CVE-2026-45078: Matrix synapse Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-8q93-326v-3m7g
Share
CVSS 5.5
CWE-400 CWE-770
Published
Advisory Summary

CVE-2026-45078 is a HIGH (CVSS 5.5) denial of service vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Matrix synapse libraries in the PyPI ecosystem. The issue involves synapse cpu starvation (denial of service). Remediation is available by upgrading affected packages to version 1.152.1.

CVE-2026-45078 at a Glance

CVE Identifier CVE-2026-45078
GHSA Identifier GHSA-8q93-326v-3m7g
Severity HIGH
CVSS v3.1 Base Score 5.5
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product Matrix synapse
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 5.5

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

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

How to Fix CVE-2026-45078

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: 1.152.1 
pip install matrix-synapse>=1.152.1
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-2026-45078, 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 1.152.1), 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-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. 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 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](mailto:security@element.io).

CVE-2026-45078 Affected Matrix synapse Versions

1 component(s)
Fixed Versions:
1.152.1
Product / Package Ecosystem Affected Versions Fixed Version
matrix-synapse
pypi/matrix-synapse
PyPI
< 1.152.1
1.152.1

References

GitHub Advisory / Wiki ADVISORY
https://github.com/element-hq/synapse/security/advisories/GHSA-8q93-326v-3m7g
NVD — CVE-2026-45078 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-45078
Matrix synapse Source Repository REPOSITORY
https://github.com/element-hq/synapse/issues/19394
Security Fix Commit FIX COMMIT
https://github.com/element-hq/synapse/commit/3f58bc50dfba5768ee43ce48c5e74c25ba0b078a
Matrix synapse Source Repository REPOSITORY
https://github.com/element-hq/synapse
Matrix synapse Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2026-191.yaml

Frequently Asked Questions

What is CVE-2026-45078?
CVE-2026-45078 is a security vulnerability in Matrix synapse involving Synapse CPU starvation (Denial of Service) under CWE-400: Uncontrolled Resource Consumption.
Which Matrix synapse versions are affected?
Affected versions include releases prior to 1.152.1 across affected packages: matrix-synapse.
What version fixes CVE-2026-45078?
Upgrading to 1.152.1 (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 1.152.1. Upgrade your dependency to 1.152.1 or later.
What is the CVSS score of CVE-2026-45078?
CVE-2026-45078 has a CVSS v3.1 base score of 5.5 (HIGH) with vector string CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-45078?
CVE-2026-45078 is classified under CWE-400: Uncontrolled Resource Consumption (The product does not properly control the allocation and maintenance of a limited resource.).
How can I check PyPI dependencies for CVE-2026-45078?
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-8q93-326v-3m7g",
  "modified" : "2026-06-09T10:18:03Z",
  "published" : "2026-05-14T16:18:40Z",
  "aliases" : [ "CVE-2026-45078" ],
  "summary" : "Synapse CPU starvation (Denial of Service)",
  "details" : "### Impact\n\nLocal authenticated users can cause Synapse to starve other requests of CPU and lead to other requests failing, causing other users to be denied service.\n\nHomeservers that trust all their local users are not at risk.\n\n### Patches\n\nUpdate to Synapse 1.152.1 or later.\n\n### Workarounds\n\nIf Synapse is deployed behind a reverse proxy, the reverse proxy could be configured to limit the rate of user requests,\npreventing or increasing the difficulty of the attack.\n\n### Identifiers\n\n- ELEMENTSEC-2026-1706\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:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "matrix-synapse"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.152.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/element-hq/synapse/security/advisories/GHSA-8q93-326v-3m7g"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-45078"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/element-hq/synapse/issues/19394"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/element-hq/synapse/commit/3f58bc50dfba5768ee43ce48c5e74c25ba0b078a"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/element-hq/synapse"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2026-191.yaml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400", "CWE-770" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-14T16:18:40Z",
    "nvd_published_at" : "2026-05-28T17:16:31Z"
  }
}