🛡️ CVE-2021-39177 — connector

🟠 CVSS 8.0 — High ✅ No Known Exploit CWE-287 OSV
8.0
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

User impersonation due to incorrect handling of the login JWT

Impact

<!--_What kind of vulnerability is it? Who is impacted?_-->

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.

Patches

<!--_Has the problem been patched? What versions should users upgrade to?_-->

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.

Workarounds

<!--_Is there a way for users to fix or remediate the vulnerability without upgrading?_-->

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

<!--_Are there any links users can visit to find out more?_-->

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](https://discord.gg/geysermc) server in the [#development](https://discord.com/channels/613163671870242838/613170125696270357) channel

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is high, an attacker needs no privileges on the target. No user interaction is required. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality high, integrity high, availability none.

Weakness class

CVE-2021-39177 is classified as CWE-287: Improper Authentication. The identity of the caller is not established correctly, so an attacker can act as another user.

Affected software

CVE-2021-39177 is recorded against 1 package.

  • org.geysermc:connector (fixed in 1.4.2-SNAPSHOT)

Timeline and source

Published on 7 September 2021 and last revised on 13 March 2026. No public exploit is currently recorded for this entry. Record sourced from OSV.

References

github.com (Web)
nvd.nist.gov (Advisory)
github.com (Web)
github.com (Package)
updates.playhive.com (Web)

Details

Severity HIGH
CVSS Score 8.0
CVSS Vector CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
CWE CWE-287
Public Exploit ✅ No
Source OSV
Published 2021-09-07
Updated 2026-08-12
Modified 2026-03-13
Fix URL N/A

Affected Packages

Software From version Fixed in
org.geysermc:connector — 1.4.2-SNAPSHOT

Site Security Check

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