CVE-2026-28684VulnerabilityPublished 13 Aug 2026Updated 26 Oct 2026

set_key · python-dotenv: Symlink following

Affects mcp-google-sheetsSource GCVE Database

Authoritative record:CVE-2026-28684· Medium 6.6

Not known to be exploited · CVSS 6.6 Medium : upgrade to 0.6.0_5 when convenient.

Data as of 26 Oct 2026, 17:16 UTC

CVSS 3.1 base score

6.6Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 16% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 0.6.0_5

CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:H

Attack Vector: Local
Attack Complexity: Low
Privileges Required: Low
User Interaction: Required
Scope: Unchanged
Confidentiality: None
Integrity: High
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Local: The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., tricking a legitimate user into opening a malicious document).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 20 Apr 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade mcp-google-sheets to 0.6.0_5.

Affected software

Product	Affected	Fixed in
Homebrewmcp-google-sheets`pkg:brew/mcp-google-sheets`Other threats	Affected >=0.5.1 <0.6.0_5	Fixed in 0.6.0_5

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

### Summary `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. ### Details The `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. 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 `follow_symlinks=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 `set_key()` or `unset_key()`, 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 `set_key()`, 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 set_key # Pre-condition: /tmp must be on a different device than the target directory. tmp_dev = os.stat("/tmp").st_dev home_dev = os.stat(os.path.expanduser("~")).st_dev assert tmp_dev != home_dev, "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, "victim_config.txt") with open(target, "w") as f: f.write("DB_PASSWORD=supersecret\n") # Attacker pre-places a symlink at the path the application will use as .env env_symlink = os.path.join(workdir, ".env") os.symlink(target, env_symlink) before = open(target).read() # Application writes a new key -- triggers the cross-device fallback set_key(env_symlink, "INJECTED", "attacker_value") after = open(target).read() print("Before:", repr(before)) print("After: ", repr(after)) print("Symlink target overwritten:", target) ``` **Expected output:** ``` Before: 'DB_PASSWORD=supersecret\n' After: "DB_PASSWORD=supersecret\nINJECTED='attacker_value'\n" Symlink target overwritten: /home/user/tmp806nut2g/victim_config.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 `follow_symlinks: bool = False` parameter** is added to `set_key()` and `unset_key()` 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 | | ------------ | ---------------

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