CVE-2025-30355: Matrix synapse Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-v56r-hwv5-mxg6
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CVSS 7.1
CWE-20
Published
Advisory Summary

CVE-2025-30355 is a HIGH (CVSS 7.1) denial of service vulnerability (CWE-20: Improper Input Validation) affecting Matrix synapse libraries in the PyPI ecosystem. The issue involves synapse vulnerable to federation denial of service via malformed events. Remediation is available by upgrading affected packages to version 1.127.1.

CVE-2025-30355 at a Glance

CVE Identifier CVE-2025-30355
GHSA Identifier GHSA-v56r-hwv5-mxg6
Severity HIGH
CVSS v3.1 Base Score 7.1
CWE Classification CWE-20: Improper Input Validation
Affected Product Matrix synapse
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.1

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required Low (PR:L)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity Low (I:L)
Availability High (A:H)

How to Fix CVE-2025-30355

To remediate this vulnerability, upgrade all affected Matrix synapse dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

matrix-synapse fixed: 1.127.1 
pip install matrix-synapse>=1.127.1
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show matrix-synapse

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2025-30355, inspect your installed packages using pip:

pip show matrix-synapse

Analyze the terminal output to locate any version of Matrix synapse in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.127.1), your application is affected.

Note: The pip show matrix-synapse command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-20: Improper Input Validation, where 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. In Matrix synapse, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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](mailto:security@element.io).

CVE-2025-30355 Affected Matrix synapse Versions

1 component(s)
Fixed Versions:
1.127.1
Product / Package Ecosystem Affected Versions Fixed Version
matrix-synapse
pypi/matrix-synapse
PyPI
< 1.127.1
1.127.1

References

GitHub Advisory / Wiki ADVISORY
https://github.com/element-hq/synapse/security/advisories/GHSA-v56r-hwv5-mxg6
NVD — CVE-2025-30355 NVD
https://nvd.nist.gov/vuln/detail/CVE-2025-30355
Security Fix Commit FIX COMMIT
https://github.com/element-hq/synapse/commit/2277df2a1eb685f85040ef98fa21d41aa4cdd389
Matrix synapse Source Repository REPOSITORY
https://github.com/element-hq/synapse
Matrix synapse Release RELEASE
https://github.com/element-hq/synapse/releases/tag/v1.127.1

Frequently Asked Questions

What is CVE-2025-30355?
CVE-2025-30355 is a security vulnerability in Matrix synapse involving Synapse vulnerable to federation denial of service via malformed events under CWE-20: Improper Input Validation.
Which Matrix synapse versions are affected?
Affected versions include releases prior to 1.127.1 across affected packages: matrix-synapse.
What version fixes CVE-2025-30355?
Upgrading to 1.127.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is matrix-synapse affected?
Yes, matrix-synapse is affected in releases prior to 1.127.1. Upgrade your dependency to 1.127.1 or later.
What is the CVSS score of CVE-2025-30355?
CVE-2025-30355 has a CVSS v3.1 base score of 7.1 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H.
What CWE is associated with CVE-2025-30355?
CVE-2025-30355 is classified under CWE-20: 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.).
How can I check PyPI dependencies for CVE-2025-30355?
Run the dependency tree command: 'pip show matrix-synapse'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-v56r-hwv5-mxg6",
  "modified" : "2025-10-24T19:32:06Z",
  "published" : "2025-03-27T18:02:14Z",
  "aliases" : [ "CVE-2025-30355" ],
  "summary" : "Synapse vulnerable to federation denial of service via malformed events",
  "details" : "### Impact\nA 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.\n\n### Patches\nFixed in Synapse v1.127.1.\n\n### Workarounds\nClosed federation environments of trusted servers or non-federating installations are not affected.\n\n### For more information\n\nIf you have any questions or comments about this advisory, please email us at [security at element.io](mailto:security@element.io).",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "matrix-synapse"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.127.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/element-hq/synapse/security/advisories/GHSA-v56r-hwv5-mxg6"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2025-30355"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/element-hq/synapse/commit/2277df2a1eb685f85040ef98fa21d41aa4cdd389"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/element-hq/synapse"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/element-hq/synapse/releases/tag/v1.127.1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-20" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2025-03-27T18:02:14Z",
    "nvd_published_at" : "2025-03-27T01:15:12Z"
  }
}