Vulnerability in Linux kernel (CVE-2026-43121)
Summary
Vulnerability in Linux kernel (CVE-2026-43121). Risk of unauthorized operations or information disclosure.
Original description
The description as published by the source.
In the Linux kernel, the following vulnerability has been resolved:
io_uring/zcrx: fix user_ref race between scrub and refill paths
The io_zcrx_put_niov_uref() function uses a non-atomic
check-then-decrement pattern (atomic_read followed by separate
atomic_dec) to manipulate user_refs. This is serialized against other
callers by rq_lock, but io_zcrx_scrub() modifies the same counter with
atomic_xchg() WITHOUT holding rq_lock.
On SMP systems, the following race exists:
CPU0 (refill, holds rq_lock) CPU1 (scrub, no rq_lock)
put_niov_uref:
atomic_read(uref) - 1
// window opens
atomic_xchg(uref, 0) - 1
return_niov_freelist(niov) [PUSH #1]
// window closes
atomic_dec(uref) - wraps to -1
returns true
return_niov(niov)
return_niov_freelist(niov) [PUSH #2: DOUBLE-FREE]
The same niov is pushed to the freelist twice, causing free_count to
exceed nr_iovs. Subsequent freelist pushes then perform an out-of-bounds
write (a u32 value) past the kvmalloc'd freelist array into the adjacent
slab object.
Fix this by replacing the non-atomic read-then-dec in
io_zcrx_put_niov_uref() with an atomic_try_cmpxchg loop that atomically
tests and decrements user_refs. This makes the operation safe against
concurrent atomic_xchg from scrub without requiring scrub to acquire
rq_lock.
[pavel: removed a warning and a comment]
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-43121 has been found in Linux kernel.
Risk of unauthorized operations or information disclosure. CVSS score: 4.7/10.
What to do: apply the vendor's official patched release.
If unsure, ask your IT team or search "Linux kernel CVE-2026-43121" on the vendor's site.
CVE-2026-43121 (Linux kernel) — CWE-362 / CVSS v3 4.7
Attack vector: local / no user interaction
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-362) exists in Linux kernel.
Check the vendor advisory for the affected versions of Linux kernel.
Severity: Medium (CVSS 4.7/10). Risk of unauthorized operations or information disclosure.
Update to the patched release per vendor advisory.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Audit SBOM/dependencies to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
Audit SBOM/dependencies for affected components.資産台帳・SBOM・依存マニフェストで Linux kernel を使っているサービスとバージョンを洗い出す。
-
2Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
Fix status by distribution
Fixed packages published by each distribution for this CVE
- BellSoft Alpaquita BELL-CVE-2026-43121 linux-lts Fixed in: 6.18.35-r1
- Debian DEBIAN-CVE-2026-43121 linux Fixed in: 6.19.6-1
- Ubuntu UBUNTU-CVE-2026-43121 linux-hwe-edge
References
- advisory nvd.nist.gov
- advisory github.com
- package git.kernel.org
- web git.kernel.org
- web git.kernel.org
- web git.kernel.org
- web github.com