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ShimApplication · Redhat

CVE-2023-40550

MEDIUM · 5.5 CVSS v3.1 Published 2024-01-29
Fix available
A fix is available. Upgrade to 15.8 or later.
See remediation →
57/100
Remediation priority · Elevated
Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
An out-of-bounds read flaw was found in Shim when it tried to validate the SBAT information. This issue may expose sensitive data during the system's boot phase.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

An out-of-bounds read vulnerability exists in the Shim bootloader during SBAT (Secure Boot Advanced Targeting) information validation. The flaw allows exposure of sensitive memory contents during the UEFI boot phase due to insufficient bounds checking when parsing SBAT data structures.

MitigationApply vendor-provided Shim firmware updates that include proper bounds checking in SBAT validation routines; verify UEFI secure boot configurations are current.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
ShimApplication
Affected:< 15.8
FedoraOperating system
Affected:= 39
Enterprise LinuxOperating system
Affected:= 8.0= 9.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Locate Shim bootloader file
    Check the EFI System Partition (ESP) for Shim EFI binary - typically at /boot/efi/EFI/*/shimx64.efi or /boot/efi/EFI/redhat/shimx64.efi. Use 'ls -la /boot/efi/EFI/' to find vendor directories.
    Affected if Shim bootloader file exists and is used for UEFI secure boot
  2. Determine installed Shim version
    Use 'strings shimx64.efi | grep -i "shim"' or check RPM package version with 'rpm -qa | grep shim' if installed via package manager. The version string usually appears in the binary metadata.
    Affected if Version is less than 15.8 (Red Hat) or if running Fedora 39 or RHEL 8.0/9.0 with unpatched Shim
  3. Verify UEFI Secure Boot is enabled
    Check system secure boot status with 'mokutil --sb-state' or check UEFI firmware settings. Shim is primarily used when Secure Boot is active.
    Affected if Secure Boot is enabled and the vulnerable Shim binary is in the boot chain
  4. Check if SBAT data is present
    Examine the Shim binary for SBAT metadata using 'objdump -s -j .sbat shimx64.efi' or 'strings shimx64.efi | grep sbat'. SBAT data is embedded in the .sbat section.
    Affected if SBAT data structures exist in the Shim binary and are parsed during boot - this is the attack surface for the vulnerability
  5. Confirm OS distribution and version match
    Check OS with 'cat /etc/os-release' or 'hostnamectl'. The affected versions are Fedora 39, RHEL 8.0, and RHEL 9.0 with Shim < 15.8.
    Affected if Running Fedora 39 or RHEL 8.0/9.0 with unpatched Shim version in use

A system is affected if it uses a Shim bootloader version below 15.8 on an affected distribution (Red Hat Shim < 15.8, Fedora 39, RHEL 8.0/9.0), has UEFI Secure Boot enabled, and the SBAT data structure is parsed during the boot process.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 15.8 or later
Fixed in 15.8
Interim mitigation

Apply vendor-provided Shim firmware updates that include proper bounds checking in SBAT validation routines; verify UEFI secure boot configurations are current.

Recommended fix High confidence

Shim version 15.8 or later (e.g., shim-x64-15.8-1.el9 or equivalent for your distribution)

  1. Update the shim-x64 package to version 15.8 or later via your package manager (e.g., dnf update shim-x64 for Fedora/RHEL)
  2. After updating shim, rebuild the boot images to ensure the new shim is deployed (on RHEL/Fedora this typically happens automatically with grub2-mkconfig or bootloader updates)
  3. Verify the installed shim version matches 15.8 or later using: rpm -q shim-x64
  4. Restart the system to boot with the updated shim
Caveat Shim updates require boot loader reconfiguration; ensure bootloader updates complete successfully before rebooting

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Shim Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $6,176.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2023-40550 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data