CVE-2025-21971

Local privilege escalation via Qdisc class use-after-free

Published Apr 1, 2025 · Updated Aug 5, 2026

Use-after-free in the Linux kernel allows local users to gain kernel privileges by creating a Qdisc class with TC_H_ROOT. qdisc_tree_reduce_backlog() treats the attacker-chosen class ID as the real tree root, stops backlog propagation early, and leaves a DRR active-list entry dangling after class deletion. A local user can obtain CAP_NET_ADMIN in a user-created network namespace and drive the deterministic sequence to disclose or overwrite kernel memory, hijack control flow, stall transmission, or crash the kernel.

CVSS severity7.8
High
EPSS probability0.22%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in the Linux kernel allows local users to gain kernel privileges by creating a Qdisc class with TC_H_ROOT. qdisc_tree_reduce_backlog() treats the attacker-chosen class ID as the real tree root, stops backlog propagation early, and leaves a DRR active-list entry dangling after class deletion. A local user can obtain CAP_NET_ADMIN in a user-created network namespace and drive the deterministic sequence to disclose or overwrite kernel memory, hijack control flow, stall transmission, or crash the kernel.

The record

CVE
CVE-2025-21971
Published
Apr 1, 2025
Updated
Aug 5, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
CWE-835, T1068
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Apr 1, 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

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=066a3b5b2346febf9a655b444567b7138e3bb939 <003d92c91cdb5a64b25a9a74cb8543aac9a8bb48
  2. Affected versionversion=066a3b5b2346febf9a655b444567b7138e3bb939 <0c3057a5a04d07120b3d0ec9c79568fceb9c921e
  3. Affected versionversion=066a3b5b2346febf9a655b444567b7138e3bb939 <5c3ca9cb48b51bd72bf76b8b05e24f3cd53db5e7
  4. Affected versionversion=066a3b5b2346febf9a655b444567b7138e3bb939 <78533c4a29ac3aeddce4b481770beaaa4f3bfb67
  5. Affected versionversion=066a3b5b2346febf9a655b444567b7138e3bb939 <7a82fe67a9f4d7123d8e5ba8f0f0806c28695006
  6. Affected versionversion=066a3b5b2346febf9a655b444567b7138e3bb939 <94edfdfb9505ab608e86599d1d1e38c83816fc1c
  7. Affected versionversion=066a3b5b2346febf9a655b444567b7138e3bb939 <e05d9938b1b0ac40b6054cc5fa0ccbd9afd5ed4c
  8. Affected versionversion=066a3b5b2346febf9a655b444567b7138e3bb939 <e5ee00607bbfc97ef1526ea95b6b2458ac9e7cb7
  9. Affected versionversion=2.6.25

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=066a3b5b2346febf9a655b444567b7138e3bb939 <003d92c91cdb5a64b25a9a74cb8543aac9a8bb48; version=066a3b5b2346febf9a655b444567b7138e3bb939 <0c3057a5a04d07120b3d0ec9c79568fceb9c921e; version=066a3b5b2346febf9a655b444567b7138e3bb939 <5c3ca9cb48b51bd72bf76b8b05e24f3cd53db5e7; version=066a3b5b2346febf9a655b444567b7138e3bb939 <78533c4a29ac3aeddce4b481770beaaa4f3bfb67; version=066a3b5b2346febf9a655b444567b7138e3bb939 <7a82fe67a9f4d7123d8e5ba8f0f0806c28695006; version=066a3b5b2346febf9a655b444567b7138e3bb939 <94edfdfb9505ab608e86599d1d1e38c83816fc1c; version=066a3b5b2346febf9a655b444567b7138e3bb939 <e05d9938b1b0ac40b6054cc5fa0ccbd9afd5ed4c; version=066a3b5b2346febf9a655b444567b7138e3bb939 <e5ee00607bbfc97ef1526ea95b6b2458ac9e7cb7; version=2.6.25

Published CVSS scores

7.8SUSECVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
7.8Linux Kernel CNACVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
8.5SUSECVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
5.5NIST National Vulnerability DatabaseCVSS: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 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
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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