanduser(\"~\")).st_dev\nassert tmp_dev != home_dev, \"Skipped: /tmp and ~ are on the same device (no cross-device move)\"\n\nwith tempfile.TemporaryDirectory(dir=os.path.expanduser(\"~\")) as workdir:\n    # File an attacker wants to overwrite\n    target = os.path.join(workdir, \"victim_config.txt\")\n    with open(target, \"w\") as f:\n        f.write(\"DB_PASSWORD=supersecret\\n\")\n\n    # Attacker pre-places a symlink at the path the application will use as .env\n    env_symlink = os.path.join(workdir, \".env\")\n    os.symlink(target, env_symlink)\n\n    before = open(target).read()\n\n    # Application writes a new key -- triggers the cross-device fallback\n    set_key(env_symlink, \"INJECTED\", \"attacker_value\")\n\n    after = open(target).read()\n\n    print(\"Before:\", repr(before))\n    print(\"After: \", repr(after))\n    print(\"Symlink target overwritten:\", target)\n```\n\n**Expected output:**\n```\nBefore: 'DB_PASSWORD=supersecret\\n'\nAfter:  \"DB_PASSWORD=supersecret\\nINJECTED='attacker_value'\\n\"\nSymlink target overwritten: /home/user/tmp806nut2g/victim_config.txt\n```\n\n### Remediation\n\nThe fix changes the `rewrite()` context manager in the following ways:\n\n1. **Symlinks are no longer followed by default.** When the `.env` path is a symlink, `rewrite()` now resolves it to the real path before proceeding, or (by default) operates on the symlink entry itself rather than the target.\n2. **A `follow_symlinks: bool = False` parameter** is added to `set_key()` and `unset_key()` for users who explicitly need the old behavior.\n3. **Temp files are written in the same directory** as the target `.env` file (instead of the system temp directory), eliminating the cross-device rename condition entirely.\n4. **`os.replace()` is used instead of `shutil.move()`**, providing atomic replacement without symlink-following fallback behavior.\n\nUsers are advised to upgrade to the patched version as soon as it is available on PyPI.\n\n### Timeline\n\n| Date             | Event                                                                                                                                                                 |\n| ------------ | ---------------------------------------------------------------------------------------------------- |\n| 2026-01-09  | Initial report received from Giorgos Tsigourakos regarding a separate, unrelated issue also located in `rewrite()` |\n| 2026-01-10   | Co-maintainer acknowledged report, requested clarification                                                         |\n| 2026-01-11    | Initial report assessed as not exploitable and closed                                                              |\n| 2026-02-24  | Reporter identified new, distinct cross-device symlink attack vector with deterministic exploitation               |\n| 2026-02-26  | Co-maintainer confirmed vulnerability and shared draft patch                                                       |\n| 2026-02-26  | Reporter validated fix with monkeypatched PoC, proposed CVSS                                                       |\n| 2026-03-01   | Patch merged to main                                                                                               |\n| 2026-03-01   | Patched version released to PyPI                                                                                   |\n| 2026-04-20   | Advisory published                                                                                                 |\n\n### Patches\n\nUpgrade to v.1.2.2 or use the patch from https://github.com/theskumar/python-dotenv/commit/790c5c02991100aa1bf41ee5330aca75edc51311.patch",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "python-dotenv"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.2.2"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/theskumar/python-dotenv/security/advisories/GHSA-mf9w-mj56-hr94"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-28684"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/theskumar/python-dotenv/commit/790c5c02991100aa1bf41ee5330aca75edc51311"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/theskumar/python-dotenv/commit/790c5c02991100aa1bf41ee5330aca75edc51311.patch"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/theskumar/python-dotenv"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/theskumar/python-dotenv/releases/tag/v1.2.2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-59", "CWE-61" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-21T14:38:57Z",
    "nvd_published_at" : "2026-04-20T17:16:33Z"
  }
}