Denial of service (via resource exhaustion) due to improper input validation on groups/communities endpoints
Key facts
- Attack vector
- Network
- Privileges required
- Low
- User interaction
- None
- Attack complexity
- Low
- Impact type
- —
- Confidentiality · integrity · availability
- None · None · High
- Fix status
- Fixed
Attack profile from CVSS 3.1 · NVD. Fix status from CNA and GitHub.
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 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.
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 1.28.0 | 1.28.0 | Patch available | CNA |
| — | matrix-synapse (PyPI) | before 1.28.0 | 1.28.0 | Patch available | GHSA-jrh7-mhhx-6h88 |
- synapse Vendormatrix-org Affected before 1.28.0 Fixed in 1.28.0 Patch available CNA
- matrix-synapse (PyPI) Vendor— Affected before 1.28.0 Fixed in 1.28.0 Patch available GHSA-jrh7-mhhx-6h88
CPE configurations NVD · 2
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:* | after 0.24.0, before 1.28.0 | Vulnerable | matrix-org |
| cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:* | 34 | Vulnerable | Fedoraproject |
- cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:* Versions after 0.24.0, before 1.28.0 Vulnerable ·matrix-org
- cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:* Versions 34 Vulnerable ·Fedoraproject
Remediation
- Fix · CNA synapse (matrix-org) Fixed in 1.28.0
- Fix · GHSA-jrh7-mhhx-6h88 matrix-synapse (PyPI) Fixed in 1.28.0 Advisory →
Vendor assessments
Timeline
- 2026-10-02 Threatist record created from CVE-2021-21393
- 2026-10-03 Alias added: GHSA-jrh7-mhhx-6h88
References
Patch 3
Advisory 3
Other 6
- https://github.com/matrix-org/synapse/commit/3f58fc848d0002de4605bed91603a1f9f245d128
- https://github.com/matrix-org/synapse/commit/d2f0ec12d5c8f113095408888e87e191ac546499
- https://github.com/matrix-org/synapse
- https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-26.yaml
- https://lists.fedoraproject.org/archives/list/[email protected]/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY
- https://pypi.org/project/matrix-synapse
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-2021-21393 · CVE Program
- CVE-2021-21393 · NVD
- FIRST EPSS
- GHSA-jrh7-mhhx-6h88 · GitHub Advisory Database · CC BY 4.0
- CVE-2021-21393 · 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