CVE-2026-40356: In MIT Kerberos 5 (aka krb5) before 1.22.3, there is an integer underflow and resultant out-of-bounds read if

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MEDIUM 5.9REMOTEUPGRADE

CVE-2026-40356 is a medium-severity flaw (CVSS 5.9) affecting kerberos 5. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

In MIT Kerberos 5 (aka krb5) before 1.22.3, there is an integer underflow and resultant out-of-bounds read if an application calls gss_accept_sec_context() on a system with a NegoEx mechanism registered in /etc/gss/mech. An unauthenticated remote attacker can trigger this, possibly causing the process to terminate in parse_message.

Why this priority

Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.9 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.6% (top 51%)
CISA exploited list	No
Weakness	CWE-191
Published	2026-04-28
Last modified	2026-10-26

Publisher (CNA): MITRE · Vendor: mit

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-28 07:16 UTCPublished CVE record published
2026-10-26 22:10 UTCUpdated Record last modified

mit kerberos 5 UPGRADE

Affected: >= 1.18.0 and <= 1.22.2

Fixed in: newer than 1.22.2

Safe to downgrade to: a release before 1.18.0

1. Check installed version

apt-cache policy kerberos-5 || rpm -q kerberos-5 || kerberos-5 --version

2. UPGRADE to newer than 1.22.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade kerberos-5

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade kerberos-5

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 1.18.0 (Debian / Ubuntu)

apt-cache madison kerberos-5 sudo apt install kerberos-5=<VERSION_BEFORE_1.18.0> sudo apt-mark hold kerberos-5

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

dnf --showduplicates list kerberos-5 sudo dnf downgrade kerberos-5-<VERSION_BEFORE_1.18.0> sudo dnf versionlock add kerberos-5

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-2026-40356?

kerberos 5: upgrade to newer than 1.22.2. Exact commands for each OS are on this page.

Is CVE-2026-40356 being exploited?

CVE-2026-40356 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-2026-40356 affect?

CVE-2026-40356 affects kerberos 5. Severity is medium (CVSS 5.9). In MIT Kerberos 5 (aka krb5) before 1.22.3, there is an integer underflow and resultant out-of-bounds read if an application calls gss_accept_sec_context() on a system with a NegoEx mechanism registered in /etc/gss/mech. An unauthenticated

How urgent is CVE-2026-40356?

Priority 43/100. Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-40355 MEDIUM 5.9 UPGRADE In MIT Kerberos 5 (aka krb5) before 1.22.3, there is a NULL pointer dereference if an application calls gss_ac

References

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