How severe is CVE-2021-39177?

CVE-2021-39177 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-39177?

CVE-2021-39177 affects the following packages: org.geysermc:connector (Maven). Ecosystems affected: Maven.

How do I fix CVE-2021-39177?

Update org.geysermc:connector to 1.4.2-SNAPSHOT or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39177 is resolved across your whole dependency graph.

How do I detect CVE-2021-39177 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.geysermc:connector, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-39177 if there is no patch (or I can't update yet)?

Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Is CVE-2021-39177 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-39177 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-39177?

CVE-2021-39177 has an EPSS (Exploit Prediction Scoring System) score of 1.4%, placing it in the 72th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-39177?

CVE-2021-39177 is classified as Improper Authentication (CWE-287). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-39177 published, and has it been updated?

CVE-2021-39177 was published on August 30, 2021 and was last updated on October 1, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕

☕ Maven

Not in CISA KEV

CRITICAL severity

CVE-2021-39177 — connector

CRITICALFix: GeyserMC/Geyser@b954150

CVE-2021-39177 is a critical-severity (CVSS 9.8) Improper Authentication vulnerability in org.geysermc:connector. A fix is available for org.geysermc:connector — see the affected versions and patch details below.

User impersonation due to incorrect handling of the login JWT

Also known asGHSA-h77f-xxx7-4858

Published

Aug 30, 2021

Updated

Oct 1, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs72th percentile — riskier than 72% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-39177 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

☕org.geysermc:connector

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

Impact

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

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

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/changelog/weekend-maintenance-breakdown

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

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.geysermc:connector`	all versions	1.4.2-SNAPSHOT`org.geysermc:connector:1.4.2-SNAPSHOT`

Affected Products

1 product · 1 configurations

Application

geysergeysermc

< 1.4.2

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.geysermc:connector, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update org.geysermc:connector to 1.4.2-SNAPSHOT or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39177 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Frequently Asked Questions

### Impact  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  This was patched as part of https://github.com/GeyserMC/Geyser/commit/b9541505af