CVE-2018-20217: A Reachable Assertion issue was discovered in the KDC in MIT Kerberos 5 (aka krb5) before 1.17

Q: How do I fix CVE-2018-20217?

kerberos: upgrade to 5-1.17; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2018-20217 being exploited?

CVE-2018-20217 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-2018-20217 affect?

CVE-2018-20217 affects kerberos, debian linux. Severity is medium (CVSS 5.3). A Reachable Assertion issue was discovered in the KDC in MIT Kerberos 5 (aka krb5) before 1.17. If an attacker can obtain a krbtgt ticket using an older encryption type (single-DES, triple-DES, or RC4), the attacker can crash the KDC by mak

Q: How urgent is CVE-2018-20217?

Priority 42/100. On your watchlist

This month

MEDIUM 5.3WATCHLISTUPGRADEMITIGATE

CVE-2018-20217 is a medium-severity flaw (CVSS 5.3) affecting kerberos, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A Reachable Assertion issue was discovered in the KDC in MIT Kerberos 5 (aka krb5) before 1.17. If an attacker can obtain a krbtgt ticket using an older encryption type (single-DES, triple-DES, or RC4), the attacker can crash the KDC by making an S4U2Self request.

Why this priority

On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	Low
User interaction	None
EPSS exploit chance	1.4% (top 27%)
CISA exploited list	No
Weakness	CWE-617
Published	2018-12-26
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · mit

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

Am I affected?

Timeline

2018-12-26 21:29 UTCPublished CVE record published
2026-06-17 01:52 UTCUpdated Record last modified

mit kerberos UPGRADE

Affected: < 5-1.17

Fixed in: 5-1.17

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 5-1.17 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade kerberos

Note

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

debian debian linux MITIGATE

Affected: 8.0 · 9.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

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-2018-20217?

kerberos: upgrade to 5-1.17; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2018-20217 being exploited?

CVE-2018-20217 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-2018-20217 affect?

CVE-2018-20217 affects kerberos, debian linux. Severity is medium (CVSS 5.3). A Reachable Assertion issue was discovered in the KDC in MIT Kerberos 5 (aka krb5) before 1.17. If an attacker can obtain a krbtgt ticket using an older encryption type (single-DES, triple-DES, or RC4), the attacker can crash the KDC by mak

How urgent is CVE-2018-20217?

Priority 42/100. On your watchlist

Software pages

Debian vulnerabilities

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References

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