CVE-2025-4166: HashiCorp Vault Information Disclosure Bug

CVE-2025-4166 Overview

CVE-2025-4166 is an information disclosure vulnerability affecting HashiCorp Vault Community and Vault Enterprise. The Key/Value (kv) Version 2 plugin may unintentionally expose sensitive information in server and audit logs when users submit malformed payloads during secret creation or update operations via the Vault REST API.

This vulnerability falls under CWE-209 (Generation of Error Message Containing Sensitive Information), where improper handling of malformed input causes sensitive data to be logged in error messages. Organizations relying on Vault for secrets management must prioritize patching to prevent potential exposure of confidential credentials and secrets.

Critical Impact

Sensitive secrets and credentials may be inadvertently exposed in server logs and audit logs when malformed API requests are processed, potentially allowing unauthorized access to confidential information by users with log access.

Affected Products

• HashiCorp Vault Community (versions prior to 1.19.3)
• HashiCorp Vault Enterprise (versions prior to 1.19.3, 1.18.9, 1.17.16, 1.16.20)
• OpenBao (affected versions)

Discovery Timeline

• 2025-05-02 - CVE-2025-4166 published to NVD
• 2025-12-31 - Last updated in NVD database

Technical Details for CVE-2025-4166

Vulnerability Analysis

The vulnerability resides in the Key/Value (kv) Version 2 secrets engine plugin within HashiCorp Vault. When processing secret creation or update operations through the REST API, the plugin fails to properly sanitize error messages generated from malformed payloads. This results in sensitive information being written to both server logs and audit logs.

The issue is particularly concerning because Vault is designed to be a secure secrets management solution. Audit logs are typically retained for compliance purposes and may be accessible to security teams, log aggregation systems, or third-party monitoring solutions. If sensitive data is inadvertently written to these logs, it expands the attack surface significantly beyond the intended access controls.

The vulnerability requires network access to exploit, as the attack vector involves submitting specially crafted malformed payloads to the Vault REST API. While user interaction is required for exploitation, the potential for high confidentiality impact makes this a significant concern for organizations managing sensitive credentials.

Root Cause

The root cause of CVE-2025-4166 is improper input handling and error message generation in the KV v2 plugin. When the plugin encounters a malformed payload during secret operations, the error handling logic inadvertently includes sensitive data from the request in the generated error message. This error message is then written to server logs and audit logs without proper sanitization or redaction.

This represents a classic CWE-209 pattern where verbose error messages intended for debugging purposes expose sensitive information that should remain protected.

Attack Vector

An attacker with network access to the Vault REST API can exploit this vulnerability by submitting intentionally malformed payloads during secret creation or update operations. The attack flow involves:

1. Identifying a target Vault instance with the KV v2 secrets engine enabled
2. Crafting malformed API requests that trigger error conditions in the secret handling logic
3. Submitting these requests via the Vault REST API
4. The malformed payloads cause the KV v2 plugin to generate error messages containing sensitive information
5. These error messages are written to server logs and audit logs

An attacker with access to these logs (either through legitimate access, log aggregation systems, or a separate compromise) could then extract the exposed sensitive information.

The vulnerability is exploited through the Vault REST API. Malformed payloads submitted to endpoints such as /v1/secret/data/{path} for secret creation or updates may trigger the information disclosure in error logs. Organizations should consult the HashiCorp Vault Security Advisory for specific technical details on the malformed payload structures.

Detection Methods for CVE-2025-4166

Indicators of Compromise

• Unusual or malformed requests to KV v2 secret endpoints in Vault access logs
• Error entries in Vault server logs containing unexpected sensitive data patterns
• Audit log entries with unusually large payloads or error messages during secret operations
• API requests with intentionally malformed JSON structures targeting secret paths

Detection Strategies

• Implement log monitoring to detect patterns of malformed API requests targeting KV v2 endpoints
• Review existing server and audit logs for potential prior exposure of sensitive information
• Monitor for unusual API activity patterns, particularly failed requests with error responses
• Deploy API gateway rules to detect and block obviously malformed payload structures

Monitoring Recommendations

• Enable detailed request logging on Vault API endpoints to track malformed request patterns
• Implement alerting on high volumes of API errors related to secret operations
• Regularly audit log access permissions to minimize exposure of potentially leaked data
• Consider log redaction solutions to automatically sanitize sensitive patterns in log outputs

How to Mitigate CVE-2025-4166

Immediate Actions Required

• Upgrade HashiCorp Vault Community to version 1.19.3 or later immediately
• Upgrade HashiCorp Vault Enterprise to versions 1.19.3, 1.18.9, 1.17.16, or 1.16.20 depending on your version branch
• Review and rotate any secrets that may have been exposed in logs prior to patching
• Audit access permissions to server logs and audit logs to minimize potential exposure

Patch Information

HashiCorp has released security patches addressing CVE-2025-4166 in the following versions:

• Vault Community: 1.19.3
• Vault Enterprise: 1.19.3, 1.18.9, 1.17.16, 1.16.20

Organizations should upgrade to the appropriate patched version for their deployment. Refer to the HashiCorp Vault Security Advisory for complete upgrade instructions and additional details.

Workarounds

• Restrict network access to Vault API endpoints using firewall rules or network segmentation
• Implement strict access controls on server logs and audit logs to limit exposure
• Enable log rotation with shorter retention periods to minimize the window of potential exposure
• Consider temporarily disabling or restricting access to KV v2 operations until patching is complete

# Example: Restrict Vault API access via iptables
# Allow only trusted networks to access Vault API
iptables -A INPUT -p tcp --dport 8200 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 8200 -j DROP

# Verify current Vault version
vault version

# Review Vault server logs for potential exposure
grep -i "error" /var/log/vault/vault.log | head -50