CVE-2019-20477
CRITICALCVE-2019-20477 is a critical-severity (CVSS 9.8) Deserialization of Untrusted Data vulnerability in pyyaml. 1 public exploit reference exists, so weaponization risk is real. O3 Security confirms whether CVE-2019-20477 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Deserialization of Untrusted Data in PyYAML
Real-World Exposure
pyyamlReal-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
PyYAML 5.1 through 5.1.2 has insufficient restrictions on the load and load_all functions because of a class deserialization issue, e.g., Popen is a class in the subprocess module. NOTE: this issue exists because of an incomplete fix for CVE-2017-18342.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | pyyaml | ≥ 5.1&&< 5.2 | 5.2 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for pyyaml. 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 pyyaml to 5.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2019-20477 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-2019-20477 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-2019-20477. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This issue did not affect the versions of PyYAML as shipped with Red Hat Enterprise Linux 7, and 8 as they did not include the class FullLoader, which contains this vulnerability. The PyYAML libary that is provided in the Red Hat OpenStack repositories is vulnerable. However, because there are no instances where this…
Frequently Asked Questions
Is CVE-2019-20477 in your dependencies?
O3 detects CVE-2019-20477 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.