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CVE-2021-39163

Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.

3.1 LowFix available

Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where the vulnerable homeserver is in the room and untrusted users are permitted to create groups (communities). By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential workarounds. Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators. Administrators that are using a reverse proxy could, with partial loss of group functionality, block the endpoints /_matrix/client/r0/groups/{group_id}/rooms and /_matrix/client/unstable/groups/{group_id}/rooms.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
High
Privileges needed
Low
User interaction
None

What an attacker can affect

Confidentiality
Low
Integrity
None
Availability
None

Exploit likelihood

0.90%Up 0.1 points since Sep 1, 2021
90%50%10%1%

What each source reports

CVE List Score used 3.1

Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.

Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where the vulnerable homeserver is in the room and untrusted users are permitted to create groups (communities). By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential workarounds. Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators. Administrators that are using a reverse proxy could, with partial loss of group functionality, block the endpoints /_matrix/client/r0/groups/{group_id}/rooms and /_matrix/client/unstable/groups/{group_id}/rooms.

Record
CVE-2021-39163
CVSS 3.1
3.1 LowCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
Weakness
CWE-200
Updated
2026-11-02
EUVDEUVD-2021-0131 3.1

Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where the vulnerable homeserver is in the room and untrusted users are permitted to create groups (communities). By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential workarounds. Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators. Administrators that are using a reverse proxy could, with partial loss of group functionality, block the endpoints /_matrix/client/r0/groups/{group_id}/rooms and /_matrix/client/unstable/groups/{group_id}/rooms.

Record
EUVD-2021-0131
CVSS 3.1
3.1 LowCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
Updated
2026-11-03
GitHub AdvisoriesGHSA-jj53-8fmw-f2w2 2.3

Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.

Impact

Unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where:

  • the vulnerable homeserver is in the room; and
  • untrusted users are permitted to create groups (communities).

By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted.

Patches

Server administrators should upgrade to 1.41.1 or higher.

Workarounds

Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators.

Administrators that are using a reverse proxy could, with partial loss of group functionality, block the following endpoints:

  • /_matrix/client/r0/groups/{group_id}/rooms
  • /_matrix/client/unstable/groups/{group_id}/rooms

References

n/a

For more information

If you have any questions or comments about this advisory, e-mail us at [email protected].

Record
GHSA-jj53-8fmw-f2w2
CVSS 4.0
2.3 LowCVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N
CVSS 3.1
3.1 LowCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
Weakness
CWE-200
Updated
2026-11-02
NVD 3.1

Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where the vulnerable homeserver is in the room and untrusted users are permitted to create groups (communities). By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential workarounds. Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators. Administrators that are using a reverse proxy could, with partial loss of group functionality, block the endpoints /_matrix/client/r0/groups/{group_id}/rooms and /_matrix/client/unstable/groups/{group_id}/rooms.

Record
CVE-2021-39163
CVSS 3.1
3.1 LowCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
CVSS 2.0
3.5 LowAV:N/AC:M/Au:S/C:P/I:N/A:N
Weakness
CWE-200, CWE-863
Updated
2026-11-02
OSVALPINE-CVE-2021-39163 3.1

Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where the vulnerable homeserver is in the room and untrusted users are permitted to create groups (communities). By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential workarounds. Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators. Administrators that are using a reverse proxy could, with partial loss of group functionality, block the endpoints /_matrix/client/r0/groups/{group_id}/rooms and /_matrix/client/unstable/groups/{group_id}/rooms.

Record
ALPINE-CVE-2021-39163
CVSS 3.1
3.1 LowCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
Updated
2026-11-03
OSV 3.1

Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where the vulnerable homeserver is in the room and untrusted users are permitted to create groups (communities). By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential workarounds. Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators. Administrators that are using a reverse proxy could, with partial loss of group functionality, block the endpoints /_matrix/client/r0/groups/{group_id}/rooms and /_matrix/client/unstable/groups/{group_id}/rooms.

Record
CVE-2021-39163
CVSS 3.1
3.1 LowCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
Updated
2026-11-02
Show 5 moreShow less
OSVDEBIAN-CVE-2021-39163 3.1

Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where the vulnerable homeserver is in the room and untrusted users are permitted to create groups (communities). By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential workarounds. Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators. Administrators that are using a reverse proxy could, with partial loss of group functionality, block the endpoints /_matrix/client/r0/groups/{group_id}/rooms and /_matrix/client/unstable/groups/{group_id}/rooms.

Record
DEBIAN-CVE-2021-39163
CVSS 3.1
3.1 LowCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
Updated
2026-11-02
OSVGHSA-jj53-8fmw-f2w2 2.3

Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.

Impact

Unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where:

  • the vulnerable homeserver is in the room; and
  • untrusted users are permitted to create groups (communities).

By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted.

Patches

Server administrators should upgrade to 1.41.1 or higher.

Workarounds

Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators.

Administrators that are using a reverse proxy could, with partial loss of group functionality, block the following endpoints:

  • /_matrix/client/r0/groups/{group_id}/rooms
  • /_matrix/client/unstable/groups/{group_id}/rooms

References

n/a

For more information

If you have any questions or comments about this advisory, e-mail us at [email protected].

Record
GHSA-jj53-8fmw-f2w2
CVSS 4.0
2.3 LowCVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N
CVSS 3.1
3.1 LowCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
Weakness
CWE-200
Updated
2026-11-02
OSVUBUNTU-CVE-2021-39163 3.1

Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where the vulnerable homeserver is in the room and untrusted users are permitted to create groups (communities). By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential workarounds. Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators. Administrators that are using a reverse proxy could, with partial loss of group functionality, block the endpoints /_matrix/client/r0/groups/{group_id}/rooms and /_matrix/client/unstable/groups/{group_id}/rooms.

Record
UBUNTU-CVE-2021-39163
CVSS 3.1
3.1 LowCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
Updated
2026-11-02
OSVopenSUSE-SU-2024:11041-1 No score

matrix-synapse-1.43.0-1.1 on GA media

These are all security issues fixed in the matrix-synapse-1.43.0-1.1 package on the GA media of openSUSE Tumbleweed.

Record
openSUSE-SU-2024:11041-1
Updated
2026-11-02
OSVPYSEC-2021-424 No score

Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, unauthorised users can access the name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where the vulnerable homeserver is in the room and untrusted users are permitted to create groups (communities). By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting enable_group_creation has been set to true are impacted. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential workarounds. Server administrators can set enable_group_creation to false in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators. Administrators that are using a reverse proxy could, with partial loss of group functionality, block the endpoints /_matrix/client/r0/groups/{group_id}/rooms and /_matrix/client/unstable/groups/{group_id}/rooms.

Record
PYSEC-2021-424
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
matrix-synapse
PyPI
 before 1.41.1
1.41.1
synapse
Alpine:v3.14
 before 1.41.1-r0
1.41.1-r0
synapse
Alpine:v3.15
 before 1.41.1-r0
1.41.1-r0
synapse
Alpine:v3.16
 before 1.41.1-r0
1.41.1-r0
synapse
Alpine:v3.17
 before 1.41.1-r0
1.41.1-r0
synapse
Alpine:v3.18
 before 1.41.1-r0
1.41.1-r0
synapse
Alpine:v3.19
 before 1.41.1-r0
1.41.1-r0
synapse
Alpine:v3.20
 before 1.41.1-r0
1.41.1-r0
synapse
Alpine:v3.21
 before 1.41.1-r0
1.41.1-r0
synapse
Alpine:v3.22
 before 1.41.1-r0
1.41.1-r0
Show 7 moreShow less
Affected packages
PackageAffectedFixed
synapse
Alpine:v3.23
 before 1.41.1-r0
1.41.1-r0
synapse
Alpine:v3.24
 before 1.41.1-r0
1.41.1-r0
matrix-synapse
PyPI
 before 1.41.1, plus listed versions
0.33.50.33.5.10.33.60.33.6rc10.33.7
Show 45 moreShow less
0.33.7rc10.33.7rc20.33.80.33.8rc20.33.90.34.00.34.0.10.34.0rc10.34.0rc20.34.1.10.99.00.99.0rc10.99.0rc20.99.0rc30.99.0rc40.99.10.99.1.10.99.1rc10.99.1rc20.99.20.99.2rc10.99.30.99.3.10.99.3.20.99.3rc10.99.40.99.4rc10.99.50.99.5.10.99.5.20.99.5rc11.0.01.0.0rc11.0.0rc21.0.0rc31.1.01.1.0rc11.1.0rc21.10.01.10.0rc11.10.0rc21.10.0rc31.10.0rc51.10.11.11.0
1.41.1
matrix-synapse
Ubuntu:Pro:18.04:LTS
 All versions, plus listed versions
0.19.2+dfsg-60.24.0+dfsg-10.24.0+dfsg-1ubuntu0.1~esm10.24.0+dfsg-1ubuntu0.1~esm4
 None known
matrix-synapse
Ubuntu:Pro:20.04:LTS
 All versions, plus listed versions
1.3.0-11.4.0-11.5.0-11.5.1-11.6.0-1
Show 12 moreShow less
1.6.1-11.7.0-21.7.1-11.7.2-11.7.3-11.8.0-11.9.0-11.9.1-11.10.0-11.10.0-21.11.0-11.11.0-1ubuntu0.1~esm2
 None known
matrix-synapse
Ubuntu:Pro:22.04:LTS
 All versions, plus listed versions
1.39.0-11.47.0-21.47.1-11.48.0-11.49.0-1
Show 7 moreShow less
1.49.2-11.50.1-11.50.2-11.51.0-11.52.0-11.53.0-11.53.0-1ubuntu0.1~esm2
 None known
matrix-synapse
openSUSE:Tumbleweed
From an advisory covering 11 CVEs
 before 1.43.0-1.1
1.43.0-1.1

History

History
RecordedChangedSources
affected, facts
affected, aliases, referencesOSV
affected
aliases, productsEUVD
first indexedCVE List, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2021-39163 is a low vulnerability (CVSS 3.1) in Element-hq Synapse, published August 31, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.90%, the 59th percentile. Fixed in matrix-synapse 1.41.1.

References

Show 21 moreShow less
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Left out here: 147 versions. The JSON record has them.

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