CVE-2021-47635

Kernel crash via missing private-page reference

Published Feb 26, 2025 · Updated Aug 5, 2026

Linux kernel UBIFS allows local users to crash affected systems during page migration through a page-refcount race. The UBIFS write-end path sets PG_private without taking the required extra page reference, so migration miscounts users and races put_page against page_ref_unfreeze. Exploitation requires local access, an active UBIFS filesystem on flash-backed UBI storage, and concurrent write, read, and page-compaction activity; the resulting refcount underflow triggers a kernel BUG and service interruption.

CVSS severity7.8
High
EPSS probability0.25%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Linux kernel UBIFS allows local users to crash affected systems during page migration through a page-refcount race. The UBIFS write-end path sets PG_private without taking the required extra page reference, so migration miscounts users and races put_page against page_ref_unfreeze. Exploitation requires local access, an active UBIFS filesystem on flash-backed UBI storage, and concurrent write, read, and page-compaction activity; the resulting refcount underflow triggers a kernel BUG and service interruption.

The record

CVE
CVE-2021-47635
Published
Feb 26, 2025
Updated
Aug 5, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
T1499
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Feb 26, 2025CVE publishedPublication date reported by the CVE source.
  2. Aug 5, 2026Record 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=1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d <3b67db8a6ca83e6ff90b756d3da0c966f61cd37b
  2. Affected versionversion=1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d <4f75bab98565afd4f905059c56ec4caba88a7eec
  3. Affected versionversion=1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d <5aaa2c0f0052b02c4a982993d4c5bb68fb7cbe22
  4. Affected versionversion=1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d <c34ae24a2590fee96a3a7735ba2fa6cc52306221
  5. Affected versionversion=1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d <fbeb2139eed65e929ce806c6468e6601ade01b1b
  6. Affected versionversion=2.6.27

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d <3b67db8a6ca83e6ff90b756d3da0c966f61cd37b; version=1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d <4f75bab98565afd4f905059c56ec4caba88a7eec; version=1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d <5aaa2c0f0052b02c4a982993d4c5bb68fb7cbe22; version=1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d <c34ae24a2590fee96a3a7735ba2fa6cc52306221; version=1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d <fbeb2139eed65e929ce806c6468e6601ade01b1b; version=2.6.27

Published CVSS scores

7.8kernel.orgCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
5.5NIST NVDCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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 exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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