CVE-2021-39177: User impersonation due to incorrect handling of the login JWT

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-08-30 · updated 2026-06-17
Weakness: CWE-287

Description

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.

Affected products

GeyserMC Geyser: before 1.4.2 — fixed in 1.4.2

How to fix

Upgrade to a fixed version: GeyserMC Geyser 1.4.2. Check the vendor advisory for mitigations.

References

https://github.com/GeyserMC/Geyser/commit/b9541505af68ac7b7c093206ac7b1ba88957a5a6
https://github.com/GeyserMC/Geyser/security/advisories/GHSA-h77f-xxx7-4858
https://updates.playhive.com/weekend-maintenance-disclosure-2kJMaY
https://github.com/GeyserMC/Geyser/commit/b9541505af68ac7b7c093206ac7b1ba88957a5a6
https://github.com/GeyserMC/Geyser/security/advisories/GHSA-h77f-xxx7-4858
https://updates.playhive.com/weekend-maintenance-disclosure-2kJMaY

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