MinIO is a popular, high-performance object storage system released under the GNU Affero General Public License v3.. Organizations and developers rely on it as a robust S3-compatible alternative for storing large amounts of unstructured data. In early 2025, a significant security vulnerability was discovered and assigned CVE-2025-31489, raising concerns for teams running older MinIO deployments.

This post breaks down what CVE-2025-31489 is, how it can be exploited, who is at risk, and provides a code snippet showing just how easy it is to abuse this flaw under the right circumstances.

What Is CVE-2025-31489?

This vulnerability resides in the authorization logic of MinIO’s API. The key problem? The so-called "signature" that helps authenticate clients *could* be entirely invalid. Yet, as long as you know a valid access key (the user's identifier, similar to a username) and target a bucket where that user has WRITE permissions, you can upload anything you want to that bucket—*regardless of whether you've got the real secret key* or not.

In simple terms:
If you know an access key and a bucket name, and the access key really does have write rights, *the system will happily let you in,* even if you provide a completely wrong signature.

Severity:
While you need prior knowledge of an access key and the relevant bucket, this is a critical risk in shared environments where such information may be leaked or guessed.

The attacker can upload arbitrary objects (files, data) to the bucket.

Note:
This doesn’t open up read or admin permissions, but can still wreak havoc—examples include poisoning data lakes or storage used for logs, machine learning, or data sharing.

How Does the Exploit Work?

MinIO expects clients to sign API requests (using their secret key) as a way to prove their identity. Due to this flaw, MinIO’s signature verification logic can be bypassed: the server only checks if the access key is valid and has WRITE permissions, not if the signature matches.

That means, you can pass *any* string as the secret/signature part of the request, and your upload succeeds as long as the access-key is write-enabled.

A bucket you want to target: demo-bucket

- The MinIO endpoint: https://minio.company.com

With the broken signature check (on a MinIO version prior to RELEASE.2025-04-03T14-56-28Z), you can upload a file using curl with a garbage secret.

Here is how it looks in practice

FILE_TO_UPLOAD="my-malicious-file.txt"
BUCKET="demo-bucket"
ACCESS_KEY="testuser"
FAKE_SECRET="lolnotarealsecret"
MINIO_ENDPOINT="https://minio.company.com"

curl -X PUT \
  -T "$FILE_TO_UPLOAD" \
  -H "Host: $MINIO_ENDPOINT" \
  -u "$ACCESS_KEY:$FAKE_SECRET" \
  "$MINIO_ENDPOINT/$BUCKET/$FILE_TO_UPLOAD"

What happens here

- The -u flag provides the access-key:secret for HTTP basic authentication, but the secret (FAKE_SECRET) is trash.

References

- Original MinIO Security Advisory *(placeholder, see official repo for final advisory)*
- MinIO Release Notes
- CVE-2025-31489 at MITRE *(pending publication)*
- About MinIO

What Should You Do?

The MinIO team patched this issue in RELEASE.2025-04-03T14-56-28Z.

Conclusion

CVE-2025-31489 demonstrates how small mistakes in authentication logic can have big consequences. If you’re self-hosting MinIO, take this as a wake-up call to stay current with security releases and keep a close eye on access credentials. Unattended upgrades and regular audits can save you a world of trouble.

If you’re interested in more details, follow the official MinIO GitHub Security Advisory page and keep up-to-date with CVE databases as more information becomes available.

Stay safe, patch early, and never trust user input!

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Timeline

Published on: 04/03/2025 20:15:25 UTC
Last modified on: 04/07/2025 14:18:34 UTC