CVE-2021-39656

Local privilege escalation via configfs refcount imbalance

Published Dec 15, 2021 · Updated Aug 4, 2024

Linux Kernel configfs use-after-free allows local users with System privileges to gain kernel privileges without user interaction. __configfs_open_file in fs/configfs/file.c calls config_item_put after the matching get was removed, unbalancing the config_item reference count so concurrent opens can access freed memory. Exploitation requires System-level execution and concurrent access to a mounted configfs instance; success compromises kernel execution integrity.

CVSS severity6.7
Medium
EPSS probability0.12%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Linux Kernel configfs use-after-free allows local users with System privileges to gain kernel privileges without user interaction. __configfs_open_file in fs/configfs/file.c calls config_item_put after the matching get was removed, unbalancing the config_item reference count so concurrent opens can access freed memory. Exploitation requires System-level execution and concurrent access to a mounted configfs instance; success compromises kernel execution integrity.

The record

CVE
CVE-2021-39656
Published
Dec 15, 2021
Updated
Aug 4, 2024
Vendor
The Linux Kernel Organization
Product
Linux Kernel
Classifications
CWE-416, CWE-667, T1068
Attack vector
local
Privileges
admin

Timeline

How it unfolded

  1. Dec 15, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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=Android kernel

What conditions does exploitation require?

Attack vectorlocal
Required privilegesadmin

What is affected?

The Linux Kernel Organization · Linux Kernelversion=Android kernel

Published CVSS scores

6.7NIST NVDCVSS: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-416Use After Free
CWE-667Improper Locking
T1068Exploitation for Privilege Escalation

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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