CVE-2021-39164Network attack vector

Room-membership disclosure via historical-state authorization bypass

Published Aug 31, 2021 · Updated Aug 4, 2024

Incorrect authorization in Matrix.org Synapse 1.41.0 and earlier allows remote authenticated users to read room membership data. The /members handler accepts an at token after checking only one historical event's visibility, then returns full historical room state without confirming that the requester belonged to the room. Exploitation requires the room ID, shared history visibility, and an account on a vulnerable homeserver participating in the room; exposure is limited to member identifiers and display names.

CVSS severity3.1
Low
EPSS probability1.46%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Incorrect authorization in Matrix.org Synapse 1.41.0 and earlier allows remote authenticated users to read room membership data. The /members handler accepts an at token after checking only one historical event's visibility, then returns full historical room state without confirming that the requester belonged to the room. Exploitation requires the room ID, shared history visibility, and an account on a vulnerable homeserver participating in the room; exposure is limited to member identifiers and display names.

The record

CVE
CVE-2021-39164
Published
Aug 31, 2021
Updated
Aug 4, 2024
Vendor
Matrix.org
Product
Synapse
Classifications
CWE-200, CWE-863, T1087
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Aug 31, 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=< 1.41.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Matrix.org · Synapseversion=< 1.41.1

Published CVSS scores

3.1GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
3.5NISTAV:N/AC:M/Au:S/C:P/I:N/A:N
3.1NISTCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N

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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-200Exposure of Sensitive Information to an Unauthorized Actor
CWE-863Incorrect Authorization
T1087Account Discovery

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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