CVE-2026-45078: Synapse CPU starvation (Denial of Service)
Impact
Local authenticated users can cause Synapse to starve other requests of CPU and lead to other requests failing, causing other users to be denied service.
Homeservers that trust all their local users are not at risk.
Patches
Update to Synapse 1.152.1 or later.
Workarounds
If Synapse is deployed behind a reverse proxy, the reverse proxy could be configured to limit the rate of user requests, preventing or increasing the difficulty of the attack.
Identifiers
- ELEMENTSEC-2026-1706
For more information
If you have any questions or comments about this advisory, please email us at security at element.io.
Other sources
Synapse is an open source Matrix homeserver implementation. Prior to 1.152.1, local authenticated users can cause Synapse to starve other requests of CPU and lead to other requests failing, causing other users to be denied service. This vulnerability is fixed in 1.152.1.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/matrix-synapseto a version that resolves this vulnerability.Fixed in 1.152.1 - Compensating control
If Synapse is deployed behind a reverse proxy, configure the reverse proxy to limit the rate of user requests (rate limiting) to prevent or increase the difficulty of the CPU starvation (DoS) attack.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-45078?
CVE-2026-45078 has a moderate severity level as it allows local authenticated users to degrade the service for others.
How do I fix CVE-2026-45078?
To fix CVE-2026-45078, update to Synapse version 1.152.1 or later.
Who is affected by CVE-2026-45078?
CVE-2026-45078 affects homeservers that do not trust all their local users.
What is the impact of CVE-2026-45078?
The impact of CVE-2026-45078 includes CPU starvation for other requests, leading to potential denial of service for other users.
Are there any workarounds for CVE-2026-45078?
Currently, the recommended approach is to update the Synapse software, as specific workarounds are not detailed.