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CVE Record

CVE-2025-21959: netfilter: nf_conncount: Fully initialize struct nf_conncount_tuple in insert_tree()

In the Linux kernel, the following vulnerability has been resolved: netfilter: nf_conncount: Fully initialize struct nf_conncount_tuple in insert_tree() Since commit b36e4523d4d5 ("netfilter: nf_conncount: fix garbage collection confirm race"), `cpu` and `jiffies32` were introduced to the struct nf_conncount_tuple. The commit made nf_conncount_add() initialize `conn->cpu` and `conn->jiffies32` when allocating the struct. In contrast, count_tree() was not changed to initialize them. By commit 34848d5c896e ("netfilter: nf_conncount: Split insert and traversal"), count_tree() was split and the relevant allocation code now resides in insert_tree(). Initialize `conn->cpu` and `conn->jiffies32` in insert_tree(). BUG: KMSAN: uninit-value in find_or_evict net/netfilter/nf_conncount.c:117 [inline] BUG: KMSAN: uninit-value in __nf_conncount_add+0xd9c/0x2850 net/netfilter/nf_conncount.c:143 find_or_evict net/netfilter/nf_conncount.c:117 [inline] __nf_conncount_add+0xd9c/0x2850 net/netfilter/nf_conncount.c:143 count_tree net/netfilter/nf_conncount.c:438 [inline] nf_conncount_count+0x82f/0x1e80 net/netfilter/nf_conncount.c:521 connlimit_mt+0x7f6/0xbd0 net/netfilter/xt_connlimit.c:72 __nft_match_eval net/netfilter/nft_compat.c:403 [inline] nft_match_eval+0x1a5/0x300 net/netfilter/nft_compat.c:433 expr_call_ops_eval net/netfilter/nf_tables_core.c:240 [inline] nft_do_chain+0x426/0x2290 net/netfilter/nf_tables_core.c:288 nft_do_chain_ipv4+0x1a5/0x230 net/netfilter/nft_chain_filter.c:23 nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline] nf_hook_slow+0xf4/0x400 net/netfilter/core.c:626 nf_hook_slow_list+0x24d/0x860 net/netfilter/core.c:663 NF_HOOK_LIST include/linux/netfilter.h:350 [inline] ip_sublist_rcv+0x17b7/0x17f0 net/ipv4/ip_input.c:633 ip_list_rcv+0x9ef/0xa40 net/ipv4/ip_input.c:669 __netif_receive_skb_list_ptype net/core/dev.c:5936 [inline] __netif_receive_skb_list_core+0x15c5/0x1670 net/core/dev.c:5983 __netif_receive_skb_list net/core/dev.c:6035 [inline] netif_receive_skb_list_internal+0x1085/0x1700 net/core/dev.c:6126 netif_receive_skb_list+0x5a/0x460 net/core/dev.c:6178 xdp_recv_frames net/bpf/test_run.c:280 [inline] xdp_test_run_batch net/bpf/test_run.c:361 [inline] bpf_test_run_xdp_live+0x2e86/0x3480 net/bpf/test_run.c:390 bpf_prog_test_run_xdp+0xf1d/0x1ae0 net/bpf/test_run.c:1316 bpf_prog_test_run+0x5e5/0xa30 kernel/bpf/syscall.c:4407 __sys_bpf+0x6aa/0xd90 kernel/bpf/syscall.c:5813 __do_sys_bpf kernel/bpf/syscall.c:5902 [inline] __se_sys_bpf kernel/bpf/syscall.c:5900 [inline] __ia32_sys_bpf+0xa0/0xe0 kernel/bpf/syscall.c:5900 ia32_sys_call+0x394d/0x4180 arch/x86/include/generated/asm/syscalls_32.h:358 do_syscall_32_irqs_on arch/x86/entry/common.c:165 [inline] __do_fast_syscall_32+0xb0/0x110 arch/x86/entry/common.c:387 do_fast_syscall_32+0x38/0x80 arch/x86/entry/common.c:412 do_SYSENTER_32+0x1f/0x30 arch/x86/entry/common.c:450 entry_SYSENTER_compat_after_hwframe+0x84/0x8e Uninit was created at: slab_post_alloc_hook mm/slub.c:4121 [inline] slab_alloc_node mm/slub.c:4164 [inline] kmem_cache_alloc_noprof+0x915/0xe10 mm/slub.c:4171 insert_tree net/netfilter/nf_conncount.c:372 [inline] count_tree net/netfilter/nf_conncount.c:450 [inline] nf_conncount_count+0x1415/0x1e80 net/netfilter/nf_conncount.c:521 connlimit_mt+0x7f6/0xbd0 net/netfilter/xt_connlimit.c:72 __nft_match_eval net/netfilter/nft_compat.c:403 [inline] nft_match_eval+0x1a5/0x300 net/netfilter/nft_compat.c:433 expr_call_ops_eval net/netfilter/nf_tables_core.c:240 [inline] nft_do_chain+0x426/0x2290 net/netfilter/nf_tables_core.c:288 nft_do_chain_ipv4+0x1a5/0x230 net/netfilter/nft_chain_filter.c:23 nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline] nf_hook_slow+0xf4/0x400 net/netfilter/core.c:626 nf_hook_slow_list+0x24d/0x860 net/netfilter/core.c:663 NF_HOOK_LIST include/linux/netfilter.h:350 [inline] ip_sublist_rcv+0x17b7/0x17f0 net/ipv4/ip_input.c:633 ip_list_rcv+0x9ef/0xa40 net/ip ---truncated---

MediumCVSS 5.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2025-21959 is a Linux kernel netfilter bug where a connection-counting structure was not fully initialized. The published impact is denial of service, not data theft or privilege escalation. It requires local, low-privileged access according to CVSS, so business urgency is moderate unless many untrusted users can run code on affected systems.

Executive priority

Treat as a moderate-priority kernel availability issue. Patch during the next planned kernel maintenance window, faster for shared systems or environments where untrusted local users can run code. No source provided evidence of active exploitation or confidentiality impact.

Technical view

The issue is CWE-908 in net/netfilter/nf_conncount.c. After earlier changes added cpu and jiffies32 fields, insert_tree() did not initialize them. KMSAN reported uninitialized-value use in find_or_evict during nf_conncount_count/connlimit processing. CVSS 3.1 is 5.5: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H.

Likely exposure

Linux systems running affected kernel versions and using netfilter connection limiting paths are the relevant exposure. The CVE record lists Linux as affected across multiple kernel lines and provides stable kernel fixes. Distribution backports may differ, so package-level vendor advisories are the best exposure source.

Exploitation context

The sources do not report active exploitation, and this CVE is not marked KEV in the provided bundle. The documented impact is local denial of service with low privileges. The stack trace involves nftables/connlimit and BPF test-run paths, but the sources do not provide a public exploit claim.

Researcher notes

Key fix direction is initializing conn->cpu and conn->jiffies32 in insert_tree(). The root cause follows commit b36e4523d4d5 and later refactoring that moved allocation into insert_tree(). Validate by source or vendor backport, not only upstream version numbers.

Mitigation direction

  • Apply a vendor kernel update containing the referenced stable fixes.
  • Check Debian LTS advisories if running Debian LTS kernels.
  • Prioritize shared-user systems where local low-privileged access is common.
  • If no update is available, follow your distribution or vendor guidance.

Validation and detection

  • Inventory Linux kernel versions across servers, appliances, and images.
  • Compare installed kernel packages against vendor advisories for CVE-2025-21959.
  • Review whether nftables/iptables connlimit functionality is enabled.
  • Confirm the running kernel includes the relevant stable fix or vendor backport.
Prepared
Confidence
high
Sources
7

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.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-908: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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cve · low confidence lookup

CVE-2025-21959 mapping review

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Vulnerability profileCVE Program record
Severity
Medium
CVSS
5.5 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:L/AC:L/PR:L/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
3ADP providers
12Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: partial

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
5.5CVSS 3.1MediumCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H1.83.6CISA-ADP

Vulnerability scoring details

Base CVSS 3.1 score

5.5Medium
CVSS 3.1 vector shape for CVE-2025-21959Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:L/AC:L/PR:L/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

CISA-ADPCISA ADP Vulnrichment
cvssV3_1other:ssvc
CVECVE Program Container
siemens-SADPADP container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinuxb36e4523d4d56e2595e28f16f6ccf1cd6a9fc452, b36e4523d4d56e2595e28f16f6ccf1cd6a9fc452, b36e4523d4d56e2595e28f16f6ccf1cd6a9fc452, b36e4523d4d56e2595e28f16f6ccf1cd6a9fc452, b36e4523d4d56e2595e28f16f6ccf1cd6a9fc452, b36e4523d4d56e2595e28f16f6ccf1cd6a9fc452, b36e4523d4d56e2595e28f16f6ccf1cd6a9fc452, b36e4523d4d56e2595e28f16f6ccf1cd6a9fc452, 75af3d78168e654a5cd8bbc4c774f97be836165f, 4.14.92unaffected
LinuxLinux4.18, 0, 5.4.292, 5.10.236, 5.15.180, 6.1.132, 6.6.84, 6.12.20, 6.13.8, 6.14affected
Weakness

CWE details

CWE-908

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-908 · source CWE mapping

Use of Uninitialized Resource

Use of Uninitialized Resource represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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