What is CVE-2021-39177?

CVE-2021-39177 is a high-severity vulnerability in Geysermc Geyser, classified under Improper Authentication. CVSS score: 7.4/10. Published 2021-08-30.

How severe is CVE-2021-39177?

High severity. CVSS v3 base score is 7.4 out of 10.

Auth bypass in Geysermc Geyser

CVE-2021-39177

Geyser is a bridge between Minecraft: Bedrock Edition and Minecraft: Java Edition. Versions of Geyser prior to 1.4.2-SNAPSHOT allow anyone that can connect to the server to forge a LoginPacket with manipulated JWT token allowing impersonation as any user. Version 1.4.2-SNAPSHOT contains a patch for the issue. There are no known workarounds aside from upgrading.

Vulnerability class: Broken Authentication

Published 2021-08-30 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 7.4 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N.

EPSS exploit prediction

EPSS: 0.014 (72.0th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 72nd percentile — 72.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.014 · newest: 0.014 · change: 0.000

Affected products

Geysermc Geyser — versions <= 1.4.1-SNAPSHOT

Weakness classification (CWE)

CWE-287 — Improper Authentication

References

github.com/GeyserMC/Geyser/security/advisories/GHSA-h77f-xxx7-4858 (Patch, Third Party Advisory)
github.com/GeyserMC/Geyser/commit/b9541505af68ac7b7c093206ac7b1ba88957a5a6 (Patch, Third Party Advisory)
updates.playhive.com/weekend-maintenance-disclosure-2kJMaY (Third Party Advisory)

Frequently asked questions

What is CVE-2021-39177?: CVE-2021-39177 is a high-severity vulnerability in Geysermc Geyser, classified under Improper Authentication. CVSS score: 7.4/10. Published 2021-08-30.
How severe is CVE-2021-39177?: High severity. CVSS v3 base score is 7.4 out of 10.