MinIO LDAP login brute-force via user enumeration and missing rate limit

Impact

What kind of vulnerability is it? Who is impacted?

MinIO AIStor's STS (Security Token Service) AssumeRoleWithLDAPIdentity endpoint is vulnerable to LDAP credential brute-forcing due to two combined weaknesses: (1) distinguishable error responses that enable username enumeration, and (2) absence of rate limiting on authentication attempts. An unauthenticated network attacker can enumerate valid LDAP usernames and then perform unlimited password guessing to obtain temporary AWS-style STS credentials, gaining access to the victim's S3 buckets and objects.

All deployments with LDAP configured running an affected version are impacted.

There are two vulnerabilities:

1. User Enumeration via Distinguishable Error Messages (CWE-204)
2. Missing Rate Limiting on STS Authentication Endpoints (CWE-307)

When exploited together, an attacker can:

1. Enumerate valid LDAP usernames by observing error message differences.
2. Perform high-speed password brute-force attacks against confirmed valid users.
3. Upon finding valid credentials, obtain temporary AWS-style STS credentials (AccessKeyId, SecretAccessKey, SessionToken) with full access to the victim user's S3 resources.

Affected Versions

All MinIO releases through the final release of the minio/minio open-source project.

Patches

Fixed in: MinIO AIStor RELEASE.2026-03-17T21-25-16Z

Binary Downloads

FIPS Binaries

Package Downloads

Container Images

# Standard
docker pull quay.io/minio/aistor/minio:RELEASE.2026-03-17T21-25-16Z
podman pull quay.io/minio/aistor/minio:RELEASE.2026-03-17T21-25-16Z

# FIPS
docker pull quay.io/minio/aistor/minio:RELEASE.2026-03-17T21-25-16Z.fips
podman pull quay.io/minio/aistor/minio:RELEASE.2026-03-17T21-25-16Z.fips

Homebrew (macOS)

brew install minio/aistor/minio

Workarounds

• Users of the open-source minio/minio project should upgrade to MinIO AIStor RELEASE.2026-03-17T21-25-16Z or later.

If upgrading is not immediately possible:

• Network-level rate limiting: Use a reverse proxy (e.g., nginx, HAProxy) or WAF to rate-limit requests to the /?Action=AssumeRoleWithLDAPIdentity endpoint.
• Firewall restrictions: Restrict access to the STS endpoint to trusted networks/IP ranges only.
• LDAP account lockout: Configure account lockout policies on the LDAP server itself (e.g., Active Directory lockout threshold). Note: this protects against brute-force but not enumeration, and may cause denial-of-service for legitimate users.