CVE-2021-29471: Matrix synapse Denial of Service Vulnerability

MODERATE
Advisory Identifier: GHSA-x345-32rc-8h85
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CVSS 3.7
CWE-331 CWE-400
Published
Advisory Summary

CVE-2021-29471 is a MODERATE (CVSS 3.7) denial of service vulnerability (CWE-331: Insufficient Entropy) affecting Matrix synapse libraries in the PyPI ecosystem. The issue involves denial of service attack via push rule patterns in matrix-synapse. Remediation is available by upgrading affected packages to version 1.33.2.

CVE-2021-29471 at a Glance

CVE Identifier CVE-2021-29471
GHSA Identifier GHSA-x345-32rc-8h85
Severity MODERATE
CVSS v3.1 Base Score 3.7
CWE Classification CWE-331: Insufficient Entropy
Affected Product Matrix synapse
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 3.7

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

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

How to Fix CVE-2021-29471

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: 1.33.2 
pip install matrix-synapse>=1.33.2
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-2021-29471, 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 1.33.2), 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-331: Insufficient Entropy, where the product uses an algorithm or scheme that produces insufficient entropy, leaving patterns or clusters of values that are more likely to occur than others. 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 "Push rules" can specify [conditions](https://matrix.org/docs/spec/client_server/r0.6.1#conditions) under which they will match, including `event_match`, which matches event content against a pattern including wildcards. Certain patterns can cause very poor performance in the matching engine, leading to a denial-of-service when processing moderate length events. ### Patches The issue is patched by https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c. ### Workarounds A potential workaround might be to prevent users from making custom push rules, by blocking such requests at a reverse-proxy. ### For more information If you have any questions or comments about this advisory, email us at security@matrix.org.

CVE-2021-29471 Affected Matrix synapse Versions

1 component(s)
Fixed Versions:
1.33.2
Product / Package Ecosystem Affected Versions Fixed Version
matrix-synapse
pypi/matrix-synapse
PyPI
< 1.33.2
1.33.2

References

GitHub Advisory / Wiki ADVISORY
https://github.com/matrix-org/synapse/security/advisories/GHSA-x345-32rc-8h85
NVD — CVE-2021-29471 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-29471
Security Fix Commit FIX COMMIT
https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c
Matrix synapse Source Repository REPOSITORY
https://github.com/matrix-org/synapse
Matrix synapse Release RELEASE
https://github.com/matrix-org/synapse/releases/tag/v1.33.2
Matrix synapse Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-135.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-2021-29471?
CVE-2021-29471 is a security vulnerability in Matrix synapse involving Denial of service attack via push rule patterns in matrix-synapse under CWE-331: Insufficient Entropy.
Which Matrix synapse versions are affected?
Affected versions include releases prior to 1.33.2 across affected packages: matrix-synapse.
What version fixes CVE-2021-29471?
Upgrading to 1.33.2 (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 1.33.2. Upgrade your dependency to 1.33.2 or later.
What is the CVSS score of CVE-2021-29471?
CVE-2021-29471 has a CVSS v3.1 base score of 3.7 (MODERATE) with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L.
What CWE is associated with CVE-2021-29471?
CVE-2021-29471 is classified under CWE-331: Insufficient Entropy (The product uses an algorithm or scheme that produces insufficient entropy, leaving patterns or clusters of values that are more likely to occur than others.).
How can I check PyPI dependencies for CVE-2021-29471?
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-x345-32rc-8h85",
  "modified" : "2024-09-30T20:43:40Z",
  "published" : "2021-05-13T20:22:51Z",
  "aliases" : [ "CVE-2021-29471" ],
  "summary" : "Denial of service attack via push rule patterns in matrix-synapse",
  "details" : "### Impact\n\n\"Push rules\" can specify [conditions](https://matrix.org/docs/spec/client_server/r0.6.1#conditions) under which they will match, including `event_match`, which matches event content against a pattern including wildcards.\n\nCertain patterns can cause very poor performance in the matching engine, leading to a denial-of-service when processing moderate length events.\n\n### Patches\n\nThe issue is patched by https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c.\n\n### Workarounds\n\nA potential workaround might be to prevent users from making custom push rules, by blocking such requests at a reverse-proxy.\n\n### For more information\n\nIf you have any questions or comments about this advisory, email us at security@matrix.org.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/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" : "1.33.2"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/security/advisories/GHSA-x345-32rc-8h85"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-29471"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/matrix-org/synapse"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/releases/tag/v1.33.2"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-135.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-331", "CWE-400" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-05-11T17:19:15Z",
    "nvd_published_at" : "2021-05-11T15:15:00Z"
  }
}