Skip to content

Synapse vulnerable to federation denial of service via malformed events

High severity GitHub Reviewed Published Mar 26, 2025 in element-hq/synapse • Updated Oct 24, 2025

Package

pip matrix-synapse (pip)

Affected versions

< 1.127.1

Patched versions

1.127.1

Description

Impact

A malicious server can craft events with a depth outside the integer range allowed by Canonical JSON. When such an event is received by Synapse version up to 1.127.0, it prevents it from federating with other servers. The vulnerability has been exploited in the wild.

Patches

Fixed in Synapse v1.127.1.

Workarounds

Closed federation environments of trusted servers or non-federating installations are not affected.

For more information

If you have any questions or comments about this advisory, please email us at security at element.io.

References

@davidegirardi davidegirardi published to element-hq/synapse Mar 26, 2025
Published by the National Vulnerability Database Mar 27, 2025
Published to the GitHub Advisory Database Mar 27, 2025
Reviewed Mar 27, 2025
Last updated Oct 24, 2025

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(91st percentile)

Weaknesses

Improper Input Validation

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. Learn more on MITRE.

CVE ID

CVE-2025-30355

GHSA ID

GHSA-v56r-hwv5-mxg6

Source code

Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.