CVE-2021-3696

Heap corruption via Huffman table underflow

Published Jul 6, 2022 · Updated Aug 3, 2024

Heap out-of-bounds write in GNU GRUB before 2.12 allows local users to corrupt heap memory through a crafted PNG image. In grub_png_insert_huff_item, a malformed Huffman code can place an insertion before the table start, shifting entries through negative indexes and writing outside the allocated array. Meaningful code execution or Secure Boot circumvention requires privileged local control and precise heap-layout manipulation.

CVSS severity5.0
Medium
EPSS probability0.47%
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 2.12 allows local users to corrupt heap memory through a crafted PNG image. In grub_png_insert_huff_item, a malformed Huffman code can place an insertion before the table start, shifting entries through negative indexes and writing outside the allocated array. Meaningful code execution or Secure Boot circumvention requires privileged local control and precise heap-layout manipulation.

The record

CVE
CVE-2021-3696
Published
Jul 6, 2022
Updated
Aug 3, 2024
Vendor
The GnuPG Project
Product
GNU GRUB
Classifications
CWE-787, T1068
Attack vector
local
Privileges
admin

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

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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

6.9NIST NVDAV:L/AC:M/Au:N/C:C/I:C/A:C
5.0GNU GRUB projectCVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:C/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.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 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
T1068Exploitation for Privilege Escalation

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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