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CVE Record

CVE-2026-31504: net: fix fanout UAF in packet_release() via NETDEV_UP race

In the Linux kernel, the following vulnerability has been resolved: net: fix fanout UAF in packet_release() via NETDEV_UP race `packet_release()` has a race window where `NETDEV_UP` can re-register a socket into a fanout group's `arr[]` array. The re-registration is not cleaned up by `fanout_release()`, leaving a dangling pointer in the fanout array. `packet_release()` does NOT zero `po->num` in its `bind_lock` section. After releasing `bind_lock`, `po->num` is still non-zero and `po->ifindex` still matches the bound device. A concurrent `packet_notifier(NETDEV_UP)` that already found the socket in `sklist` can re-register the hook. For fanout sockets, this re-registration calls `__fanout_link(sk, po)` which adds the socket back into `f->arr[]` and increments `f->num_members`, but does NOT increment `f->sk_ref`. The fix sets `po->num` to zero in `packet_release` while `bind_lock` is held to prevent NETDEV_UP from linking, preventing the race window. This bug was found following an additional audit with Claude Code based on CVE-2025-38617.

HighCVSS 7.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

A race in the Linux kernel’s packet-socket cleanup can leave a freed socket referenced by a fanout group. A local, low-privileged attacker able to reach the relevant packet-socket behavior could potentially corrupt kernel memory, threatening confidentiality, integrity, and availability. The supplied evidence does not establish active exploitation.

Executive priority

Treat this as a high-priority kernel update for multi-user hosts, container platforms, and appliances exposing relevant local packet-socket capabilities. It is not evidenced as an emergency internet-scale threat, but possible kernel compromise warrants prompt exposure assessment and deployment through normal tested maintenance procedures.

Technical view

During packet_release(), a concurrent NETDEV_UP notification can relink a socket into the fanout array after cleanup has begun, without increasing the associated reference count. This leaves a dangling pointer and creates a use-after-free condition. The fix clears po->num while bind_lock is held, preventing the relink race.

Likely exposure

Exposure is limited to Linux systems where a local account or workload can exercise the affected packet fanout socket path during network-device activation. The supplied affected-version encoding is ambiguous and should not be interpreted as clean version ranges. Confirm exposure using distribution or appliance-vendor advisories and backport records.

Exploitation context

CVSS 3.1 is 7.8: local access, low complexity, low privileges, no user interaction, and potentially high confidentiality, integrity, and availability impact. The CVE is not listed as KEV in the bundle, and no supplied source establishes active exploitation, a public exploit, or reliable real-world exploitation.

Researcher notes

The security defect is a fanout-array use-after-free caused by asymmetric relinking and reference accounting during a NETDEV_UP race. Practical exploitability is not demonstrated in the supplied evidence. Version applicability needs vendor confirmation because the record’s version list mixes releases and commit identifiers without clearly expressed ranges.

Mitigation direction

  • Install the applicable vendor kernel update containing this fix or an equivalent backport.
  • Prioritize shared systems where untrusted local users or workloads can access packet fanout sockets.
  • If updates are unavailable, obtain product-specific mitigation guidance from the Linux distribution or appliance vendor.

Validation and detection

  • Record each system’s kernel release, distribution build, and vendor patch level.
  • Check vendor advisories or changelogs for CVE-2026-31504 or the referenced stable commits.
  • Verify the applied patch clears po->num inside packet_release() while bind_lock is held.
  • Reassess systems after reboot to confirm the updated kernel is running.
Prepared
Confidence
high
Sources
10

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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CVE-2026-31504 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.8 (3.1)
Known Exploited
No
Published

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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
3Timeline events
1ADP providers
11Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.8CVSS 3.1HighCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H1.85.9Linux

Vulnerability scoring details

Base CVSS 3.1 score

7.8High
CVSS 3.1 vector shape for CVE-2026-31504Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

siemens-SADPADP container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinuxce06b03e60fc19c680d1bf873e779bf11c2fc518, ce06b03e60fc19c680d1bf873e779bf11c2fc518, ce06b03e60fc19c680d1bf873e779bf11c2fc518, ce06b03e60fc19c680d1bf873e779bf11c2fc518, ce06b03e60fc19c680d1bf873e779bf11c2fc518, ce06b03e60fc19c680d1bf873e779bf11c2fc518, ce06b03e60fc19c680d1bf873e779bf11c2fc518, ce06b03e60fc19c680d1bf873e779bf11c2fc518unaffected
LinuxLinux3.1, 0, 5.10.253, 5.15.203, 6.1.168, 6.6.131, 6.12.80, 6.18.21, 6.19.11, 7.0affected
Weakness

CWE details

No CWE listed

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