CVE-2021-20225

Secure Boot bypass via repeated short options

Published Mar 3, 2021 · Updated Aug 3, 2024

Heap out-of-bounds write in GNU GRUB before 2.06 allows local users to execute arbitrary code and bypass UEFI Secure Boot restrictions. In grub_arg_parse, allocation tracks discrete arguments, but short-option parsing continues through combined options that require values, overrunning the option array. High-privilege access to the GRUB command line or a crafted grub.cfg is required; successful exploitation runs code before the operating system.

CVSS severity6.7
Medium
EPSS probability1.01%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Heap out-of-bounds write in GNU GRUB before 2.06 allows local users to execute arbitrary code and bypass UEFI Secure Boot restrictions. In grub_arg_parse, allocation tracks discrete arguments, but short-option parsing continues through combined options that require values, overrunning the option array. High-privilege access to the GRUB command line or a crafted grub.cfg is required; successful exploitation runs code before the operating system.

The record

CVE
CVE-2021-20225
Published
Mar 3, 2021
Updated
Aug 3, 2024
Vendor
The GnuPG Project
Product
GNU GRUB
Classifications
CWE-787, T1542.003
Attack vector
local
Privileges
admin

Timeline

How it unfolded

  1. Mar 3, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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  1. Affected versionversion=grub 2.06

What conditions does exploitation require?

Attack vectorlocal
Required privilegesadmin

What is affected?

The GnuPG Project · GNU GRUBversion=grub 2.06

Published CVSS scores

7.5GNU GRUB upstream security noticeCVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H
6.7NIST NVDCVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
7.2NIST NVDAV:L/AC:L/Au:N/C:C/I:C/A:C
6.7Canonical Ubuntu Security TeamCVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesadmin
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-787Out-of-bounds Write
T1542.003Pre-OS Boot: Bootkit

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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