CVE-2025-30355 Synapse vulnerable to federation denial of service via malformed events
A malicious server can cause Synapse to stop federating with other servers by sending malformed events, requiring an upgrade to fix.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-30355?
CVE-2025-30355 is a high-severity denial of service vulnerability in element-hq synapse, published on 27 Mar 2025. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
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.
Weakness: CWE-20
Which versions are affected by CVE-2025-30355?
3 open source packages ship an affected version, including matrix-synapse. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
matrix-synapse PyPI | All versions | 1.127.1 |
matrix-synapse PyPI | All versions | 1.127.1 |
matrix-synapse PyPI | All versions | 1.127.1 |
Linux distribution packages (1)
matrix-synapse openSUSE:Tumbleweed | All versions | 1.127.1-1.1 |
Products include Matrix Synapse.
How do I fix CVE-2025-30355?
Upgrade matrix-synapse to 1.127.1 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-30355 being exploited?
There is no known exploitation of CVE-2025-30355 in the wild and no public exploit code.
Other vulnerabilities in element-hq synapse
- CVE-2019-18835 Improper Verification of Cryptographic Signature in matrix-synapse
- CVE-2024-52815 Synapse allows a a malformed invite to break the invitee's `/sync`
- CVE-2024-52805 Synapse allows unsupported content types to lead to memory exhaustion
- CVE-2024-53863 Synapse can be forced to thumbnail unexpected file formats, invoking external, potentially untrustworthy decoders
- CVE-2024-37302 Synapse denial of service through media disk space consumption
Sources
Also known as GHSA-v56r-hwv5-mxg6, PYSEC-2026-1614, UBUNTU-CVE-2025-30355, openSUSE-SU-2025:14939-1.