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Not in CISA KEV
MEDIUM severity

Cross-site scripting (XSS) vulnerability in the password reset endpointGHSA-246w-56m2-5899

MEDIUMFix: matrix-org/synapse#9200

GHSA-246w-56m2-5899 is a medium-severity (CVSS 6.9) Cross-site Scripting (XSS) vulnerability in matrix-synapse. A fix is available for matrix-synapse — see the affected versions and patch details below.

Also known asCVE-2021-21332PYSEC-2021-133
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.2%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs68th percentile — riskier than 68% of all scored CVEsHighest risk
0.72%1.05%1.39%1.72%1.2%1.2%1.2%Sep 26Oct 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

GHSA-246w-56m2-5899 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
🐍matrix-synapse

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Impact

The password reset 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 in #9200.

Workarounds

Depending on the needs and configuration of the homeserver a few options are available:

  1. Password resets can be disabled by delegating email to a third-party service (via the account_threepid_delegates.email setting) or disabling email (by not configuring the email setting).

  2. If the homeserver is not configured to use passwords (via the password_config.enabled setting) then the affected endpoint can be blocked at a reverse proxy:

    • /_synapse/client/password_reset/email/submit_token

  3. The password_reset_confirmation.html template can be overridden with a custom template that manually escapes the variables using JInja2's escape filter. See the email.template_dir setting.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPImatrix-synapseall versions1.27.0pip install --upgrade 'matrix-synapse==1.27.0'

Affected Products

2 products · 2 configurations
OS
fedorafedoraproject
1 version
34
Application
synapsematrix
< 1.27.0
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-synapse, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update matrix-synapse to 1.27.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-246w-56m2-5899 is resolved across your whole dependency graph.

  3. Workarounds

    Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

### Impact The password reset 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 in #9200. ### Workarounds Depending on the needs and configuration of the homeserver a few options are available: 1. Password resets can be disabled by delegating email to a third-party service (via the `account_threepid_de
O3 Security · Impact-Aware SCA

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Cross-site scripting (XSS) vulnerability in the password…