GHSA-c5f8-35qr-q4fm is a medium-severity (CVSS 6.1) vulnerability in matrix-synapse. O3 Security confirms whether GHSA-c5f8-35qr-q4fm is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
HTML injection in email and account expiry notifications
Real-World Exposure
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Description
Impact
The 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.
The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker.
Patches
This issue is fixed in #9200.
Workarounds
For the missed messages notifications:
The 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. See the email.template_dir setting.
For the account expiry notifications:
-
Account expiry can be disabled via the
account_validity.enabledsetting. -
The
notice_expiry.htmltemplate can be overridden with a custom template that manually escapes the variables using JInja2'sescapefilter. See theemail.template_dirsetting.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | matrix-synapse | all versions | 1.27.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-synapse. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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-c5f8-35qr-q4fm is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-c5f8-35qr-q4fm is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-c5f8-35qr-q4fm. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-c5f8-35qr-q4fm in your dependencies?
O3 detects GHSA-c5f8-35qr-q4fm across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.