CVE-2025-31489: MinIO Auth Bypass Vulnerability

CVE-2025-31489 Overview

CVE-2025-31489 is an authorization signature bypass vulnerability in MinIO, a High Performance Object Storage system released under GNU Affero General Public License v3.0. The vulnerability exists in the signature component of the authorization mechanism, allowing the signature to be invalid while still permitting object uploads. This means that an attacker with prior knowledge of an access-key (that has WRITE permissions on a bucket) and the bucket name can use any arbitrary secret to upload objects to that bucket.

Critical Impact

Attackers with knowledge of access-keys and bucket names can bypass signature validation to upload arbitrary objects to MinIO buckets, potentially leading to data integrity compromise, storage resource abuse, or injection of malicious content.

Affected Products

• MinIO Object Storage (versions prior to RELEASE.2025-04-03T14-56-28Z)

Discovery Timeline

• April 3, 2025 - CVE-2025-31489 published to NVD
• April 7, 2025 - Last updated in NVD database

Technical Details for CVE-2025-31489

Vulnerability Analysis

This vulnerability is classified as CWE-347 (Improper Verification of Cryptographic Signature). The flaw resides in MinIO's authorization signature validation logic, where the signature component of an authorization request may not be properly verified. This creates a scenario where the cryptographic integrity check that should prevent unauthorized access can be bypassed.

The attack requires prior knowledge of specific elements: an access-key with WRITE permissions and the name of a bucket that the access-key has permissions to access. With this information, an attacker can craft requests using any arbitrary secret value and successfully upload objects to the target bucket. The vulnerability is exploitable via simple HTTP requests using tools like curl, making it trivially easy to abuse once the prerequisites are met.

Root Cause

The root cause of this vulnerability lies in improper verification of the cryptographic signature within MinIO's authorization flow. The signature validation mechanism fails to correctly reject requests with invalid secrets when other authorization parameters (access-key, bucket name) are valid. This represents a fundamental breakdown in the authentication chain that should ensure both identity (access-key) and proof of secret knowledge (signature) are verified before granting access.

Attack Vector

The attack vector is network-based and requires no user interaction. An attacker must first obtain or discover:

1. A valid access-key with WRITE permissions on a target bucket
2. The name of a bucket accessible by that access-key

With these prerequisites, the attacker can construct HTTP requests with arbitrary secret values. The signature validation flaw allows these requests to succeed, enabling unauthorized object uploads. The attack is straightforward to execute using standard HTTP clients, and the network accessibility of MinIO deployments increases the attack surface significantly.

The exploitation path involves crafting S3-compatible API requests to MinIO's endpoint with a valid access-key but using any arbitrary secret for signature generation. Due to the improper verification, these requests pass authorization checks and allow object creation in the target bucket.

Detection Methods for CVE-2025-31489

Indicators of Compromise

• Unexpected objects appearing in MinIO buckets without corresponding legitimate upload activities
• Authorization logs showing successful WRITE operations with signature anomalies
• Multiple upload requests from unusual source IPs targeting specific buckets
• Discrepancies between expected signature patterns and actual request signatures in audit logs

Detection Strategies

• Enable and review MinIO audit logs for unusual PUT/POST operations and signature validation patterns
• Implement integrity monitoring on critical buckets to detect unauthorized object additions
• Deploy network monitoring to identify unusual traffic patterns to MinIO endpoints
• Configure alerting for high-volume upload activities from unexpected sources or access-keys

Monitoring Recommendations

• Enable comprehensive MinIO audit logging and forward logs to a centralized SIEM for analysis
• Monitor bucket object counts and storage utilization for unexpected growth
• Implement real-time alerting on WRITE operations to sensitive buckets
• Review access-key usage patterns and correlate with expected application behavior

How to Mitigate CVE-2025-31489

Immediate Actions Required

• Upgrade MinIO to RELEASE.2025-04-03T14-56-28Z or later immediately
• Audit existing buckets for any unauthorized or unexpected objects
• Review and rotate access-keys, especially those with WRITE permissions
• Implement network segmentation to limit MinIO exposure to trusted sources only

Patch Information

MinIO has addressed this vulnerability in RELEASE.2025-04-03T14-56-28Z. Organizations should upgrade to this version or later to remediate the signature bypass vulnerability. The fix properly validates the cryptographic signature component of authorization requests.

For detailed information about the fix, refer to the GitHub Pull Request #21103 and the GitHub Security Advisory GHSA-wg47-6jq2-q2hh.

Workarounds

• Restrict network access to MinIO endpoints using firewall rules to allow only trusted IP addresses
• Implement additional authentication layers such as VPN or reverse proxy with authentication
• Apply strict bucket policies limiting WRITE access to the minimum necessary principals
• Consider temporarily revoking WRITE permissions from less critical access-keys until patching is complete

# Example: Restrict MinIO access via firewall (iptables)
# Allow only trusted IP ranges to access MinIO port
iptables -A INPUT -p tcp --dport 9000 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 9000 -j DROP