What is CVE-2021-21390?

CVE-2021-21390 is a medium-severity vulnerability in Minio, classified under Improper Enforcement of Message Integrity During Transmission in a Communication Channel. CVSS score: 6.5/10. Published 2021-03-19.

How severe is CVE-2021-21390?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Is CVE-2021-21390 known to be exploited?

2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Minio

CVE-2021-21390

MinIO is an open-source high performance object storage service and it is API compatible with Amazon S3 cloud storage service. In MinIO before version RELEASE.2021-03-17T02-33-02Z, there is a vulnerability which enables MITM modification of request bodies that are meant to have integrity guaranteed by chunk signatures. In a PUT request using aws-chunked encoding, MinIO ordinarily verifies signatures at the end of a chunk. This check can be skipped if the client sends a false chunk size that is much greater than the actual data sent: the server accepts and completes the request without ever reaching the end of the chunk + thereby without ever checking the chunk signature. This is fixed in version RELEASE.2021-03-17T02-33-02Z. As a workaround one can avoid using "aws-chunked" encoding-based chunk signature upload requests instead use TLS. MinIO SDKs automatically disable chunked encoding signature when the server endpoint is configured with TLS.

Published 2021-03-19 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N.

EPSS exploit prediction

EPSS: 0.009 (56.5th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 57th percentile — 56.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.009 · newest: 0.009 · change: 0.000

Affected products

Minio — versions < RELEASE.2021-03-17T02-33-02Z

Weakness classification (CWE)

CWE-924 — Improper Enforcement of Message Integrity During Transmission in a Communication Channel

Public proof-of-concept exploits

References

github.com/minio/minio/security/advisories/GHSA-xr7r-7gpj-5pgp (Exploit, Patch, Third Party Advisory)
github.com/minio/minio/pull/11801 (Exploit, Patch, Third Party Advisory)
github.com/minio/minio/commit/e197800f9055489415b53cf137e31e194aaf7ba0 (Exploit, Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2021-21390?: CVE-2021-21390 is a medium-severity vulnerability in Minio, classified under Improper Enforcement of Message Integrity During Transmission in a Communication Channel. CVSS score: 6.5/10. Published 2021-03-19.
How severe is CVE-2021-21390?: Medium severity. CVSS v3 base score is 6.5 out of 10.
Is CVE-2021-21390 known to be exploited?: 2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.