CVE-2021-46912

Host-wide TCP policy modification via global sysctl

Published Feb 27, 2024 · Updated Aug 5, 2026

Improper namespace isolation in the Linux kernel allows local users to alter the host-wide TCP congestion-control allowlist. The per-network-namespace ipv4_net_table exposes tcp_allowed_congestion_control with a NULL data pointer, so its handler updates global TCP_CONG_NON_RESTRICTED flags instead of namespace-specific state. A user who can create user and network namespaces gains CAP_NET_ADMIN there and can rewrite the policy across namespaces, impairing algorithm selection without crashing the kernel.

CVSS severity7.3
High
EPSS probability0.23%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper namespace isolation in the Linux kernel allows local users to alter the host-wide TCP congestion-control allowlist. The per-network-namespace ipv4_net_table exposes tcp_allowed_congestion_control with a NULL data pointer, so its handler updates global TCP_CONG_NON_RESTRICTED flags instead of namespace-specific state. A user who can create user and network namespaces gains CAP_NET_ADMIN there and can rewrite the policy across namespaces, impairing algorithm selection without crashing the kernel.

The record

CVE
CVE-2021-46912
Published
Feb 27, 2024
Updated
Aug 5, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
CWE-476, T1611
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Feb 27, 2024CVE 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=5.7
  2. Affected versionversion=9cb8e048e5d93825ec5e8dfb5b8df4987ea25745 <1ccdf1bed140820240e383ba0accc474ffc7f006
  3. Affected versionversion=9cb8e048e5d93825ec5e8dfb5b8df4987ea25745 <35d7491e2f77ce480097cabcaf93ed409e916e12
  4. Affected versionversion=9cb8e048e5d93825ec5e8dfb5b8df4987ea25745 <97684f0970f6e112926de631fdd98d9693c7e5c1

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=5.7; version=9cb8e048e5d93825ec5e8dfb5b8df4987ea25745 <1ccdf1bed140820240e383ba0accc474ffc7f006; version=9cb8e048e5d93825ec5e8dfb5b8df4987ea25745 <35d7491e2f77ce480097cabcaf93ed409e916e12; version=9cb8e048e5d93825ec5e8dfb5b8df4987ea25745 <97684f0970f6e112926de631fdd98d9693c7e5c1

Published CVSS scores

5.5NIST NVDCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
7.3kernel.orgCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:H/A:L

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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-476NULL Pointer Dereference
T1611Escape to Host

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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