CVE-2026-45076: Matrix synapse Denial of Service Vulnerability

MODERATE
Advisory Identifier: GHSA-6qf2-7x63-mm6v
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CWE-20
Published
Advisory Summary

CVE-2026-45076 is a MODERATE denial of service vulnerability (CWE-20: Improper Input Validation) affecting Matrix synapse libraries in the PyPI ecosystem. The issue involves synapse pagination denial of service. Remediation is available by upgrading affected packages to version 1.152.1.

CVE-2026-45076 at a Glance

CVE Identifier CVE-2026-45076
GHSA Identifier GHSA-6qf2-7x63-mm6v
Severity MODERATE
CVSS v3.1 Base Score N/A
CWE Classification CWE-20: Improper Input Validation
Affected Product Matrix synapse
Package Ecosystem PyPI
Published Date
Last Updated

How to Fix CVE-2026-45076

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-2026-45076, 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-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 In federated rooms, malicious homeservers can craft room events in such a way that prevents Synapse from providing full history to paginating clients. Clients could therefore fail to display room history. ### Patches Update to Synapse 1.152.1 or later. ### Workarounds There are no known workarounds for this issue. ### Identifiers - ELEMENTSEC-2025-1636 ### 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-2026-45076 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-6qf2-7x63-mm6v
NVD — CVE-2026-45076 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-45076
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-2026-194.yaml

Frequently Asked Questions

What is CVE-2026-45076?
CVE-2026-45076 is a security vulnerability in Matrix synapse involving Synapse pagination Denial of Service under CWE-20: Improper Input Validation.
Which Matrix synapse versions are affected?
Affected versions include releases prior to 1.152.1 across affected packages: matrix-synapse.
What version fixes CVE-2026-45076?
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 CWE is associated with CVE-2026-45076?
CVE-2026-45076 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-2026-45076?
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-6qf2-7x63-mm6v",
  "modified" : "2026-06-09T10:18:35Z",
  "published" : "2026-05-14T16:18:46Z",
  "aliases" : [ "CVE-2026-45076" ],
  "summary" : "Synapse pagination Denial of Service",
  "details" : "### Impact\n\nIn federated rooms, malicious homeservers can craft room events in such a way that prevents Synapse from providing full history to paginating clients.\n\nClients could therefore fail to display room history.\n\n### Patches\n\nUpdate to Synapse 1.152.1 or later.\n\n### Workarounds\n\nThere are no known workarounds for this issue.\n\n### Identifiers\n\n- ELEMENTSEC-2025-1636\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_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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-6qf2-7x63-mm6v"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-45076"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/element-hq/synapse"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2026-194.yaml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-20" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-14T16:18:46Z",
    "nvd_published_at" : "2026-05-28T17:16:31Z"
  }
}