CVE-2021-21333: Matrix synapse HTML Injection Vulnerability
MODERATECVE-2021-21333 is a MODERATE (CVSS 6.1) html injection 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 html injection in email and account expiry notifications. Remediation is available by upgrading affected packages to version 1.27.0.
CVE-2021-21333 at a Glance
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:H/PR:N/UI:R/S:C/C:N/I:H/A:N).
How to Fix CVE-2021-21333
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.
pip install matrix-synapse>=1.27.0Run the following command in your project directory to verify that all resolved dependencies use the patched release:
pip show matrix-synapseHow to Check if You Are Affected
To determine whether your Python environment is affected by CVE-2021-21333, inspect your installed packages using pip:
pip show matrix-synapseAnalyze 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 1.27.0), 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
CVE-2021-21333 Affected Matrix synapse Versions
1 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
matrix-synapse
|
PyPI |
< 1.27.0
|
1.27.0
|
References
Frequently Asked Questions
What is CVE-2021-21333?
Which Matrix synapse versions are affected?
What version fixes CVE-2021-21333?
Is matrix-synapse affected?
What is the CVSS score of CVE-2021-21333?
What CWE is associated with CVE-2021-21333?
How can I check PyPI dependencies for CVE-2021-21333?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-c5f8-35qr-q4fm",
"modified" : "2024-09-30T20:35:33Z",
"published" : "2021-03-26T19:53:04Z",
"aliases" : [ "CVE-2021-21333" ],
"summary" : "HTML injection in email and account expiry notifications",
"details" : "### Impact\nThe notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email.\n\nThe account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker.\n\n### Patches\nThis issue is fixed in #9200.\n\n### Workarounds\nFor the missed messages notifications:\n\nThe `notif.html`, `notif_mail.html`, and `room.html` templates can be overridden with custom templates that manually escapes the variables using [JInja2's `escape` filter](https://jinja.palletsprojects.com/en/2.11.x/templates/#escape). See the `email.template_dir` setting.\n\nFor the account expiry notifications:\n\n1. Account expiry can be disabled via the `account_validity.enabled` setting.\n\n2. The `notice_expiry.html` template can be overridden with a custom template that manually escapes the variables using [JInja2's `escape` filter](https://jinja.palletsprojects.com/en/2.11.x/templates/#escape). See the `email.template_dir` setting.",
"severity" : [ {
"type" : "CVSS_V3",
"score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N"
}, {
"type" : "CVSS_V4",
"score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:N/SI:H/SA:N"
} ],
"affected" : [ {
"package" : {
"ecosystem" : "PyPI",
"name" : "matrix-synapse"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "1.27.0"
} ]
} ]
} ],
"references" : [ {
"type" : "WEB",
"url" : "https://github.com/matrix-org/synapse/security/advisories/GHSA-c5f8-35qr-q4fm"
}, {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-21333"
}, {
"type" : "WEB",
"url" : "https://github.com/matrix-org/synapse/pull/9200"
}, {
"type" : "WEB",
"url" : "https://github.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73df"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/matrix-org/synapse"
}, {
"type" : "WEB",
"url" : "https://github.com/matrix-org/synapse/releases/tag/v1.27.0"
}, {
"type" : "WEB",
"url" : "https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-134.yaml"
}, {
"type" : "WEB",
"url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-74", "CWE-79" ],
"severity" : "MODERATE",
"github_reviewed" : true,
"github_reviewed_at" : "2021-03-26T19:52:46Z",
"nvd_published_at" : "2021-03-26T20:15:00Z"
}
}