Open redirects on some federation and push requests
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- Required
- Attack complexity
- Low
- Impact type
- Spoofing
- Confidentiality · integrity · availability
- Low · Low · None
- Fix status
- Fixed
Attack profile from CVSS 3.1 · NVD. Fix status from CNA and GitHub. Impact type from CWE-601.
Summary
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 1.25.0, 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.
Why it matters now
SSVC decision points
Not available: CISA has not published SSVC decision points for this record.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| matrix-org | synapse | before 1.25.0 | 1.25.0 | Patch available | CNA |
| — | matrix-synapse (PyPI) | before 1.25.0 | 1.25.0 | Patch available | GHSA-v936-j8gp-9q3p |
- synapse Vendormatrix-org Affected before 1.25.0 Fixed in 1.25.0 Patch available CNA
- matrix-synapse (PyPI) Vendor— Affected before 1.25.0 Fixed in 1.25.0 Patch available GHSA-v936-j8gp-9q3p
CPE configurations NVD · 2
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:* | before 1.25.0 | Vulnerable | matrix-org |
| cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:* | 34 | Vulnerable | Fedoraproject |
- cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:* Versions before 1.25.0 Vulnerable ·matrix-org
- cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:* Versions 34 Vulnerable ·Fedoraproject
Remediation
- Fix · CNA synapse (matrix-org) Fixed in 1.25.0
- Fix · GHSA-v936-j8gp-9q3p matrix-synapse (PyPI) Fixed in 1.25.0 Advisory →
Vendor assessments
Timeline
- 2026-10-02 Threatist record created from CVE-2021-21273
- 2026-10-03 Alias added: GHSA-v936-j8gp-9q3p
References
Patch 3
Advisory 3
More matrix-org Synapse vulnerabilities
- TI-2023-15617 Synapse Outgoing federation to specific hosts can be disabled by sending malicious invites CVSS 3.1MEDIUM 5.0
- TI-2024-35327 Synapse allows a a malformed invite to break the invitee's `/sync` CVSS 4.0HIGH 8.7
- TI-2019-13571 Matrix Synapse before 1.5.0 mishandles signature checking on some federation APIs. CVSS 3.1CRITICAL 9.8
- TI-2024-36056 Synapse can be forced to thumbnail unexpected file formats, invoking external, potentially untrustworthy decoders CVSS 4.0HIGH 8.2
- TI-2024-35318 Synapse allows unsupported content types to lead to memory exhaustion CVSS 4.0HIGH 8.2
- TI-2018-11846 Matrix Synapse before 0.33.3.1 allows remote attackers to spoof events and possibly have… CVSS 3.0HIGH 8.8
Sources
- CVE-2021-21273 · CVE Program
- CVE-2021-21273 · NVD
- FIRST EPSS
- GHSA-v936-j8gp-9q3p · GitHub Advisory Database · CC BY 4.0
- CVE-2021-21273 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences