User impersonation due to incorrect handling of the login JWT

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.4 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS

• CVE-2021-39177: 1.44% estimated exploitation activity in the next 30 days; percentile 73.28 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

7 Sep 2021, 23:04 UTC

Last modified by the source

1 Oct 2026, 18:41 UTC

Source last checked

26 Oct 2026, 16:56 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-h77f-xxx7-4858

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# Patch Next advisory brief: CVE-2021-39177

Advisory: User impersonation due to incorrect handling of the login JWT
IDs: CVE-2021-39177, GHSA-h77f-xxx7-4858
Record: https://patchnext.com/advisories/cve-2021-39177 (revision 1, source checked 26 Oct 2026, 16:56 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: org.geysermc:connector 1.4.2-SNAPSHOT. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.geysermc:connector: affected < 1.4.2-SNAPSHOT; fixed in 1.4.2-SNAPSHOT

## Suggested inspection (read-only)
- Maven org.geysermc:connector: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on org.geysermc:connector.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-h77f-xxx7-4858 (GitHub-reviewed advisory GHSA-h77f-xxx7-4858 lists affected ranges: Maven org.geysermc:connector >=0 <1.4.2-SNAPSHOT)

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