How severe is CVE-2021-21332?

CVE-2021-21332 has a CVSS score of 8.2/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-21332?

CVE-2021-21332 affects the following packages: matrix-synapse (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-21332?

Update matrix-synapse to 1.27.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21332 is resolved across your whole dependency graph.

How do I detect CVE-2021-21332 in my PyPI dependencies?

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.

How do I mitigate CVE-2021-21332 if there is no patch (or I can't update yet)?

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.

Is CVE-2021-21332 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21332 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21332?

CVE-2021-21332 has an EPSS (Exploit Prediction Scoring System) score of 1.2%, placing it in the 68th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21332?

CVE-2021-21332 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-21332 published, and has it been updated?

CVE-2021-21332 was published on March 26, 2021 and was last updated on July 8, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

HIGH severity

CVE-2021-21332 — matrix-synapse

HIGHFix: matrix-org/synapse@e54746b

CVE-2021-21332 is a high-severity (CVSS 8.2) Cross-site Scripting (XSS) vulnerability in matrix-synapse. A fix is available for matrix-synapse — see the affected versions and patch details below.

Cross-site scripting (XSS) vulnerability in the password reset endpoint

Also known asGHSA-246w-56m2-5899PYSEC-2021-133

Published

Mar 26, 2021

Updated

Jul 8, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 10, 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

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

How urgent is this, really

CVE-2021-21332 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:

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).
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
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

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`matrix-synapse`	all versions	1.27.0`pip install --upgrade 'matrix-synapse==1.27.0'`

Affected Products

2 products · 2 configurations

fedorafedoraproject

1 version

Application

synapsematrix

< 1.27.0

range

Detection & mitigation playbook

Open-source dependency

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.
Fix
Update matrix-synapse to 1.27.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21332 is resolved across your whole dependency graph.
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