CVE-2021-47591

Local kernel crash via TCP_ULP on MPTCP

Published Jun 19, 2024 · Updated May 11, 2026

NULL pointer dereference in Linux 5.13 through 5.15.10 and 5.16-rc1 through 5.16-rc5 allows local users to crash the kernel. The mptcp_setsockopt path accepts TCP_ULP even though MPTCP already uses that option to connect TCP subflows to its layer, leading tls_build_proto to dereference a null protocol pointer. Local low-privileged code must create an MPTCP connection in fallback mode and invoke setsockopt; successful triggering crashes the host without reported data disclosure or modification.

CVSS severity5.5
Medium
EPSS probability0.21%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

NULL pointer dereference in Linux 5.13 through 5.15.10 and 5.16-rc1 through 5.16-rc5 allows local users to crash the kernel. The mptcp_setsockopt path accepts TCP_ULP even though MPTCP already uses that option to connect TCP subflows to its layer, leading tls_build_proto to dereference a null protocol pointer. Local low-privileged code must create an MPTCP connection in fallback mode and invoke setsockopt; successful triggering crashes the host without reported data disclosure or modification.

The record

CVE
CVE-2021-47591
Published
Jun 19, 2024
Updated
May 11, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
CWE-476, T1499
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Jun 19, 2024CVE publishedPublication date reported by the CVE source.
  2. May 11, 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=5.13
  2. Affected versionversion=d9e4c129181004ec94b315b0c9db5eeb09da75e6 <3de0c86d42f841d1d64f316cd949e65c566f0734
  3. Affected versionversion=d9e4c129181004ec94b315b0c9db5eeb09da75e6 <404cd9a22150f24acf23a8df2ad0c094ba379f57

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=5.13; version=d9e4c129181004ec94b315b0c9db5eeb09da75e6 <3de0c86d42f841d1d64f316cd949e65c566f0734; version=d9e4c129181004ec94b315b0c9db5eeb09da75e6 <404cd9a22150f24acf23a8df2ad0c094ba379f57

Published CVSS scores

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.

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

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

CWE-476NULL Pointer Dereference
T1499Endpoint Denial of Service

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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