Synapse vulnerable to federation denial of service via malformed events
Priority LOW 35CWE-20Published · Updated
Key facts
- Attack vector
- Network
- Privileges required
- None
- 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 an open source Matrix homeserver implementation. A malicious server can craft events which, when received, prevent Synapse version up to 1.127.0 from federating with other servers. The vulnerability has been exploited in the wild and has been fixed in Synapse v1.127.1. No known workarounds are available.
Why it matters now
Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.01226 69th percentileScored 2026-11-07
Fix Fixed synapse (element-hq): 1.127.1matrix-synapse (PyPI): 1.127.1
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- No
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| element-hq | synapse | before 1.127.1 | 1.127.1 | Patch available | CNA |
| — | matrix-synapse (PyPI) | before 1.127.1 | 1.127.1 | Patch available | GHSA-v56r-hwv5-mxg6 |
- synapse Vendorelement-hq Affected before 1.127.1 Fixed in 1.127.1 Patch available CNA
- matrix-synapse (PyPI) Vendor— Affected before 1.127.1 Fixed in 1.127.1 Patch available GHSA-v56r-hwv5-mxg6
CPE configurations NVD · 1
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:* | before 1.127.1 | Vulnerable | matrix-org |
- cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:* Versions before 1.127.1 Vulnerable ·matrix-org
Remediation
- Fix · CNA synapse (element-hq) Fixed in 1.127.1
- Fix · GHSA-v56r-hwv5-mxg6 matrix-synapse (PyPI) Fixed in 1.127.1 Advisory →
Timeline
- 2026-10-02 Threatist record created from CVE-2025-30355
- 2026-10-03 Alias added: GHSA-v56r-hwv5-mxg6
References
Advisory 2
More element-hq vulnerabilities
- TI-2024-35327 Synapse allows a a malformed invite to break the invitee's `/sync` CVSS 4.0HIGH 8.7
- 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-2022-07650 Remote program execution with user interaction CVSS 3.1HIGH 8.8
- TI-2026-19372 ESS Community Helm Chart has a weak server key generation method CVSS 4.0CRITICAL 9.2
- TI-2024-24814 Synapse denial of service through media disk space consumption CVSS 3.1HIGH 7.5
Sources
- CVE-2025-30355 · CVE Program
- CVE-2025-30355 · NVD
- FIRST EPSS
- GHSA-v56r-hwv5-mxg6 · GitHub Advisory Database · CC BY 4.0
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