CVE-2026-28684: python-dotenv: Symlink following in set_key allows arbitrary file overwrite via cross-device rename fallback

Summary

setkey() and unsetkey() 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.

Details

The rewrite() context manager in dotenv/main.py is used by both setkey() and unsetkey() 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.

When 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:

1. shutil.move() first attempts os.rename(), which fails with an OSError because atomic renames cannot cross device boundaries. 2. On failure, shutil.move() falls back to shutil.copy2() followed by os.unlink(). 3. shutil.copy2() calls shutil.copyfile() with followsymlinks=True by default. 4. This causes the content to be written to the symlink target rather than replacing the symlink itself.

An 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 setkey() or unsetkey(), the symlink target is overwritten with the new .env content.

This vulnerability does not require a race condition and is fully deterministic once the preconditions are met.

Impact The primary impacts are to integrity and availability:

- 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. - Integrity violation: The target file's original content is replaced with .env-formatted content controlled by the attacker. - Potential privilege escalation: In scenarios where a privileged process (running as root or a service account) calls setkey(), the attacker can leverage this to write to files beyond their own access level.

The scope of impact depends on the application using python-dotenv and the privileges under which it runs.

Proof of Concept

The 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).

python import os import sys import tempfile from dotenv import setkey

Pre-condition: /tmp must be on a different device than the target directory. tmpdev = os.stat("/tmp").stdev homedev = os.stat(os.path.expanduser("~")).stdev assert tmpdev != homedev, "Skipped: /tmp and ~ are on the same device (no cross-device move)"

with tempfile.TemporaryDirectory(dir=os.path.expanduser("~")) as workdir: # File an attacker wants to overwrite target = os.path.join(workdir, "victimconfig.txt") with open(target, "w") as f: f.write("DBPASSWORD=supersecret\n")

# Attacker pre-places a symlink at the path the application will use as .env envsymlink = os.path.join(workdir, ".env") os.symlink(target, envsymlink)

before = open(target).read()

# Application writes a new key -- triggers the cross-device fallback setkey(envsymlink, "INJECTED", "attackervalue")

after = open(target).read()

print("Before:", repr(before)) print("After: ", repr(after)) print("Symlink target overwritten:", target)

Expected output: Before: 'DBPASSWORD=supersecret\n' After: "DBPASSWORD=supersecret\nINJECTED='attackervalue'\n" Symlink target overwritten: /home/user/tmp806nut2g/victimconfig.txt

Remediation

The fix changes the rewrite() context manager in the following ways:

1. 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. 2. A followsymlinks: bool = False parameter is added to setkey() and unsetkey() for users who explicitly need the old behavior. 3. 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. 4. os.replace() is used instead of shutil.move(), providing atomic replacement without symlink-following fallback behavior.

Users are advised to upgrade to the patched version as soon as it is available on PyPI.

Timeline

| Date | Event | | ------------ | ---------------------------------------------------------------------------------------------------- | | 2026-01-09 | Initial report received from Giorgos Tsigourakos regarding a separate, unrelated issue also located in rewrite() | | 2026-01-10 | Co-maintainer acknowledged report, requested clarification | | 2026-01-11 | Initial report assessed as not exploitable and closed | | 2026-02-24 | Reporter identified new, distinct cross-device symlink attack vector with deterministic exploitation | | 2026-02-26 | Co-maintainer confirmed vulnerability and shared draft patch | | 2026-02-26 | Reporter validated fix with monkeypatched PoC, proposed CVSS | | 2026-03-01 | Patch merged to main | | 2026-03-01 | Patched version released to PyPI | | 2026-04-20 | Advisory published |

Patches

Upgrade to v.1.2.2 or use the patch from https://github.com/theskumar/python-dotenv/commit/790c5c02991100aa1bf41ee5330aca75edc51311.patch