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CVE-2021-21333

HTML injection in email and account expiry notifications

6.1 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 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
High
Privileges needed
None
User interaction
Required

What an attacker can affect

Confidentiality
None
Integrity
High
Availability
None

Exploit likelihood

1.35%Up 0.3 points since Apr 14, 2021
90%50%10%1%

What each source reports

CVE List Score used 6.1

HTML injection in email and account expiry notifications

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 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.

Record
CVE-2021-21333
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N
Weakness
CWE-74
Updated
2026-11-02
EUVDEUVD-2021-0126 6.1

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 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.

Record
EUVD-2021-0126
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N
Updated
2026-11-03
GitHub AdvisoriesGHSA-c5f8-35qr-q4fm 4.9

HTML injection in email and account expiry notifications

Impact

The notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email.

The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker.

Patches

This issue is fixed in #9200.

Workarounds

For the missed messages notifications:

The notif.html, notif_mail.html, and room.html templates can be overridden with custom templates that manually escapes the variables using JInja2's escape filter. See the email.template_dir setting.

For the account expiry notifications:

  1. Account expiry can be disabled via the account_validity.enabled setting.

  2. The notice_expiry.html template can be overridden with a custom template that manually escapes the variables using JInja2's escape filter. See the email.template_dir setting.

Record
GHSA-c5f8-35qr-q4fm
CVSS 4.0
4.9 MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:N/SI:H/SA:N
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N
Weakness
CWE-74, CWE-79
Updated
2026-11-02
NVD 6.1

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 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.

Record
CVE-2021-21333
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N
CVSS 2.0
2.6 LowAV:N/AC:H/Au:N/C:N/I:P/A:N
Weakness
CWE-74, CWE-79
Updated
2026-11-02
OSV 6.1

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 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.

Record
CVE-2021-21333
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N
Updated
2026-11-02
OSVDEBIAN-CVE-2021-21333 6.1

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 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.

Record
DEBIAN-CVE-2021-21333
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N
Updated
2026-11-02
OSVGHSA-c5f8-35qr-q4fm 4.9

HTML injection in email and account expiry notifications

Impact

The notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email.

The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker.

Patches

This issue is fixed in #9200.

Workarounds

For the missed messages notifications:

The notif.html, notif_mail.html, and room.html templates can be overridden with custom templates that manually escapes the variables using JInja2's escape filter. See the email.template_dir setting.

For the account expiry notifications:

  1. Account expiry can be disabled via the account_validity.enabled setting.

  2. The notice_expiry.html template can be overridden with a custom template that manually escapes the variables using JInja2's escape filter. See the email.template_dir setting.

Record
GHSA-c5f8-35qr-q4fm
CVSS 4.0
4.9 MediumCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:N/SI:H/SA:N
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N
Weakness
CWE-74, CWE-79
Updated
2026-11-02
OSVUBUNTU-CVE-2021-21333 6.1

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 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.

Record
UBUNTU-CVE-2021-21333
CVSS 3.1
6.1 MediumCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N
Updated
2026-11-02
OSVPYSEC-2021-134 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 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.

Record
PYSEC-2021-134
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
matrix-synapse
PyPI
 before 1.27.0
1.27.0
matrix-synapse
PyPI
 before 1.27.0, 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
1.27.0
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

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-2021-21333 is a medium vulnerability (CVSS 6.1) in Element-hq Synapse, published March 26, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 1.35%, the 71st percentile. Fixed in matrix-synapse 1.27.0.

References

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