CVE-2020-26891: Matrix synapse Cross-Site Scripting Vulnerability

MODERATE
Advisory Identifier: GHSA-3x8c-fmpc-5rmq
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CVSS 6.1
CWE-79
Published
Advisory Summary

CVE-2020-26891 is a MODERATE (CVSS 6.1) cross-site scripting vulnerability (CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) affecting Matrix synapse libraries in the PyPI ecosystem. The issue involves cross-site scripting (xss) vulnerability in the fallback authentication endpoint. Remediation is available by upgrading affected packages to version 1.21.0.

CVE-2020-26891 at a Glance

CVE Identifier CVE-2020-26891
GHSA Identifier GHSA-3x8c-fmpc-5rmq
Severity MODERATE
CVSS v3.1 Base Score 6.1
CWE Classification CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Affected Product Matrix synapse
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 6.1

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:C/C:L/I:L/A:N).

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

How to Fix CVE-2020-26891

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: 1.21.0 
pip install matrix-synapse>=1.21.0
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-2020-26891, 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 1.21.0), 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-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'), where the product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. 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 The fallback authentication endpoint served via Synapse was vulnerable to cross-site scripting (XSS) attacks. The impact depends on the configuration of the domain that Synapse is deployed on, but may allow access to cookies and other browser data, CSRF vulnerabilities, and access to other resources served on the same domain or parent domains. ### Patches This is fixed by #8444, which is included in Synapse v1.21.0. ### Workarounds If the homeserver is not configured to use reCAPTCHA, consent (terms of service), or single sign-on then the affected endpoint can be blocked at a reverse proxy: * `/_matrix/client/r0/auth/.*/fallback/web` * `/_matrix/client/unstable/auth/.*/fallback/web`

CVE-2020-26891 Affected Matrix synapse Versions

1 component(s)
Fixed Versions:
1.21.0
Product / Package Ecosystem Affected Versions Fixed Version
matrix-synapse
pypi/matrix-synapse
PyPI
< 1.21.0
1.21.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/matrix-org/synapse/security/advisories/GHSA-3x8c-fmpc-5rmq
NVD — CVE-2020-26891 NVD
https://nvd.nist.gov/vuln/detail/CVE-2020-26891
Matrix synapse Source Repository REPOSITORY
https://github.com/matrix-org/synapse/pull/8444
Matrix synapse Source Repository REPOSITORY
https://github.com/matrix-org/synapse
Matrix synapse Source Repository REPOSITORY
https://github.com/matrix-org/synapse/releases
Matrix synapse Release RELEASE
https://github.com/matrix-org/synapse/releases/tag/v1.21.2
Matrix synapse Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2020-238.yaml
Matrix synapse Release RELEASE
https://matrix.org/blog/2020/10/15/synapse-1-21-2-released-and-security-advisory

Frequently Asked Questions

What is CVE-2020-26891?
CVE-2020-26891 is a security vulnerability in Matrix synapse involving Cross-site scripting (XSS) vulnerability in the fallback authentication endpoint under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').
Which Matrix synapse versions are affected?
Affected versions include releases prior to 1.21.0 across affected packages: matrix-synapse.
What version fixes CVE-2020-26891?
Upgrading to 1.21.0 (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 1.21.0. Upgrade your dependency to 1.21.0 or later.
What is the CVSS score of CVE-2020-26891?
CVE-2020-26891 has a CVSS v3.1 base score of 6.1 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.
What CWE is associated with CVE-2020-26891?
CVE-2020-26891 is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.).
How can I check PyPI dependencies for CVE-2020-26891?
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-3x8c-fmpc-5rmq",
  "modified" : "2024-09-24T17:41:06Z",
  "published" : "2020-10-16T16:56:04Z",
  "aliases" : [ "CVE-2020-26891" ],
  "summary" : "Cross-site scripting (XSS) vulnerability in the fallback authentication endpoint",
  "details" : "### Impact\nThe fallback authentication endpoint served via Synapse was vulnerable to cross-site scripting (XSS) attacks. The impact depends on the configuration of the domain that Synapse is deployed on, but may allow access to cookies and other browser data, CSRF vulnerabilities, and access to other resources served on the same domain or parent domains.\n\n### Patches\nThis is fixed by #8444, which is included in Synapse v1.21.0.\n\n### Workarounds\nIf the homeserver is not configured to use reCAPTCHA, consent (terms of service), or single sign-on then the affected endpoint can be blocked at a reverse proxy:\n\n* `/_matrix/client/r0/auth/.*/fallback/web`\n* `/_matrix/client/unstable/auth/.*/fallback/web`",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "matrix-synapse"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.21.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/security/advisories/GHSA-3x8c-fmpc-5rmq"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2020-26891"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/pull/8444"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/matrix-org/synapse"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/releases"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/releases/tag/v1.21.2"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2020-238.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://matrix.org/blog/2020/10/15/synapse-1-21-2-released-and-security-advisory"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2020-10-16T16:55:42Z",
    "nvd_published_at" : null
  }
}