CVE-2021-3695

Secure Boot bypass via grayscale PNG heap overwrite

Published Jul 6, 2022 · Updated Aug 3, 2024

Heap out-of-bounds write in GNU GRUB before 2.12 allows local users to execute code and bypass Secure Boot via a crafted grayscale PNG. In grub-core/video/readers/png.c, the 16-bit grayscale conversion advances a three-byte destination pixel by four bytes, overwriting three of every four bytes beyond the image buffer. Exploitation requires local control of a grayscale PNG without alpha and precise manipulation of GRUB's heap layout.

CVSS severity4.5
Medium
EPSS probability0.46%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Heap out-of-bounds write in GNU GRUB before 2.12 allows local users to execute code and bypass Secure Boot via a crafted grayscale PNG. In grub-core/video/readers/png.c, the 16-bit grayscale conversion advances a three-byte destination pixel by four bytes, overwriting three of every four bytes beyond the image buffer. Exploitation requires local control of a grayscale PNG without alpha and precise manipulation of GRUB's heap layout.

The record

CVE
CVE-2021-3695
Published
Jul 6, 2022
Updated
Aug 3, 2024
Vendor
The GnuPG Project
Product
GNU GRUB
Classifications
CWE-787, T1542.003
Attack vector
local
Privileges
Unavailable

Timeline

How it unfolded

  1. Jul 6, 2022CVE 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

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=grub-2.06

What conditions does exploitation require?

Attack vectorlocal
Required privilegesUnavailable in this record.

What is affected?

The GnuPG Project · GNU GRUBversion=grub-2.06

Published CVSS scores

7.5GNU GRUB upstream security disclosureCVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H
4.4NIST NVDAV:L/AC:M/Au:N/C:P/I:P/A:P
4.5Canonical UbuntuCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L
4.5NIST NVDCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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
PrivilegesUnavailable
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.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo