CVE-2021-39177Network attack vector

User impersonation via trailing untrusted JWT

Published Aug 30, 2021 · Updated Aug 4, 2024

Improper authentication in Geyser before 1.4.2-SNAPSHOT allows remote attackers to impersonate any Bedrock user through a forged LoginPacket. LoginEncryptionUtils.validateChainData stopped at the first Mojang-signed JWT but accepted later attacker-signed entries, allowing a trailing token to replace the trusted identity key. Any user able to reach the Bedrock listener can assume another player identity; online mode remains protected unless credentials are saved.

CVSS severity7.4
High
EPSS probability1.43%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper authentication in Geyser before 1.4.2-SNAPSHOT allows remote attackers to impersonate any Bedrock user through a forged LoginPacket. LoginEncryptionUtils.validateChainData stopped at the first Mojang-signed JWT but accepted later attacker-signed entries, allowing a trailing token to replace the trusted identity key. Any user able to reach the Bedrock listener can assume another player identity; online mode remains protected unless credentials are saved.

The record

CVE
CVE-2021-39177
Published
Aug 30, 2021
Updated
Aug 4, 2024
Vendor
GeyserMC
Product
Geyser
Classifications
CWE-287, T1078
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 30, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=<= 1.4.1-SNAPSHOT

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

GeyserMC · Geyserversion=<= 1.4.1-SNAPSHOT

Published CVSS scores

7.4GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-287Improper Authentication
T1078Valid Accounts

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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