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Linux Kernel CVE-2026-31551

| EUVDEUVD-2026-25444 MEDIUM
Integer Underflow (CWE-191)
2026-04-24 Linux GHSA-gmmg-2vvm-27c6
Information Disclosure Linux Google Integer Overflow
5.5
CVSS 3.1 · NVD
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Severity by source

NVD PRIMARY
5.5 MEDIUM
AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
vuln.today AI
4.1 MEDIUM

Race condition requires concurrent access (AC:H); debugfs writes typically require root/CAP_SYS_ADMIN (PR:H); no confidentiality or integrity impact.

3.1 AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H
4.0 AV:L/AC:H/AT:P/PR:H/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
SUSE
MEDIUM
qualitative
Red Hat
5.5 MEDIUM
qualitative

Primary rating from NVD.

CVSS VectorNVD

Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

Lifecycle Timeline

6
Analysis Generated
Jul 24, 2026 - 03:13 vuln.today
CVSS changed
Apr 27, 2026 - 20:22 NVD
5.5 (MEDIUM)
Patch released
Apr 27, 2026 - 20:15 nvd
Patch available
Patch available
Apr 24, 2026 - 16:01 EUVD
EUVD ID Assigned
Apr 24, 2026 - 15:00 euvd
EUVD-2026-25444
CVE Published
Apr 24, 2026 - 14:33 nvd
MEDIUM 5.5

DescriptionCVE.org

In the Linux kernel, the following vulnerability has been resolved:

wifi: mac80211: Fix static_branch_dec() underflow for aql_disable.

syzbot reported static_branch_dec() underflow in aql_enable_write(). [0]

The problem is that aql_enable_write() does not serialise concurrent write()s to the debugfs.

aql_enable_write() checks static_key_false(&aql_disable.key) and later calls static_branch_inc() or static_branch_dec(), but the state may change between the two calls.

aql_disable does not need to track inc/dec.

Let's use static_branch_enable() and static_branch_disable().

[0]: val == 0 WARNING: kernel/jump_label.c:311 at __static_key_slow_dec_cpuslocked.part.0+0x107/0x120 kernel/jump_label.c:311, CPU#0: syz.1.3155/20288 Modules linked in: CPU: 0 UID: 0 PID: 20288 Comm: syz.1.3155 Tainted: G U L syzkaller #0 PREEMPT(full) Tainted: [U]=USER, [L]=SOFTLOCKUP Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026 RIP: 0010:__static_key_slow_dec_cpuslocked.part.0+0x107/0x120 kernel/jump_label.c:311 Code: f2 c9 ff 5b 5d c3 cc cc cc cc e8 54 f2 c9 ff 48 89 df e8 ac f9 ff ff eb ad e8 45 f2 c9 ff 90 0f 0b 90 eb a2 e8 3a f2 c9 ff 90 <0f> 0b 90 eb 97 48 89 df e8 5c 4b 33 00 e9 36 ff ff ff 0f 1f 80 00 RSP: 0018:ffffc9000b9f7c10 EFLAGS: 00010293 RAX: 0000000000000000 RBX: ffffffff9b3e5d40 RCX: ffffffff823c57b4 RDX: ffff8880285a0000 RSI: ffffffff823c5846 RDI: ffff8880285a0000 RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: 000000000000000a R13: 1ffff9200173ef88 R14: 0000000000000001 R15: ffffc9000b9f7e98 FS: 00007f530dd726c0(0000) GS:ffff8881245e3000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000200000001140 CR3: 000000007cc4a000 CR4: 00000000003526f0 Call Trace: <TASK> __static_key_slow_dec_cpuslocked kernel/jump_label.c:297 [inline] __static_key_slow_dec kernel/jump_label.c:321 [inline] static_key_slow_dec+0x7c/0xc0 kernel/jump_label.c:336 aql_enable_write+0x2b2/0x310 net/mac80211/debugfs.c:343 short_proxy_write+0x133/0x1a0 fs/debugfs/file.c:383 vfs_write+0x2aa/0x1070 fs/read_write.c:684 ksys_pwrite64 fs/read_write.c:793 [inline] __do_sys_pwrite64 fs/read_write.c:801 [inline] __se_sys_pwrite64 fs/read_write.c:798 [inline] __x64_sys_pwrite64+0x1eb/0x250 fs/read_write.c:798 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0xc9/0xf80 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7f530cf9aeb9 Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007f530dd72028 EFLAGS: 00000246 ORIG_RAX: 0000000000000012 RAX: ffffffffffffffda RBX: 00007f530d215fa0 RCX: 00007f530cf9aeb9 RDX: 0000000000000003 RSI: 0000000000000000 RDI: 0000000000000010 RBP: 00007f530d008c1f R08: 0000000000000000 R09: 0000000000000000 R10: 4200000000000005 R11: 0000000000000246 R12: 0000000000000000 R13: 00007f530d216038 R14: 00007f530d215fa0 R15: 00007ffde89fb978 </TASK>

AnalysisAI

Race condition in the Linux kernel's mac80211 WiFi subsystem allows a local low-privileged attacker to trigger a static_branch_dec() integer underflow via concurrent writes to the AQL debugfs control file, causing a kernel WARNING and potential availability impact. Affected kernels span from the introduction of the aql_disable static key (v5.12) through multiple stable branches, with fixes backported to 5.15.203, 6.1.167, 6.6.130, 6.12.78, 6.18.20, 6.19.10, and 7.0. EPSS is 0.02% (7th percentile) and no active exploitation is recorded in CISA KEV, placing this firmly in the low-real-world-risk category despite its availability impact.

Technical ContextAI

The vulnerability exists in net/mac80211/debugfs.c within the aql_enable_write() function, which controls Airtime Queue Limits (AQL) - a WiFi QoS scheduling feature in the mac80211 wireless stack. The flaw is a classic TOCTOU (time-of-check-time-of-use) race: aql_enable_write() reads the state of the static_key_false(&aql_disable.key) branch, then independently calls static_branch_inc() or static_branch_dec() without holding a lock between the two operations. When two concurrent pwrite64() syscalls race on the same debugfs file, both can observe the same initial state and both attempt to decrement the reference counter, driving it below zero - a CWE-191 integer underflow. The kernel's jump_label infrastructure detects this underflow and emits a WARNING from kernel/jump_label.c:311 (__static_key_slow_dec_cpuslocked). The fix replaces the reference-counted static_branch_inc/dec with the idempotent static_branch_enable/disable, which are safe to call regardless of current state. Affected CPE: cpe:2.3:a:linux:linux:*:*:*:*:*:*:*:* from commit e908435e402aff23c9b0b3c59c7cd12b08b681b0.

RemediationAI

The primary fix is to upgrade to a patched kernel version: 5.15.203, 6.1.167, 6.6.130, 6.12.78, 6.18.20, 6.19.10, or 7.0, as documented in the upstream stable commits at https://git.kernel.org/stable/c/787152497ac763deab16f6f4b7ce79aaeb3eb7e8 (6.x branch), https://git.kernel.org/stable/c/5ba05436f15d16ae7ab04b880e8bf8d440be892b, and related commits. The Ubuntu advisory is at https://ubuntu.com/security/notices/USN-8567-1. If patching is not immediately possible, a targeted workaround is to restrict or unmount debugfs (umount /sys/kernel/debug), which eliminates the attack surface entirely; the trade-off is loss of all kernel debugging visibility, which may impact observability tooling. Alternatively, restricting write access to the debugfs AQL file via filesystem permissions or a MAC policy (SELinux/AppArmor) limits exposure to privileged users only, reducing but not eliminating risk.

Same weakness CWE-191 – Integer Underflow

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Vendor StatusVendor

SUSE

Severity: Medium
Product Status
SUSE Linux Enterprise Desktop 15 SP7 Fixed
SUSE Linux Enterprise Desktop 15 SP7 Fixed
SUSE Linux Enterprise High Availability Extension 15 SP7 Fixed
SUSE Linux Enterprise High Availability Extension 15 SP7 Fixed
SUSE Linux Enterprise High Performance Computing 15 SP7 Fixed

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CVE-2026-31551 vulnerability details – vuln.today
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