CVE-2021-3492Known exploitationPublic exploit

Local privilege escalation via shiftfs ioctl double-free

Published Apr 17, 2021 · Updated Sep 17, 2024

Double-free in Ubuntu Linux kernel 5.4 before 5.4.0-72.80 and 5.8 before 5.8.0-50.56 allows local users to gain root access. shiftfs_btrfs_ioctl_fd_replace treats a positive copy_to_user fault count as success, so shiftfs_btrfs_ioctl_fd_restore closes and frees the same objects twice. Reachability requires a shiftfs mount with Btrfs snapshot passthrough in an unprivileged user namespace; exploitation grants kernel code execution, while an invalid descriptor can exhaust kernel memory.

CVSS severity8.8
High
EPSS probability1.54%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Double-free in Ubuntu Linux kernel 5.4 before 5.4.0-72.80 and 5.8 before 5.8.0-50.56 allows local users to gain root access. shiftfs_btrfs_ioctl_fd_replace treats a positive copy_to_user fault count as success, so shiftfs_btrfs_ioctl_fd_restore closes and frees the same objects twice. Reachability requires a shiftfs mount with Btrfs snapshot passthrough in an unprivileged user namespace; exploitation grants kernel code execution, while an invalid descriptor can exhaust kernel memory.

The record

CVE
CVE-2021-3492
Published
Apr 17, 2021
Updated
Sep 17, 2024
Vendor
Linux Foundation
Product
Linux Kernel
Classifications
CWE-401, CWE-415, T1068
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Apr 17, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 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=5.4 kernel <5.4.0-72.80
  2. Affected versionversion=5.8 kernel <5.8.0-50.56

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

Linux Foundation · Linux Kernelversion=5.4 kernel <5.4.0-72.80; version=5.8 kernel <5.8.0-50.56

Published CVSS scores

8.8Canonical Ltd.CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
7.2NIST NVDAV:L/AC:L/Au:N/C:C/I:C/A:C
7.8NIST NVDCVSS:3.1/AV:L/AC:L/PR:L/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationReported
Public exploitPublished

Weakness, pattern, technique

CWE-401Missing Release of Memory after Effective Lifetime
CWE-415Double Free
T1068Exploitation for Privilege Escalation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Reported exploitation

Technologies

Your stack

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Your stack

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