CVE-2021-21392
Open redirect via transitional IPv6 addresses on dual-stack networks
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 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- Low
- Privileges needed
- Low
- User interaction
- Required
What an attacker can affect
- Confidentiality
- Low
- Integrity
- High
- Availability
- None
Exploit likelihood
What each source reports
CVE List Score used
Open redirect via transitional IPv6 addresses on dual-stack networks
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 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.
- Record
CVE-2021-21392- CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N - Weakness
- CWE-601
- Updated
- 2026-11-02
EUVDEUVD-2021-0128
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 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.
- Record
EUVD-2021-0128- CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N - Updated
- 2026-11-03
GitHub AdvisoriesGHSA-5wrh-4jwv-5w78
Open redirect via transitional IPv6 addresses on dual-stack networks
Impact
Requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks.
Patches
This issue is fixed by #9240.
Workarounds
Outbound requests to the following address ranges can be blocked by a firewall, if unused for internal communication between systems:
::ffff/80::0000/80(note that this IP range is considered deprecated by the IETF)2002::/16(note that this IP range is considered deprecated by the IETF)
References
- Record
GHSA-5wrh-4jwv-5w78- CVSS 4.0
- 7.1 High
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:H/VA:N/SC:N/SI:N/SA:N - CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N - Weakness
- CWE-601
- Updated
- 2026-11-02
NVD
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 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.
- Record
CVE-2021-21392- CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N - CVSS 2.0
- 4.9 Medium
AV:N/AC:M/Au:S/C:P/I:P/A:N - Weakness
- CWE-601
- Updated
- 2026-11-02
OSV
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 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.
- Record
CVE-2021-21392- CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N - Updated
- 2026-11-02
OSVDEBIAN-CVE-2021-21392
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 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.
- Record
DEBIAN-CVE-2021-21392- CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N - Updated
- 2026-11-02
OSVGHSA-5wrh-4jwv-5w78
Open redirect via transitional IPv6 addresses on dual-stack networks
Impact
Requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks.
Patches
This issue is fixed by #9240.
Workarounds
Outbound requests to the following address ranges can be blocked by a firewall, if unused for internal communication between systems:
::ffff/80::0000/80(note that this IP range is considered deprecated by the IETF)2002::/16(note that this IP range is considered deprecated by the IETF)
References
- Record
GHSA-5wrh-4jwv-5w78- CVSS 4.0
- 7.1 High
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:H/VA:N/SC:N/SI:N/SA:N - CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N - Weakness
- CWE-601
- Updated
- 2026-11-02
OSVUBUNTU-CVE-2021-21392
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 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.
- Record
UBUNTU-CVE-2021-21392- CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N - Updated
- 2026-11-02
OSVPYSEC-2021-25 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 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.
- Record
PYSEC-2021-25- Updated
- 2026-11-02
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
matrix-synapsePyPI |
before 1.28.0rc1 | 1.28.0rc1 |
matrix-synapsePyPI |
before 1.28.0rc1, plus listed versions 0.33.50.33.5.10.33.60.33.6rc10.33.7 Show 45 moreShow less0.33.7rc10.33.7rc20.33.80.33.8rc20.33.90.34.00.34.0.10.34.0rc10.34.0rc20.34.1.10.99.00.99.0rc10.99.0rc20.99.0rc30.99.0rc40.99.10.99.1.10.99.1rc10.99.1rc20.99.20.99.2rc10.99.30.99.3.10.99.3.20.99.3rc10.99.40.99.4rc10.99.50.99.5.10.99.5.20.99.5rc11.0.01.0.0rc11.0.0rc21.0.0rc31.1.01.1.0rc11.1.0rc21.10.01.10.0rc11.10.0rc21.10.0rc31.10.0rc51.10.11.11.0 |
1.28.0rc1 |
matrix-synapsePyPI |
before 1.28.0, plus listed versions 0.33.50.33.5.10.33.6rc10.33.60.33.7rc1 Show 45 moreShow less0.33.7rc20.33.70.33.8rc20.33.80.33.90.34.0rc10.34.0rc20.34.00.34.0.10.34.1.10.99.0rc10.99.0rc20.99.0rc30.99.0rc40.99.00.99.1rc10.99.1rc20.99.10.99.1.10.99.2rc10.99.20.99.3rc10.99.30.99.3.10.99.3.20.99.4rc10.99.40.99.5rc10.99.50.99.5.10.99.5.21.0.0rc11.0.0rc21.0.0rc31.0.01.1.0rc11.1.0rc21.1.01.2.0rc11.2.0rc21.2.01.2.11.3.0rc11.3.01.3.1 |
1.28.0 |
matrix-synapseUbuntu: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-synapseUbuntu: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 less1.6.1-11.7.0-21.7.1-11.7.2-11.7.3-11.8.0-11.9.0-11.9.1-11.10.0-11.10.0-21.11.0-11.11.0-1ubuntu0.1~esm2 |
None known |
matrix-synapseUbuntu: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 less1.49.2-11.50.1-11.50.2-11.51.0-11.52.0-11.53.0-11.53.0-1ubuntu0.1~esm2 |
None known |
History
| Recorded | Changed | Sources |
|---|---|---|
| affected | ||
| aliases, products | EUVD | |
| first indexed | CVE List, GitHub Advisories, NVD, OSV |
Dates are when Every CVE recorded each change. Upstream dates may differ.
In short
CVE-2021-21392 is a medium vulnerability (CVSS 6.3) 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 0.94%, the 61st percentile. Fixed in matrix-synapse 1.28.0rc1.
References
pypi.org/project/matrix-synapse
github.com/matrix-org/synapse/security/advisories/GHSA-5wrh-4jwv-5w78
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY
nvd.nist.gov/vuln/detail/CVE-2021-21392
Show 7 moreShow less
security-tracker.debian.org/tracker/CVE-2021-21392
ubuntu.com/security/CVE-2021-21392
cve.org/CVERecord?id=CVE-2021-21392
Left out here: 201 versions. The JSON record has them.