CVE-2021-21394: Matrix synapse Remote Code Execution Vulnerability
MODERATECVE-2021-21394 is a MODERATE (CVSS 5.3) remote code execution vulnerability (CWE-20: Improper Input Validation) affecting Matrix synapse libraries in the PyPI ecosystem. The issue involves denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints. Remediation is available by upgrading affected packages to version 1.28.0.
CVE-2021-21394 at a Glance
Security Impact
CVSS: 5.3
The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H).
How to Fix CVE-2021-21394
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>=1.28.0Run 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-2021-21394, 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 1.28.0), 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-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
CVE-2021-21394 Affected Matrix synapse Versions
1 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
matrix-synapse
|
PyPI |
< 1.28.0
|
1.28.0
|
References
Frequently Asked Questions
What is CVE-2021-21394?
Which Matrix synapse versions are affected?
What version fixes CVE-2021-21394?
Is matrix-synapse affected?
What is the CVSS score of CVE-2021-21394?
What CWE is associated with CVE-2021-21394?
How can I check PyPI dependencies for CVE-2021-21394?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-w9fg-xffh-p362",
"modified" : "2024-09-24T20:08:23Z",
"published" : "2021-04-13T15:12:51Z",
"aliases" : [ "CVE-2021-21394" ],
"summary" : "Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints",
"details" : "### Impact\nMissing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion.\n\n### Patches\nThe issue is fixed by #9321.\n\n### Workarounds\nDepending on the needs and configuration of the homeserver a few options are available:\n\n1. Using email as third-party identifiers be disabled by not configuring the `email` setting.\n2. Using phone numbers as third-party identifiers can be disabled by ensuring that `account_threepid_delegates.msisdn` is not configured.\n3. Additionally, the affected endpoint patterns can be blocked at a reverse proxy:\n\n * `^/_matrix/client/(r0|unstable)/register/email`\n * `^/_matrix/client/(r0|unstable)/register/msisdn`\n * `^/_matrix/client/(r0|unstable)/account/password`\n * `^/_matrix/client/(r0|unstable)/account/3pid`",
"severity" : [ {
"type" : "CVSS_V3",
"score" : "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H"
}, {
"type" : "CVSS_V4",
"score" : "CVSS:4.0/AV:N/AC:H/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
} ],
"affected" : [ {
"package" : {
"ecosystem" : "PyPI",
"name" : "matrix-synapse"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "1.28.0"
} ]
} ]
} ],
"references" : [ {
"type" : "WEB",
"url" : "https://github.com/matrix-org/synapse/security/advisories/GHSA-w9fg-xffh-p362"
}, {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-21394"
}, {
"type" : "WEB",
"url" : "https://github.com/matrix-org/synapse/pull/9321"
}, {
"type" : "WEB",
"url" : "https://github.com/matrix-org/synapse/pull/9393"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/matrix-org/synapse"
}, {
"type" : "WEB",
"url" : "https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-27.yaml"
}, {
"type" : "WEB",
"url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY"
}, {
"type" : "WEB",
"url" : "https://pypi.org/project/matrix-synapse"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-20" ],
"severity" : "MODERATE",
"github_reviewed" : true,
"github_reviewed_at" : "2021-04-12T21:01:40Z",
"nvd_published_at" : "2021-04-12T21:15:00Z"
}
}