CVE-2021-21392: Matrix synapse Open Redirect Vulnerability

HIGH
Advisory Identifier: GHSA-5wrh-4jwv-5w78
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CVSS 6.3
CWE-601
Published
Advisory Summary

CVE-2021-21392 is a HIGH (CVSS 6.3) open redirect vulnerability (CWE-601: URL Redirection to Untrusted Site ('Open Redirect')) affecting Matrix synapse libraries in the PyPI ecosystem. The issue involves open redirect via transitional ipv6 addresses on dual-stack networks. Remediation is available by upgrading affected packages to version 1.28.0rc1.

CVE-2021-21392 at a Glance

CVE Identifier CVE-2021-21392
GHSA Identifier GHSA-5wrh-4jwv-5w78
Severity HIGH
CVSS v3.1 Base Score 6.3
CWE Classification CWE-601: URL Redirection to Untrusted Site ('Open Redirect')
Affected Product Matrix synapse
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 6.3

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

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

How to Fix CVE-2021-21392

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: 1.28.0rc1 
pip install matrix-synapse>=1.28.0rc1
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-2021-21392, 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 1.28.0rc1), 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-601: URL Redirection to Untrusted Site ('Open Redirect'), where the web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a redirect. 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 Requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. ### Patches This issue is fixed by #9240. ### Workarounds Outbound requests to the following address ranges can be blocked by a firewall, if unused for internal communication between systems: * `::ffff/80` * `::0000/80` (note that this IP range is considered deprecated by the IETF) * `2002::/16` (note that this IP range is considered deprecated by the IETF) ### References * [RFC3056](https://tools.ietf.org/html/rfc3056) * [RFC4291](https://tools.ietf.org/html/rfc4291)

CVE-2021-21392 Affected Matrix synapse Versions

1 component(s)
Fixed Versions:
1.28.0rc1
Product / Package Ecosystem Affected Versions Fixed Version
matrix-synapse
pypi/matrix-synapse
PyPI
< 1.28.0rc1
1.28.0rc1

References

GitHub Advisory / Wiki ADVISORY
https://github.com/matrix-org/synapse/security/advisories/GHSA-5wrh-4jwv-5w78
NVD — CVE-2021-21392 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-21392
Matrix synapse Source Repository REPOSITORY
https://github.com/matrix-org/synapse/pull/9240
Security Fix Commit FIX COMMIT
https://github.com/matrix-org/synapse/commit/4ca054a4eaa714d0befb4fc30b19a1131e52c9cc
Matrix synapse Source Repository REPOSITORY
https://github.com/matrix-org/synapse
Matrix synapse Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-25.yaml
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY
pypi.org/project/matrix-syna... WEB
https://pypi.org/project/matrix-synapse

Frequently Asked Questions

What is CVE-2021-21392?
CVE-2021-21392 is a security vulnerability in Matrix synapse involving Open redirect via transitional IPv6 addresses on dual-stack networks under CWE-601: URL Redirection to Untrusted Site ('Open Redirect').
Which Matrix synapse versions are affected?
Affected versions include releases prior to 1.28.0rc1 across affected packages: matrix-synapse.
What version fixes CVE-2021-21392?
Upgrading to 1.28.0rc1 (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 1.28.0rc1. Upgrade your dependency to 1.28.0rc1 or later.
What is the CVSS score of CVE-2021-21392?
CVE-2021-21392 has a CVSS v3.1 base score of 6.3 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N.
What CWE is associated with CVE-2021-21392?
CVE-2021-21392 is classified under CWE-601: URL Redirection to Untrusted Site ('Open Redirect') (The web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a redirect.).
How can I check PyPI dependencies for CVE-2021-21392?
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-5wrh-4jwv-5w78",
  "modified" : "2024-09-30T20:28:18Z",
  "published" : "2021-04-13T15:13:08Z",
  "aliases" : [ "CVE-2021-21392" ],
  "summary" : "Open redirect via transitional IPv6 addresses on dual-stack networks",
  "details" : "### Impact\nRequests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks.\n\n### Patches\nThis issue is fixed by #9240.\n\n### Workarounds\nOutbound requests to the following address ranges can be blocked by a firewall, if unused for internal communication between systems:\n\n* `::ffff/80`\n* `::0000/80` (note that this IP range is considered deprecated by the IETF)\n* `2002::/16` (note that this IP range is considered deprecated by the IETF)\n\n### References\n* [RFC3056](https://tools.ietf.org/html/rfc3056)\n* [RFC4291](https://tools.ietf.org/html/rfc4291)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:H/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "matrix-synapse"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.28.0rc1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/security/advisories/GHSA-5wrh-4jwv-5w78"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-21392"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/pull/9240"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/matrix-org/synapse/commit/4ca054a4eaa714d0befb4fc30b19a1131e52c9cc"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/matrix-org/synapse"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-25.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-601" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-04-12T21:51:49Z",
    "nvd_published_at" : "2021-04-12T22:15:00Z"
  }
}