CVE-2017-18342: In PyYAML before 5.1, the yaml.load() API could execute arbitrary code if used with untrusted data

Q: How do I fix CVE-2017-18342?

pyyaml: upgrade to 5.1; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2017-18342 being exploited?

CVE-2017-18342 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2017-18342 affect?

CVE-2017-18342 affects pyyaml, fedora. Severity is critical (CVSS 9.8). In PyYAML before 5.1, the yaml.load() API could execute arbitrary code if used with untrusted data. The load() function has been deprecated in version 5.1 and the 'UnsafeLoader' has been introduced for backward compatibility with the functi

Q: How urgent is CVE-2017-18342?

Priority 63/100. Critical severity; EPSS top 7%: very likely to be exploited; Remotely exploitable, no login, no user action

This week

CRITICAL 9.8REMOTEUPGRADEMITIGATE

CVE-2017-18342 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.

In PyYAML before 5.1, the yaml.load() API could execute arbitrary code if used with untrusted data. The load() function has been deprecated in version 5.1 and the 'UnsafeLoader' has been introduced for backward compatibility with the function.

Why this priority

Critical severity
EPSS top 7%: very likely to be exploited
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	5.7% (top 7%)
CISA exploited list	No
Weakness	CWE-502
Published	2018-06-27
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

2018-06-27 12:29 UTCPublished CVE record published
2026-06-17 01:12 UTCUpdated Record last modified

pyyaml pyyaml UPGRADE

Affected: < 5.1

Fixed in: 5.1

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 5.1 (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.

fedoraproject fedora MITIGATE

Affected: 28 · 29 · 30

Fixed in: no fixed release known

Safe to downgrade to: a release before 28

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-2017-18342?

pyyaml: upgrade to 5.1; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2017-18342 being exploited?

CVE-2017-18342 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-2017-18342 affect?

CVE-2017-18342 affects pyyaml, fedora. Severity is critical (CVSS 9.8). In PyYAML before 5.1, the yaml.load() API could execute arbitrary code if used with untrusted data. The load() function has been deprecated in version 5.1 and the 'UnsafeLoader' has been introduced for backward compatibility with the functi

How urgent is CVE-2017-18342?

Priority 63/100. Critical severity; EPSS top 7%: very likely to be exploited; 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-2019-20477 CRITICAL 9.8 UPGRADE MITIGATE PyYAML 5.1 through 5.1.2 has insufficient restrictions on the load and load_all functions because of a class d

References

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