CVE-2021-1136

Unsigned code execution via unlocked GRUB menu

Published Feb 4, 2021 · Updated Nov 8, 2024

Improper signature verification in the GRUB boot loader of Cisco IOS XR Software allows local administrators to execute unsigned code at boot. The unlocked loader accepts changes made through its GRUB menu without preserving the boot chain of trust. Exploitation requires authenticated local administrative access and permits code to run before trusted IOS XR startup.

CVSS severity6.7
Medium
EPSS probability0.20%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper signature verification in the GRUB boot loader of Cisco IOS XR Software allows local administrators to execute unsigned code at boot. The unlocked loader accepts changes made through its GRUB menu without preserving the boot chain of trust. Exploitation requires authenticated local administrative access and permits code to run before trusted IOS XR startup.

The record

CVE
CVE-2021-1136
Published
Feb 4, 2021
Updated
Nov 8, 2024
Vendor
ClamAV
Product
Cisco IOS XR Software
Classifications
CWE-347, T1542.003
Attack vector
local
Privileges
admin

Timeline

How it unfolded

  1. Feb 4, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 8, 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?

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

  1. Affected versionversion=n/a

What conditions does exploitation require?

Attack vectorlocal
Required privilegesadmin

What is affected?

ClamAV · Cisco IOS XR Softwareversion=n/a

Published CVSS scores

6.7Cisco Systems, Inc.CVSS: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-347Improper Verification of Cryptographic Signature
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

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Your stack

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