CVE-2021-3418

2021-03-0200:00:00

ubuntu.com

grub boot vulnerability

kernel signature validation

cve-2021-3418

secureboot mode

tampered kernel

shim_lock mechanism

bios grub bits

ubuntu

sbeattie

CVSS2

4.4

Attack Vector

LOCAL

Attack Complexity

MEDIUM

Authentication

NONE

Confidentiality Impact

PARTIAL

Integrity Impact

PARTIAL

Availability Impact

PARTIAL

AV:L/AC:M/Au:N/C:P/I:P/A:P

CVSS3

6.4

Attack Vector

LOCAL

Attack Complexity

HIGH

Privileges Required

HIGH

User Interaction

NONE

Scope

UNCHANGED

Confidentiality Impact

HIGH

Integrity Impact

HIGH

Availability Impact

HIGH

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

EPSS

0.001

Percentile

31.4%

JSON

If certificates that signed grub are installed into db, grub can be booted
directly. It will then boot any kernel without signature validation. The
booted kernel will think it was booted in secureboot mode and will
implement lockdown, yet it could have been tampered. This flaw is a
reintroduction of CVE-2020-15705 and only affects grub2 versions prior to
2.06 and upstream and distributions using the shim_lock mechanism.

Notes

Author	Note
sbeattie	does not affect grub2 as packaged in Ubuntu, we already had to address CVE-2020-15705.
sbeattie	grub2-unsigned will contain fixes and supersede grub2, which will contain only BIOS grub bits.