CVE-2019-20477: PyYAML 5.1 through 5.1.2 has insufficient restrictions on the load and load_all functions because of a class d

This week

CRITICAL 9.8REMOTEUPGRADEMITIGATE

CVE-2019-20477 is a critical-severity flaw (CVSS 9.8) affecting pyyaml, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

Critical severity
EPSS top 8%: very likely to be exploited
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	5.1% (top 8%)
CISA exploited list	No
Weakness	CWE-502
Published	2020-02-19
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: fedoraproject · pyyaml

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2020-02-19 04:15 UTCPublished CVE record published
2026-06-17 02:30 UTCUpdated Record last modified

pyyaml pyyaml UPGRADE

Affected: >= 5.1 and <= 5.1.2

Fixed in: newer than 5.1.2

Safe to downgrade to: a release before 5.1

1. Check installed version

apt-cache policy pyyaml || rpm -q pyyaml || pyyaml --version

2. UPGRADE to newer than 5.1.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade pyyaml

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade pyyaml

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 5.1 (Debian / Ubuntu)

apt-cache madison pyyaml sudo apt install pyyaml=<VERSION_BEFORE_5.1> sudo apt-mark hold pyyaml

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list pyyaml sudo dnf downgrade pyyaml-<VERSION_BEFORE_5.1> sudo dnf versionlock add pyyaml

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

fedoraproject fedora MITIGATE

Affected: 30 · 31

Fixed in: no fixed release known

Safe to downgrade to: a release before 30

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2019-20477?

pyyaml: upgrade to newer than 5.1.2; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2019-20477 being exploited?

CVE-2019-20477 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2019-20477 affect?

CVE-2019-20477 affects pyyaml, fedora. Severity is critical (CVSS 9.8). 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 CV

How urgent is CVE-2019-20477?

Priority 67/100. Critical severity; EPSS top 8%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2020-1747 CRITICAL 9.8 UPGRADE MITIGATE A vulnerability was discovered in the PyYAML library in versions before 5.3.1, where it is susceptible to arbi
CVE-2020-14343 CRITICAL 9.8 UPGRADE DOWNGRADE A vulnerability was discovered in the PyYAML library in versions before 5.4, where it is susceptible to arbitr
CVE-2017-18342 CRITICAL 9.8 UPGRADE MITIGATE In PyYAML before 5.1, the yaml.load() API could execute arbitrary code if used with untrusted data

References

← Back to latest CVEs