CVE-2021-39177: Geysermc Geyser

Critical severity, CVSS 9.8. EPSS: 1.4% chance of exploitation in the next 30 days.

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)

Published 2021-08-30. Last modified 2026-06-17.