Denial of service attack via .well-known lookups
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- Required
- Attack complexity
- Low
- Impact type
- Denial of service
- Confidentiality · integrity · availability
- None · None · High
- Fix status
- Fixed
Attack profile from CVSS 3.1 · NVD. Fix status from CNA and GitHub. Impact type from CWE-400.
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, a malicious homeserver could redirect requests to their .well-known file to a large file. This can lead to a denial of service attack where homeservers will consume significantly more resources when requesting the .well-known file of a malicious homeserver. This affects any server which accepts federation requests from untrusted servers. Issue is resolved in version 1.25.0. As a workaround the `federation_domain_whitelist` setting can be used to restrict the homeservers communicated with over federation.
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 | 0.99.0 to before 1.25.0 | 1.25.0 | Patch available | CNA |
| — | matrix-synapse (PyPI) | 0.99.0 to before 1.25.0 | 1.25.0 | Patch available | GHSA-2hwx-mjrm-v3g8 |
- synapse Vendormatrix-org Affected 0.99.0 to before 1.25.0 Fixed in 1.25.0 Patch available CNA
- matrix-synapse (PyPI) Vendor— Affected 0.99.0 to before 1.25.0 Fixed in 1.25.0 Patch available GHSA-2hwx-mjrm-v3g8
CPE configurations NVD · 2
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:* | 0.99.0 to before 1.25.0 | Vulnerable | matrix-org |
| cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:* | 34 | Vulnerable | Fedoraproject |
- cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:* Versions 0.99.0 to 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-2hwx-mjrm-v3g8 matrix-synapse (PyPI) Fixed in 1.25.0 Advisory →
Vendor assessments
Timeline
- 2026-10-02 Threatist record created from CVE-2021-21274
- 2026-10-03 Alias added: GHSA-2hwx-mjrm-v3g8
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-2021-21274 · CVE Program
- CVE-2021-21274 · NVD
- FIRST EPSS
- GHSA-2hwx-mjrm-v3g8 · GitHub Advisory Database · CC BY 4.0
- CVE-2021-21274 · 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