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ShimApplication · Redhat

CVE-2023-40546

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
A flaw was found in Shim when an error happened while creating a new ESL variable. If Shim fails to create the new variable, it tries to print an error message to the user; however, the number of parameters used by the logging function doesn't match the format string used by it, leading to a crash under certain circumstances.

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 · moderate confidence

A format string vulnerability exists in Shim where the error logging function receives a mismatched number of parameters compared to what the format string expects. When Shim fails to create a new ESL variable and attempts to log an error message, the parameter/format string mismatch causes a crash.

MitigationFix the parameter count in the error logging function call to match the format string specifiers, ensuring proper error handling without crashing.

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
None
Integrity
None
Availability
High

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

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. Confirm Shim bootloader is in use
    Check if the system uses UEFI boot and has Shim installed. On Linux systems, look for shimx64.efi in the EFI system partition (typically /boot/efi/EFI/*/shimx64.efi) or check for the shim package via package manager.
    Affected if The system boots using Shim as the initial UEFI bootloader and the shim package is present.
  2. Identify the installed Shim version
    Use the system's package manager to query the Shim package version. On RHEL/Fedora systems, run: rpm -q shim-x64 (or rpm -qa | grep shim). On Debian-based systems, run: dpkg -l | grep shim.
    Affected if A Shim package is installed and its version can be retrieved.
  3. Compare version against affected Red Hat range
    If using Red Hat Shim, compare the installed version number to 15.8. The version string is typically in the format like 15.7-1.el9 or similar.
    Affected if The installed version is less than 15.8 (for example, 15.7, 15.6, etc.).
  4. Compare version against affected Fedora 39
    If using Fedora 39, verify the exact Shim version packaged with this specific Fedora release.
    Affected if Running Fedora 39 with a Shim version that differs from the fixed release for that distribution.
  5. Check if ESL variable creation failure scenario applies
    This is a runtime condition that triggers when Shim attempts to create a new ESL (Event Sequence Log) variable and fails. This is not directly detectable via static inspection but indicates the vulnerability is triggerable on affected versions.
    Affected if The Shim version is within the affected range and the system encounters ESL variable creation failures during boot.

A user is affected if their installed Shim version is less than 15.8 (Red Hat) or matches the specific vulnerable versions for Fedora 39 or RHEL 8.0/9.0, and the system uses Shim as its UEFI bootloader.

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

Fix the parameter count in the error logging function call to match the format string specifiers, ensuring proper error handling without crashing.

Recommended fix High confidence

Shim 15.8 or later

  1. Identify the current Shim version installed: `rpm -q shim-x64` or `rpm -qa | grep shim`
  2. Update Shim to version 15.8 or later using the package manager: `sudo dnf update shim-x64`
  3. If the initramfs needs to be regenerated (which typically happens automatically with the shim update), rebuild it: `sudo dracut -f` or wait for the next boot where it will be regenerated
  4. Reboot the system to load the updated Shim: `sudo reboot`
  5. Verify the installed Shim version after reboot: `rpm -q shim-x64`

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

Fix this in Shim Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing4.0 h
  • Review / QA2.0 h
10.0 hours of engineering $1,720
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,752.

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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-2023-40546 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
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