CVE-2021-39177

User impersonation due to incorrect handling of the login JWT

Workarounds

Geyser strongly recommends updating to fix this issue. If this isn't possible:

• Use online mode and don't save credentials in your Geyser configuration
• Use an additional authentication method on the Java server

References

This was disclosed to us by a staff member over at Hive; you can read their disclosure here: https://updates.playhive.com/weekend-maintenance-disclosure-2kJMaY

For more information

If you have any questions or comments about this advisory:

• Come talk to us over on our Discord server in the #development channel

This allows anyone that can connect to the server to forge a LoginPacket with manipulated JWT token allowing impersonation as any Bedrock user. Unless credentials are saved in your configuration, online mode is not affected as users are still required to log in separately. If your credentials are saved, there is no risk of exposing your email or password.

The application does not adequately verify the identity of a user, device, or process before granting access. Typical impact: unauthorized access to functions or data reserved for authenticated parties.

CVE-2021-39177 has a CVSS score of 7.4 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.4.2-SNAPSHOT); upgrading removes the vulnerable code path.

Kodem intelligence

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This was patched as part of https://github.com/GeyserMC/Geyser/commit/b9541505af68ac7b7c093206ac7b1ba88957a5a6 and https://github.com/GeyserMC/Geyser/commit/ab2f5b326fe590e09167e8b45b4b165ac06ecd13. if your Geyser version is 1.4.2-SNAPSHOT or later, the issue has been addressed on your build.

1. What is CVE-2021-39177? CVE-2021-39177 is a high-severity improper authentication vulnerability in org.geysermc:connector (maven), affecting versions <= 1.4.1-SNAPSHOT. It is fixed in 1.4.2-SNAPSHOT. The application does not adequately verify the identity of a user, device, or process before granting access.
2. How severe is CVE-2021-39177? CVE-2021-39177 has a CVSS score of 7.4 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of org.geysermc:connector are affected by CVE-2021-39177? org.geysermc:connector (maven) versions <= 1.4.1-SNAPSHOT is affected.
4. Is there a fix for CVE-2021-39177? Yes. CVE-2021-39177 is fixed in 1.4.2-SNAPSHOT. Upgrade to this version or later.
5. Is CVE-2021-39177 exploitable, and should I be worried? Whether CVE-2021-39177 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-39177 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-39177? Upgrade org.geysermc:connector to 1.4.2-SNAPSHOT or later.