CVE-2026-34204: MinIO Auth Bypass Vulnerability

CVE-2026-34204 Overview

CVE-2026-34204 is an authentication bypass vulnerability in MinIO, a high-performance object storage system. A flaw in the extractMetadataFromMime() function allows any authenticated user with s3:PutObject permission to inject internal server-side encryption metadata into objects by sending crafted X-Minio-Replication-* headers on a normal PutObject request. This vulnerability enables attackers to manipulate internal metadata that should be restricted to server-side operations only.

Critical Impact

Authenticated users can inject internal server-side encryption metadata, potentially compromising data integrity and causing denial of service through manipulation of replication and encryption settings.

Affected Products

• MinIO versions prior to RELEASE.2026-03-26T21-24-40Z

Discovery Timeline

• 2026-03-31 - CVE CVE-2026-34204 published to NVD
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2026-34204

Vulnerability Analysis

This vulnerability stems from improper authentication controls in MinIO's metadata extraction process. The extractMetadataFromMime() function fails to properly validate and restrict certain internal headers when processing PutObject requests. Specifically, the function does not adequately filter X-Minio-Replication-* headers, which are intended for internal server-side operations only.

When an authenticated user sends a standard PutObject request with maliciously crafted replication headers, the server incorrectly accepts and processes these headers as legitimate internal metadata. This allows the attacker to inject server-side encryption metadata into objects, bypassing the intended access controls for these sensitive internal settings.

The vulnerability is classified under CWE-287 (Improper Authentication), as the system fails to properly authenticate the source of metadata that should only originate from trusted internal components.

Root Cause

The root cause lies in insufficient input validation within the extractMetadataFromMime() function. The function processes HTTP headers during object uploads but does not properly distinguish between user-supplied headers and internal system headers. The X-Minio-Replication-* header namespace, which should be reserved for internal replication operations, is not adequately filtered when processing standard PutObject requests from authenticated users.

Attack Vector

This vulnerability is exploitable over the network by any authenticated user who possesses s3:PutObject permissions. The attack requires:

1. Valid authentication credentials with s3:PutObject permission on at least one bucket
2. The ability to send HTTP requests with custom headers to the MinIO server
3. Crafted X-Minio-Replication-* headers in the PutObject request

The attacker sends a normal PutObject request but includes specially crafted X-Minio-Replication-* headers. The extractMetadataFromMime() function processes these headers without proper validation, allowing the injected metadata to be associated with the uploaded object. This can lead to data integrity violations and potential denial of service by manipulating encryption and replication settings.

For detailed technical analysis, refer to the GitHub Security Advisory.

Detection Methods for CVE-2026-34204

Indicators of Compromise

• Unusual X-Minio-Replication-* headers appearing in PutObject requests from non-replication sources
• Objects with unexpected server-side encryption metadata that was not configured by administrators
• Anomalous replication metadata on objects in buckets where replication is not enabled
• Authentication logs showing PutObject requests with suspicious custom header patterns

Detection Strategies

• Implement network-level monitoring to detect PutObject requests containing X-Minio-Replication-* headers from non-trusted sources
• Review MinIO access logs for patterns of header injection attempts in object upload operations
• Deploy web application firewall (WAF) rules to flag or block requests with internal header namespaces
• Correlate S3 API audit logs to identify users attempting to set restricted metadata

Monitoring Recommendations

• Enable detailed audit logging for all S3 PutObject operations in MinIO
• Configure alerting for objects created with server-side encryption metadata by users who should not have encryption management privileges
• Monitor for sudden changes in replication metadata across buckets
• Implement real-time log analysis for anomalous header patterns in API requests

How to Mitigate CVE-2026-34204

Immediate Actions Required

• Upgrade MinIO to version RELEASE.2026-03-26T21-24-40Z or later immediately
• Audit existing objects for unexpected or unauthorized encryption metadata
• Review access logs for evidence of exploitation attempts using X-Minio-Replication-* headers
• Restrict s3:PutObject permissions to only essential users while patching is in progress

Patch Information

MinIO has released a security patch in version RELEASE.2026-03-26T21-24-40Z that addresses this vulnerability. The patch adds proper validation to the extractMetadataFromMime() function to ensure that internal header namespaces, including X-Minio-Replication-*, cannot be injected by standard user requests. Organizations should update to this version or later as soon as possible. For more details, see the GitHub Security Advisory.

Workarounds

• Deploy a reverse proxy or WAF in front of MinIO to strip or reject requests containing X-Minio-Replication-* headers from client connections
• Implement network segmentation to restrict direct API access to trusted sources only
• Temporarily disable public or untrusted access to buckets while awaiting patch deployment
• Monitor and audit all PutObject operations for suspicious header injection attempts

# Example: Nginx configuration to strip malicious headers
# Add to your nginx server block proxying to MinIO
proxy_set_header X-Minio-Replication-Actual-Size "";
proxy_set_header X-Minio-Replication-Proxy-Request "";
# Block all X-Minio-Replication-* headers from clients
if ($http_x_minio_replication_actual_size) {
    return 403;
}