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Linux KernelOperating system · Linux

CVE-2026-31669

CRITICAL · 9.8 CVSS v3.1 Published 2026-04-24
Fix available
A fix is available. Upgrade to 5.15.203 / 6.1.169 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
In the Linux kernel, the following vulnerability has been resolved: mptcp: fix slab-use-after-free in __inet_lookup_established The ehash table lookups are lockless and rely on SLAB_TYPESAFE_BY_RCU to guarantee socket memory stability during RCU read-side critical sections. Both tcp_prot and tcpv6_prot have their slab caches created with this flag via proto_register(). However, MPTCP's mptcp_subflow_init() copies tcpv6_prot into tcpv6_prot_override during inet_init() (fs_initcall, level 5), before inet6_init() (module_init/device_initcall, level 6) has called proto_register(&tcpv6_prot). At that point, tcpv6_prot.slab is still NULL, so tcpv6_prot_override.slab remains NULL permanently. This causes MPTCP v6 subflow child sockets to be allocated via kmalloc (falling into kmalloc-4k) instead of the TCPv6 slab cache. The kmalloc-4k cache lacks SLAB_TYPESAFE_BY_RCU, so when these sockets are freed without SOCK_RCU_FREE (which is cleared for child sockets by design), the memory can be immediately reused. Concurrent ehash lookups under rcu_read_lock can then access freed memory, triggering a slab-use-after-free in __inet_lookup_established. Fix this by splitting the IPv6-specific initialization out of mptcp_subflow_init() into a new mptcp_subflow_v6_init(), called from mptcp_proto_v6_init() before protocol registration. This ensures tcpv6_prot_override.slab correctly inherits the SLAB_TYPESAFE_BY_RCU slab cache.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A race condition in Linux kernel's MPTCP implementation causes IPv6 subflow child sockets to be allocated via kmalloc (kmalloc-4k) instead of the proper TCPv6 slab cache. This occurs because mptcp_subflow_init() copies tcpv6_prot before proto_register() has initialized its slab field, causing the override to retain a NULL slab pointer. Since kmalloc-4k lacks SLAB_TYPESAFE_BY_RCU protection, freed socket memory can be immediately reused while concurrent ehash lookups under rcu_read_lock still reference it, resulting in a use-after-free.

MitigationApply the kernel patch that restructures MPTCP initialization by splitting IPv6-specific setup into mptcp_subflow_v6_init() called after tcpv6_prot registration, ensuring proper slab cache inheritance.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Linux KernelOperating system
Affected:>= 5.12.1, < 5.15.203>= 5.16, < 6.1.169>= 6.2, < 6.6.135>= 6.7, < 6.12.82>= 6.13, < 6.18.23>= 6.19, < 6.19.13= 5.12= 7.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check kernel version against affected ranges
    Run 'uname -r' or 'cat /proc/version' and compare the kernel version to the affected ranges: >= 5.12.1, < 5.15.203; >= 5.16, < 6.1.169; >= 6.2, < 6.6.135; >= 6.7, < 6.12.82; >= 6.13, < 6.18.23; >= 6.19, < 6.19.13; = 5.12; = 7.0
    Affected if The installed kernel version falls within any of the listed affected ranges
  2. Verify MPTCP kernel support is enabled
    Check for CONFIG_MPTCP in /boot/config-$(uname -r) or /proc/config.gz, or check if /sys/module/mptcp exists
    Affected if MPTCP support is compiled into the running kernel (CONFIG_MPTCP=y or mptcp module is loaded)
  3. Confirm IPv6 is enabled in the kernel
    Check if IPv6 is loaded: 'lsmod | grep ipv6' or check /proc/sys/net/ipv6/conf/*/disable_ipv6 shows 0, or attempt 'ip -6 addr'
    Affected if IPv6 support is loaded or enabled in the kernel
  4. Check if MPTCP is available at runtime
    Run 'ss -m' or check /proc/net/protocols for MPTCP, or check 'cat /proc/sys/net/mptcp/enabled'
    Affected if MPTCP is reported as enabled and available for use on the system

The system is affected if the kernel version is in the affected list, MPTCP support is enabled, and IPv6 is active, as only then will the vulnerable IPv6 subflow socket allocation code path be exercised.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 5.15.203 / 6.1.169 / 6.6.135 or later
Fixed in 5.15.2036.1.1696.6.135
Vendor patch git.kernel.org →
Interim mitigation

Apply the kernel patch that restructures MPTCP initialization by splitting IPv6-specific setup into mptcp_subflow_v6_init() called after tcpv6_prot registration, ensuring proper slab cache inheritance.

Recommended fix High confidence

Upgrade to kernel version 5.15.203 or later (for 5.15.x branch), 6.1.169 or later (for 6.1.x branch), 6.6.135 or later (for 6.6.x branch), or 6.12.82 or later (for 6.12.x branch or later)

  1. 1. Identify the current running kernel version using `uname -r`
  2. 2. Determine the appropriate upgrade path based on the current kernel version (see upgrade_path)
  3. 3. Update the Linux kernel package to a version >= 5.15.203, >= 6.1.169, >= 6.6.135, or >= 6.12.82 depending on your current branch
  4. 4. Reboot the system to load the fixed kernel
  5. 5. Verify the fix is applied by checking the kernel version with `uname -r` and confirming it matches a fixed release
Caveat Kernel upgrades may require system reboot and could have compatibility implications with custom modules or specific hardware drivers; ensure all dependent modules are rebuilt for the new kernel version

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Linux Kernel Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing8.0 h
  • Review / QA3.0 h
17.0 hours of engineering $2,860
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-31669 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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