CVE-2021-21274: Matrix synapse Denial of Service Vulnerability

MODERATE
Advisory Identifier: GHSA-2hwx-mjrm-v3g8
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CVSS 4.3
CWE-400 CWE-770
Published
Advisory Summary

CVE-2021-21274 is a MODERATE (CVSS 4.3) denial of service vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Matrix synapse libraries in the PyPI ecosystem. The issue involves denial of service attack via .well-known lookups. Remediation is available by upgrading affected packages to version 1.25.0.

CVE-2021-21274 at a Glance

CVE Identifier CVE-2021-21274
GHSA Identifier GHSA-2hwx-mjrm-v3g8
Severity MODERATE
CVSS v3.1 Base Score 4.3
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product Matrix synapse
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 4.3

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

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

How to Fix CVE-2021-21274

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: 1.25.0 
pip install matrix-synapse>=1.25.0
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-2021-21274, 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 1.25.0), 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 A malicious homeserver could redirect requests to their .well-known file to a large file. This can lead to a denial of service attack where homeservers will consume significantly more resources when requesting the .well-known file of a malicious homeserver. This affects any server which accepts federation requests from untrusted servers. ### Patches Issue is resolved by #8950. A bug not affecting the security aspects of this was fixed in #9108. ### Workarounds The `federation_domain_whitelist` setting can be used to restrict the homeservers communicated with over federation.

CVE-2021-21274 Affected Matrix synapse Versions

1 component(s)
Fixed Versions:
1.25.0
Product / Package Ecosystem Affected Versions Fixed Version
matrix-synapse
pypi/matrix-synapse
PyPI
≥ 0.99.0
< 1.25.0
1.25.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8
NVD — CVE-2021-21274 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-21274
Matrix synapse Source Repository REPOSITORY
https://github.com/matrix-org/synapse/pull/8950
Security Fix Commit FIX COMMIT
https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6
Matrix synapse Source Repository REPOSITORY
https://github.com/matrix-org/synapse
Matrix synapse Release RELEASE
https://github.com/matrix-org/synapse/releases/tag/v1.25.0
Matrix synapse Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-132.yaml
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY

Frequently Asked Questions

What is CVE-2021-21274?
CVE-2021-21274 is a security vulnerability in Matrix synapse involving Denial of service attack via .well-known lookups under CWE-400: Uncontrolled Resource Consumption.
Which Matrix synapse versions are affected?
Affected versions include releases prior to 1.25.0 across affected packages: matrix-synapse.
What version fixes CVE-2021-21274?
Upgrading to 1.25.0 (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 1.25.0. Upgrade your dependency to 1.25.0 or later.
What is the CVSS score of CVE-2021-21274?
CVE-2021-21274 has a CVSS v3.1 base score of 4.3 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L.
What CWE is associated with CVE-2021-21274?
CVE-2021-21274 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-2021-21274?
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-2hwx-mjrm-v3g8",
  "modified" : "2024-09-30T20:23:54Z",
  "published" : "2021-03-01T19:34:54Z",
  "aliases" : [ "CVE-2021-21274" ],
  "summary" : "Denial of service attack via .well-known lookups",
  "details" : "### Impact\nA malicious homeserver could redirect requests to their .well-known file to a large file. This can lead to a denial of service attack where homeservers will consume significantly more resources when requesting the .well-known file of a malicious homeserver.\n\nThis affects any server which accepts federation requests from untrusted servers.\n\n### Patches\nIssue is resolved by #8950. A bug not affecting the security aspects of this was fixed in #9108.\n\n### Workarounds\nThe `federation_domain_whitelist` setting can be used to restrict the homeservers communicated with over federation.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "matrix-synapse"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.99.0"
      }, {
        "fixed" : "1.25.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-21274"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/pull/8950"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/matrix-org/synapse"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/releases/tag/v1.25.0"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-132.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400", "CWE-770" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-03-01T19:03:06Z",
    "nvd_published_at" : "2021-02-26T18:15:00Z"
  }
}