Open redirects on some federation and push requests
Impact
Requests to user provided domains were not restricted to external IP addresses when calculating the key validity for third-party invite events and sending push notifications. This could cause Synapse to make requests to internal infrastructure. The type of request was not controlled by the user, although limited modification of request bodies was possible.
For the most thorough protection server administrators should remove the deprecated federation_ip_range_blacklist from their settings after upgrading to Synapse v1.25.0 which will result in Synapse using the improved default IP address restrictions. See the new ip_range_blacklist and ip_range_whitelist settings if more specific control is necessary.
Patches
Issue is resolved by #8821. Further improvements to protect homeservers by default were made in #8870 and #8954.
Workarounds
Requests to internal IP addresses could be blocked at the system or network level.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | matrix-synapse | < 1.25.0 | 1.25.0 |
References
- https://github.com/matrix-org/synapse/releases/tag/v1.25.0
- https://github.com/matrix-org/synapse/commit/30fba6210834a4ecd91badf0c8f3eb278b72e746
- https://github.com/matrix-org/synapse/security/advisories/GHSA-v936-j8gp-9q3p
- https://github.com/matrix-org/synapse/pull/8821
- https://nvd.nist.gov/vuln/detail/CVE-2021-21273
- https://github.com/matrix-org/synapse
- https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-131.yaml
- https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY
- CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:L/A:N- Weakness (CWE)
CWE-601- Platform
- Python (PyPI)
- Package
matrix-synapse- Disclosed
- 26.02.2021
- Updated
- 11.10.2026 13:19
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2021-131 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-v936-j8gp-9q3p - CC BY 4.0
Vulnerability history: matrix-synapse
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 45
- First → latest
- 03.2019 05.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 21
- Medium 21
- Low 3
Disclosures per year
- 2019: 3
- 2020: 3
- 2021: 12
- 2022: 7
- 2023: 9
- 2024: 7
- 2025: 2
- 2026: 2
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Open redirects on some federation and push requests this entry
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