Summary:
A new vulnerability, CVE-2025-4166, affects HashiCorp Vault users worldwide. Vault Community and Vault Enterprise Key/Value (kv) Version 2 plugin may expose sensitive information in server logs and audit logs if a user accidentally or maliciously submits a malformed payload through the REST API for secret creation or update. This long-read dives into what happened, how it happens, code examples, and how to stay safe.

What Is HashiCorp Vault kv v2?

HashiCorp Vault is a popular secrets management tool for storing sensitive data like passwords, API keys, and tokens. The kv v2 (Key/Value version 2) plugin is a storage engine that allows versioning of secrets and supports CRUD via HTTP REST API.

What Is CVE-2025-4166?

- CVE ID: CVE-2025-4166

Date Announced: June 2024

- Affected: Vault Community before 1.19.3, Vault Enterprise before 1.19.3, 1.18.9, 1.17.16, 1.16.20

How Does It Happen?

If a user (or attacker) submits a malformed or incorrectly structured HTTP request body to Vault’s kv v2 plugin—for example, when writing or updating a secret—the payload contents can end up in server logs or audit logs. Any secrets or sensitive info in that malformed payload may leak, even if Vault refuses the request.

*Accidentally* leak their own secrets via malformed requests.

- *Maliciously* send crafted payloads to trigger logging of sensitive data, then steal logs if they have access.

Below is Python code illustrating a problematic request to the Vault API

import requests

vault_addr = "https://your-vault.example.com";
secret_path = "secret/data/mysecret"
token = "YOUR_VAULT_TOKEN"

# Malformed (should be '{"data": {"password": "hunter2"}}')
bad_payload = {
    "password": "hunter2"  # Missing 'data' wrapper
}

headers = {
    "X-Vault-Token": token
}

response = requests.post(
    f"{vault_addr}/v1/{secret_path}",
    json=bad_payload,  # Wrong shape!
    headers=headers
)

print("Status Code:", response.status_code)
print("Body:", response.text)

What happens?

Real-World Exploit Scenario

*Imagine you have centralized logging or SIEM processing Vault’s logs for compliance. An attacker with minimal access submits malformed updates packed with secrets or tokens. If logs are insufficiently protected—or monitored by too many people—sensitive data walks right out of your “secure” Vault.*

Here is what a typical faulty log entry might look like

2024-06-18T20:22:16.999Z [ERROR] [kv/v2] Write request failed, payload: {"password":"hunter2"}

Vault Enterprise: 1.19.3, 1.18.9, 1.17.16, 1.16.20

Upgrade right away!
Official Release Notes

References

- CVE-2025-4166 at NVD
- HashiCorp Official Vault Releases
- Vault Audit Devices Documentation
- kv Secrets Engine – Version 2

Final Advice

CVE-2025-4166 is a textbook example of how error handling and logging design can turn a simple bug into a data breach risk. Always keep your secrets management platform up-to-date, and remember: *what you log gets seen*. If you use Vault’s kv v2 API, patch now, review your logs, and enforce strict monitoring on who can see them.

Have questions about this vulnerability? Drop a comment or check the official references linked above.

Timeline

Published on: 05/02/2025 15:15:50 UTC