CVE-2023-40551: A flaw was found in the MZ binary format in Shim

Q: How do I fix CVE-2023-40551?

shim: upgrade to 15.8; fedora: mitigate or apply the listed mitigation; enterprise linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2023-40551 being exploited?

CVE-2023-40551 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-2023-40551 affect?

CVE-2023-40551 affects shim, fedora, enterprise linux. Severity is medium (CVSS 5.1). A flaw was found in the MZ binary format in Shim. An out-of-bounds read may occur, leading to a crash or possible exposure of sensitive data during the system's boot phase.

Q: How urgent is CVE-2023-40551?

Priority 31/100. On your watchlist

Routine

MEDIUM 5.1WATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2023-40551 is a medium-severity flaw (CVSS 5.1) affecting shim, fedora, enterprise linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A flaw was found in the MZ binary format in Shim. An out-of-bounds read may occur, leading to a crash or possible exposure of sensitive data during the system's boot phase.

Why this priority

On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	5.1 (Medium)
Attack vector	Local
Attack complexity	Low
Privileges needed	High
User interaction	None
EPSS exploit chance	0.4% (top 66%)
CISA exploited list	No
Weakness	CWE-125
Published	2024-01-29
Last modified	2026-06-17

Publisher (CNA): Red Hat, Inc. · Vendor: fedoraproject · redhat

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

Am I affected?

Timeline

2024-01-29 17:15 UTCPublished CVE record published
2026-06-17 06:18 UTCUpdated Record last modified

redhat shim UPGRADE

Affected: < 15.8

Fixed in: 15.8

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 15.8 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade shim

Note

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

fedoraproject fedora MITIGATE

Affected: 39

Fixed in: no fixed release known

Safe to downgrade to: a release before 39

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.

redhat enterprise 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-2023-40551?

shim: upgrade to 15.8; fedora: mitigate or apply the listed mitigation; enterprise linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2023-40551 being exploited?

CVE-2023-40551 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-2023-40551 affect?

CVE-2023-40551 affects shim, fedora, enterprise linux. Severity is medium (CVSS 5.1). A flaw was found in the MZ binary format in Shim. An out-of-bounds read may occur, leading to a crash or possible exposure of sensitive data during the system's boot phase.

How urgent is CVE-2023-40551?

Priority 31/100. On your watchlist

Related CVEs

CVE-2023-40548 HIGH 7.4 UPGRADE MITIGATE A buffer overflow was found in Shim in the 32-bit system
CVE-2023-40547 HIGH 8.3 UPGRADE MITIGATE A remote code execution vulnerability was found in Shim
CVE-2023-40550 MEDIUM 5.5 UPGRADE MITIGATE An out-of-bounds read flaw was found in Shim when it tried to validate the SBAT information
CVE-2023-40549 MEDIUM 6.2 UPGRADE MITIGATE An out-of-bounds read flaw was found in Shim due to the lack of proper boundary verification during the load o
CVE-2023-40546 MEDIUM 6.2 UPGRADE MITIGATE A flaw was found in Shim when an error happened while creating a new ESL variable

References

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