CVE-2021-29431
MEDIUMCVE-2021-29431 is a medium-severity (CVSS 6.5) Improper Input Validation vulnerability in matrix-sydent. O3 Security confirms whether CVE-2021-29431 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
SSRF in Sydent due to missing validation of hostnames
Blast Radius
matrix-sydentReal-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
Sydent is a reference Matrix identity server. Sydent can be induced to send HTTP GET requests to internal systems, due to lack of parameter validation or IP address blacklisting. It is not possible to exfiltrate data or control request headers, but it might be possible to use the attack to perform an internal port enumeration. This issue has been addressed in in 9e57334, 8936925, 3d531ed, 0f00412. A potential workaround would be to use a firewall to ensure that Sydent cannot reach internal HTTP resources.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | matrix-sydent | all versions | 2.3.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-sydent. 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-sydent to 2.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-29431 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 CVE-2021-29431 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 CVE-2021-29431. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-29431 in your dependencies?
O3 detects CVE-2021-29431 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.