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Public proof of conceptFix available

CVE-2021-20233

A public proof of concept exists; no exploitation attested.

  • Published2021-03-03
  • WeaknessCWE-787

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

A flaw was found in grub2 in versions prior to 2.06. Setparam_prefix() in the menu rendering code performs a length calculation on the assumption that expressing a quoted single quote will require 3 characters, while it actually requires 4 characters which allows an attacker to corrupt memory by one byte for each quote in the input. The highest threat from this vulnerability is to data confidentiality and integrity as well as system availability.

Timeline

dates in UTC
  1. 2021-03-0317:15CVE publishedNVD
  2. 2026-10-2319:23First public proof of conceptPoC-in-GitHub

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-20233?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-20233?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Redhat enterprise linux server aus
BranchAffectedFixed in
7.2= 7.2no fix published
7.3= 7.3no fix published
7.4= 7.4no fix published
7.6= 7.6no fix published
7.7= 7.7no fix published
8.2= 8.2no fix published
Redhat enterprise linux server tus
BranchAffectedFixed in
7.4= 7.4no fix published
7.6= 7.6no fix published
7.7= 7.7no fix published
8.2= 8.2no fix published
Redhat enterprise linux server eus
BranchAffectedFixed in
7.6= 7.6no fix published
7.7= 7.7no fix published
8.1= 8.1no fix published

5 more products in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • shim-unsigned-aarch64AlmaLinux:8all versions · fixed in 15-7.el8_1.alma
  • shim-unsigned-x64AlmaLinux:8all versions · fixed in 15.4-4.el8_1.alma
  • shim-x64AlmaLinux:8all versions · fixed in 15.4-2.el8_1.almaand 2 other packages

Full record on NVD

Fix

Advisories and patches

Detection

read 2026-10-23 19:23 UTC
Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
1 repository, unverifiedpauljrowland/BootHoleFix
PoC-in-GitHub

Sources

  • NVDCVSS 3.18.2read 2026-10-23
  • NVDCVSS 2.07.2read 2026-10-23
  • CNAweakness (CWE)CWE-787read 2026-10-23
  • NVDweakness (CWE)CWE-787read 2026-10-23
  • FIRST EPSSEPSS score0.6 %2026-10-23
  • PoC-in-GitHubproof of concept1read 2026-10-23
  • OSVaffected packages—

Technical details

CVSS v3 Vector
CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H
Published
2021-03-03
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2021-20233?

A flaw was found in grub2 in versions prior to 2.06. Setparam_prefix() in the menu rendering code performs a length calculation on the assumption that expressing a quoted single quote will require 3 characters, while it actually requires 4 characters which allows an attacker to corrupt memory by one byte for each quote in the input. The highest threat from this vulnerability is to data confidentiality and integrity as well as system availability.

Is CVE-2021-20233 actively exploited?

Active exploitation of CVE-2021-20233 has not been confirmed. Its EPSS score was 0.6% on 2026-10-23, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-20233?

CVE-2021-20233 has a CVSS v3 base score of 8.2 (HIGH severity), with vector string CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H.

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