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CVE Record

CVE-2025-39894: netfilter: br_netfilter: do not check confirmed bit in br_nf_local_in() after confirm

In the Linux kernel, the following vulnerability has been resolved: netfilter: br_netfilter: do not check confirmed bit in br_nf_local_in() after confirm When send a broadcast packet to a tap device, which was added to a bridge, br_nf_local_in() is called to confirm the conntrack. If another conntrack with the same hash value is added to the hash table, which can be triggered by a normal packet to a non-bridge device, the below warning may happen. ------------[ cut here ]------------ WARNING: CPU: 1 PID: 96 at net/bridge/br_netfilter_hooks.c:632 br_nf_local_in+0x168/0x200 CPU: 1 UID: 0 PID: 96 Comm: tap_send Not tainted 6.17.0-rc2-dirty #44 PREEMPT(voluntary) RIP: 0010:br_nf_local_in+0x168/0x200 Call Trace: <TASK> nf_hook_slow+0x3e/0xf0 br_pass_frame_up+0x103/0x180 br_handle_frame_finish+0x2de/0x5b0 br_nf_hook_thresh+0xc0/0x120 br_nf_pre_routing_finish+0x168/0x3a0 br_nf_pre_routing+0x237/0x5e0 br_handle_frame+0x1ec/0x3c0 __netif_receive_skb_core+0x225/0x1210 __netif_receive_skb_one_core+0x37/0xa0 netif_receive_skb+0x36/0x160 tun_get_user+0xa54/0x10c0 tun_chr_write_iter+0x65/0xb0 vfs_write+0x305/0x410 ksys_write+0x60/0xd0 do_syscall_64+0xa4/0x260 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK> ---[ end trace 0000000000000000 ]--- To solve the hash conflict, nf_ct_resolve_clash() try to merge the conntracks, and update skb->_nfct. However, br_nf_local_in() still use the old ct from local variable 'nfct' after confirm(), which leads to this warning. If confirm() does not insert the conntrack entry and return NF_DROP, the warning may also occur. There is no need to reserve the WARN_ON_ONCE, just remove it.

HighCVSS 7.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

A Linux bridge-netfilter flaw can trigger a kernel warning because connection tracking uses an outdated reference. The sources rate it CVSS 7.5 High for availability impact only; they do not demonstrate data theft or alteration. Service disruption is plausible on relevant bridge and tap-device hosts, but a reliable denial of service is not established.

Executive priority

Treat this as a high-priority availability issue for virtualization, container, or networking hosts matching the required configuration. Patch within the high-severity SLA, accelerating critical network infrastructure. Current sources do not justify an emergency internet-wide response because active exploitation and reliable denial-of-service behavior are not established.

Technical view

After conntrack confirmation, nf_ct_resolve_clash() can replace skb->_nfct, while br_nf_local_in() retains the old local nfct reference. Its subsequent confirmed-bit warning can therefore fire, including when confirmation returns NF_DROP. The referenced stable-kernel fix removes the obsolete warning check.

Likely exposure

Likely exposure is limited to affected Linux kernels using br_netfilter connection tracking with bridged tap devices. The supplied version data is irregular, including commit hashes and an entry named "0"; validate distribution packages against vendor advisories and the referenced stable fixes rather than relying only on version strings.

Exploitation context

The bundle marks this CVE as absent from KEV, and no supplied source reports active exploitation or a public weaponized exploit. CVSS classifies it as network-accessible without privileges or user interaction, but successful triggering depends on the relevant bridge, tap, and conntrack configuration.

Researcher notes

No CWE is assigned. The disclosed symptom is WARN_ON_ONCE in br_nf_local_in(), caused by stale conntrack state after confirmation or clash resolution. Although CVSS claims high availability impact, the bundle does not explain whether the warning reliably causes a crash, panic, or sustained outage; practical impact needs controlled validation.

Mitigation direction

  • Install a vendor-supported kernel containing the applicable referenced Linux stable fix.
  • Reboot affected systems into the updated kernel and retire vulnerable boot entries.
  • If updating is delayed, consult the Linux distribution vendor; the bundle names no alternative mitigation.

Validation and detection

  • Inventory running kernels and identify hosts using Linux bridges, tap devices, br_netfilter, and conntrack.
  • Map each distribution kernel package to its vendor advisory or applicable referenced stable fix.
  • After updating, confirm systems booted the patched kernel.
  • Monitor kernel logs for the br_nf_local_in warning signature after remediation.
Prepared
Confidence
medium
Sources
9

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

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

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CVE-2025-39894 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.5 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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
8Source 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
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6Linux

Vulnerability scoring details

Base CVSS 3.1 score

7.5High
CVSS 3.1 vector shape for CVE-2025-39894Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

CVECVE Program Container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux7c3f28599652acf431a2211168de4a583f30b6d5, 2b1414d5e94e477edff1d2c79030f1d742625ea0, 80cd0487f630b5382734997c3e5e3003a77db315, 62e7151ae3eb465e0ab52a20c941ff33bb6332e9, 62e7151ae3eb465e0ab52a20c941ff33bb6332e9, 62e7151ae3eb465e0ab52a20c941ff33bb6332e9, cb734975b0ffa688ff6cc0eed463865bf07b6c01, 5.15.151, 6.1.81, 6.6.21, 6.7.9unaffected
LinuxLinux6.8, 0, 5.15.192, 6.1.151, 6.6.105, 6.12.46, 6.16.6, 6.17affected
Weakness

CWE details

No CWE listed

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