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CVE-2021-21394

Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints

5.3 MediumFix available

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.

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
None
Integrity
None
Availability
High

Exploit likelihood

1.54%Up 0.5 points since Apr 14, 2021
90%50%10%1%

What each source reports

CVE List Score used 5.3

Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.

Record
CVE-2021-21394
CVSS 3.1
5.3 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-20
Updated
2026-11-02
EUVDEUVD-2021-0130 5.3

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.

Record
EUVD-2021-0130
CVSS 3.1
5.3 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-03
GitHub AdvisoriesGHSA-w9fg-xffh-p362 6.0

Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints

Impact

Missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion.

Patches

The issue is fixed by #9321.

Workarounds

Depending on the needs and configuration of the homeserver a few options are available:

  1. Using email as third-party identifiers be disabled by not configuring the email setting.

  2. Using phone numbers as third-party identifiers can be disabled by ensuring that account_threepid_delegates.msisdn is not configured.

  3. Additionally, the affected endpoint patterns can be blocked at a reverse proxy:

    • ^/_matrix/client/(r0|unstable)/register/email
    • ^/_matrix/client/(r0|unstable)/register/msisdn
    • ^/_matrix/client/(r0|unstable)/account/password
    • ^/_matrix/client/(r0|unstable)/account/3pid
Record
GHSA-w9fg-xffh-p362
CVSS 4.0
6.0 MediumCVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
CVSS 3.1
5.3 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-20
Updated
2026-11-02
NVD 5.3

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.

Record
CVE-2021-21394
CVSS 3.1
5.3 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
CVSS 2.0
4.0 MediumAV:N/AC:L/Au:S/C:N/I:N/A:P
Weakness
CWE-20
Updated
2026-11-02
OSV 6.5

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.

Record
CVE-2021-21394
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-02
OSVDEBIAN-CVE-2021-21394 6.5

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.

Record
DEBIAN-CVE-2021-21394
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-02
OSVGHSA-w9fg-xffh-p362 6.0

Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints

Impact

Missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion.

Patches

The issue is fixed by #9321.

Workarounds

Depending on the needs and configuration of the homeserver a few options are available:

  1. Using email as third-party identifiers be disabled by not configuring the email setting.

  2. Using phone numbers as third-party identifiers can be disabled by ensuring that account_threepid_delegates.msisdn is not configured.

  3. Additionally, the affected endpoint patterns can be blocked at a reverse proxy:

    • ^/_matrix/client/(r0|unstable)/register/email
    • ^/_matrix/client/(r0|unstable)/register/msisdn
    • ^/_matrix/client/(r0|unstable)/account/password
    • ^/_matrix/client/(r0|unstable)/account/3pid
Record
GHSA-w9fg-xffh-p362
CVSS 4.0
6.0 MediumCVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
CVSS 3.1
5.3 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-20
Updated
2026-11-02
OSVUBUNTU-CVE-2021-21394 5.3

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.

Record
UBUNTU-CVE-2021-21394
CVSS 3.1
5.3 MediumCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-02
OSVPYSEC-2021-27 No score

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.

Record
PYSEC-2021-27
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
matrix-synapse
PyPI
 before 1.28.0
1.28.0
matrix-synapse
PyPI
 before 1.28.0, 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
1.28.0
matrix-synapse
PyPI
 before 1.28.0, plus listed versions
0.33.50.33.5.10.33.6rc10.33.60.33.7rc1
Show 45 moreShow less
0.33.7rc20.33.70.33.8rc20.33.80.33.90.34.0rc10.34.0rc20.34.00.34.0.10.34.1.10.99.0rc10.99.0rc20.99.0rc30.99.0rc40.99.00.99.1rc10.99.1rc20.99.10.99.1.10.99.2rc10.99.20.99.3rc10.99.30.99.3.10.99.3.20.99.4rc10.99.40.99.5rc10.99.50.99.5.10.99.5.21.0.0rc11.0.0rc21.0.0rc31.0.01.1.0rc11.1.0rc21.1.01.2.0rc11.2.0rc21.2.01.2.11.3.0rc11.3.01.3.1
1.28.0
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

History

History
RecordedChangedSources
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-2021-21394 is a medium vulnerability (CVSS 5.3) in Element-hq Synapse, published April 12, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 1.54%, the 75th percentile. Fixed in matrix-synapse 1.28.0.

References

Show 7 moreShow less
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Left out here: 202 versions. The JSON record has them.

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