CVE-2023-26554: mstolfp in libntp/mstolfp.c in NTP 4.2.8p15 has an out-of-bounds write when adding a '\0' character

Routine

MEDIUM 5.6REMOTEDOWNGRADE

CVE-2023-26554 is a medium-severity flaw (CVSS 5.6) affecting ntp. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

mstolfp in libntp/mstolfp.c in NTP 4.2.8p15 has an out-of-bounds write when adding a '\0' character. An adversary may be able to attack a client ntpq process, but cannot attack ntpd.

Why this priority

Remotely exploitable, no login, no user action

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	5.6 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.6% (top 49%)
CISA exploited list	No
Weakness	CWE-787
Published	2023-04-11
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: ntp

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

Am I affected?

Timeline

2023-04-11 21:15 UTCPublished CVE record published
2026-06-17 05:43 UTCUpdated Record last modified

ntp ntp DOWNGRADE

Affected: 4.2.8 p15

Fixed in: no fixed release known

Safe to downgrade to: a release before 4.2.8

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 4.2.8 (Debian / Ubuntu)

apt-cache madison ntp sudo apt install ntp=<VERSION_BEFORE_4.2.8> sudo apt-mark hold ntp

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list ntp sudo dnf downgrade ntp-<VERSION_BEFORE_4.2.8> sudo dnf versionlock add ntp

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.

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-2023-26554?

ntp: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2023-26554 being exploited?

CVE-2023-26554 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-2023-26554 affect?

CVE-2023-26554 affects ntp. Severity is medium (CVSS 5.6). mstolfp in libntp/mstolfp.c in NTP 4.2.8p15 has an out-of-bounds write when adding a '\0' character. An adversary may be able to attack a client ntpq process, but cannot attack ntpd.

How urgent is CVE-2023-26554?

Priority 37/100. Remotely exploitable, no login, no user action

Related CVEs

CVE-2023-26555 MEDIUM 6.4 DOWNGRADE praecis_parse in ntpd/refclock_palisade.c in NTP 4.2.8p15 has an out-of-bounds write
CVE-2023-26553 MEDIUM 5.6 DOWNGRADE mstolfp in libntp/mstolfp.c in NTP 4.2.8p15 has an out-of-bounds write when copying the trailing number
CVE-2023-26552 MEDIUM 5.6 DOWNGRADE mstolfp in libntp/mstolfp.c in NTP 4.2.8p15 has an out-of-bounds write when adding a decimal point
CVE-2023-26551 MEDIUM 5.6 DOWNGRADE mstolfp in libntp/mstolfp.c in NTP 4.2.8p15 has an out-of-bounds write in the cp<cpdec while loop

References

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