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Linux Kernel CVE-2025-21796

CRITICAL
Use After Free (CWE-416)
2025-02-27 416baaa9-dc9f-4396-8d5f-8c081fb06d67
Information Disclosure Linux Use After Free Memory Corruption
Critical
Disputed · 9.8 Vendor: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
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Severity by source

Sources disagree (Medium–Critical)
Vendor (416baaa9-dc9f-4396-8d5f-8c081fb06d67) PRIMARY
9.8 CRITICAL
AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
vuln.today AI
5.9 MEDIUM

Network-reachable via nfsd but requires an NFSACL export, client access, and hitting an error-path race (AC:H, PR:L); demonstrated impact is a kernel panic (A:H) with limited info leak (C:L).

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

vuln.today treats the vendor’s rating as authoritative. A higher third-party CVSS (e.g. CISA-ADP) is shown for transparency but does not drive the headline severity.

CVSS VectorVendor: 416baaa9-dc9f-4396-8d5f-8c081fb06d67

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

Lifecycle Timeline

7
Analysis Updated
Jul 30, 2026 - 06:28 vuln.today
v2 (cvss_changed)
Re-analysis Queued
Jul 30, 2026 - 06:22 vuln.today
cvss_changed
Severity Changed
Jul 30, 2026 - 06:22 NVD
HIGH CRITICAL
CVSS changed
Jul 30, 2026 - 06:22 NVD
7.8 (HIGH) 9.8 (CRITICAL)
Analysis Generated
Mar 28, 2026 - 18:28 vuln.today
Patch released
Mar 28, 2026 - 18:28 nvd
Patch available
CVE Published
Feb 27, 2025 - 03:15 nvd
HIGH 7.8

DescriptionCVE.org

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

nfsd: clear acl_access/acl_default after releasing them

If getting acl_default fails, acl_access and acl_default will be released simultaneously. However, acl_access will still retain a pointer pointing to the released posix_acl, which will trigger a WARNING in nfs3svc_release_getacl like this:

------------[ cut here ]------------ refcount_t: underflow; use-after-free. WARNING: CPU: 26 PID: 3199 at lib/refcount.c:28 refcount_warn_saturate+0xb5/0x170 Modules linked in: CPU: 26 UID: 0 PID: 3199 Comm: nfsd Not tainted 6.12.0-rc6-00079-g04ae226af01f-dirty #8 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014 RIP: 0010:refcount_warn_saturate+0xb5/0x170 Code: cc cc 0f b6 1d b3 20 a5 03 80 fb 01 0f 87 65 48 d8 00 83 e3 01 75 e4 48 c7 c7 c0 3b 9b 85 c6 05 97 20 a5 03 01 e8 fb 3e 30 ff <0f> 0b eb cd 0f b6 1d 8a3 RSP: 0018:ffffc90008637cd8 EFLAGS: 00010282 RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff83904fde RDX: dffffc0000000000 RSI: 0000000000000008 RDI: ffff88871ed36380 RBP: ffff888158beeb40 R08: 0000000000000001 R09: fffff520010c6f56 R10: ffffc90008637ab7 R11: 0000000000000001 R12: 0000000000000001 R13: ffff888140e77400 R14: ffff888140e77408 R15: ffffffff858b42c0 FS: 0000000000000000(0000) GS:ffff88871ed00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000562384d32158 CR3: 000000055cc6a000 CR4: 00000000000006f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: <TASK> ? refcount_warn_saturate+0xb5/0x170 ? __warn+0xa5/0x140 ? refcount_warn_saturate+0xb5/0x170 ? report_bug+0x1b1/0x1e0 ? handle_bug+0x53/0xa0 ? exc_invalid_op+0x17/0x40 ? asm_exc_invalid_op+0x1a/0x20 ? tick_nohz_tick_stopped+0x1e/0x40 ? refcount_warn_saturate+0xb5/0x170 ? refcount_warn_saturate+0xb5/0x170 nfs3svc_release_getacl+0xc9/0xe0 svc_process_common+0x5db/0xb60 ? __pfx_svc_process_common+0x10/0x10 ? __rcu_read_unlock+0x69/0xa0 ? __pfx_nfsd_dispatch+0x10/0x10 ? svc_xprt_received+0xa1/0x120 ? xdr_init_decode+0x11d/0x190 svc_process+0x2a7/0x330 svc_handle_xprt+0x69d/0x940 svc_recv+0x180/0x2d0 nfsd+0x168/0x200 ? __pfx_nfsd+0x10/0x10 kthread+0x1a2/0x1e0 ? kthread+0xf4/0x1e0 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x34/0x60 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1a/0x30 </TASK> Kernel panic - not syncing: kernel: panic_on_warn set ...

Clear acl_access/acl_default after posix_acl_release is called to prevent UAF from being triggered.

AnalysisAI

Kernel-level use-after-free in the Linux kernel's NFS server (nfsd) NFSv3 ACL handling lets an error path leave a dangling POSIX ACL pointer, causing a refcount underflow and use-after-free reachable by NFS clients. When retrieving the default ACL fails, both acl_access and acl_default are released while acl_access still references the freed posix_acl, tripping a WARNING in nfs3svc_release_getacl and (with panic_on_warn) a kernel panic. No public exploit is identified at time of analysis, and EPSS is very low (0.03%, 10th percentile) despite the CNA-assigned CVSS of 9.8.

Technical ContextAI

The flaw lives in nfsd, the in-kernel NFS server, specifically its NFSACL sideband protocol used to serve POSIX ACLs to NFSv3 clients. POSIX ACLs in Linux are reference-counted objects (struct posix_acl) managed via posix_acl_release(). The root cause is CWE-416 (Use After Free): in the GETACL path, if fetching acl_default fails, cleanup releases both acl_access and acl_default, but acl_access is not cleared, so it keeps pointing at an already-freed object. When nfs3svc_release_getacl later runs during response cleanup, it calls posix_acl_release() on the stale pointer, driving the refcount below zero (refcount_t underflow) and touching freed memory. The CPE data lists only generic linux:linux_kernel entries plus explicit 6.14:rc1 and 6.14:rc2, indicating the bug spans multiple maintained branches; the crash trace was reproduced on 6.12.0-rc6. The fix simply nulls acl_access/acl_default after posix_acl_release.

RemediationAI

Apply your distribution's patched kernel: the upstream fix (clearing acl_access/acl_default after posix_acl_release) is merged via the stable commits at git.kernel.org/stable (e.g. 1fd94884174bd20beb1773990fd3b1aa877688d9 and the six sibling backport commits). Patch available per vendor advisory - use the exact fixed builds from Red Hat (RHSA-2025:20095, RHSA-2025:20518), SUSE (SUSE-SU-2025:1177 and the 2025:019xx series), or Debian LTS (debian-lts-announce 2025/03 and 2025/05), then reboot into the new kernel. If you cannot patch immediately and do not require it, disable the in-kernel NFS server or specifically stop exporting filesystems over NFSv3 with ACL support (unexport or drop the NFSACL/ACL option), which removes the vulnerable GETACL path at the cost of losing ACL-over-NFSv3 functionality for clients. Where nfsd must stay enabled, restrict NFS access (mountd/nfsd ports 2049 and rpcbind) to trusted client subnets via firewall rules to shrink the attacker population; this does not eliminate the bug but limits who can send the triggering NFSACL RPCs.

Same weakness CWE-416 – Use After Free

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Same technique Information Disclosure

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Vendor StatusVendor

SUSE

Severity: High
Product Status
Container suse/hpc/warewulf4-x86_64/sle-hpc-node:15.7.20.5.1 Image SLES15-SP7-BYOS-Azure Image SLES15-SP7-BYOS-EC2 Image SLES15-SP7-BYOS-GCE Image SLES15-SP7-CHOST-BYOS-Aliyun Image SLES15-SP7-CHOST-BYOS-Azure Image SLES15-SP7-CHOST-BYOS-EC2 Image SLES15-SP7-CHOST-BYOS-GCE Image SLES15-SP7-CHOST-BYOS-GDC Image SLES15-SP7-CHOST-BYOS-SAP-CCloud Image SLES15-SP7-EC2 Image SLES15-SP7-EC2-ECS-HVM Image SLES15-SP7-GCE Image SLES15-SP7-GCE-3P Image SLES15-SP7-HPC-BYOS-Azure Image SLES15-SP7-HPC-BYOS-EC2 Image SLES15-SP7-HPC-BYOS-GCE Image SLES15-SP7-Hardened-BYOS-Azure Image SLES15-SP7-Hardened-BYOS-EC2 Image SLES15-SP7-Hardened-BYOS-GCE Image SLES15-SP7-SAPCAL-Azure Image SLES15-SP7-SAPCAL-EC2 Image SLES15-SP7-SAPCAL-GCE Affected
Container suse/sl-micro/6.0/base-os-container:2.1.3-6.11 Container suse/sl-micro/6.1/base-os-container:2.2.0-4.21 Image SL-Micro Image SL-Micro-Azure Image SL-Micro-BYOS-Azure Image SL-Micro-BYOS-EC2 Image SL-Micro-BYOS-GCE Image SL-Micro-EC2 Image SLE-Micro Image SLE-Micro-Azure Image SLE-Micro-BYOS Image SLE-Micro-BYOS-Azure Image SLE-Micro-BYOS-EC2 Image SLE-Micro-BYOS-GCE Image SLE-Micro-EC2 Image SLE-Micro-GCE Image SUSE-Multi-Linux-Manager-Proxy-BYOS-Azure Image SUSE-Multi-Linux-Manager-Proxy-BYOS-EC2 Image SUSE-Multi-Linux-Manager-Proxy-BYOS-GCE Image SUSE-Multi-Linux-Manager-Server-Azure-llc Image SUSE-Multi-Linux-Manager-Server-Azure-ltd Image SUSE-Multi-Linux-Manager-Server-BYOS-Azure Image SUSE-Multi-Linux-Manager-Server-BYOS-EC2 Image SUSE-Multi-Linux-Manager-Server-BYOS-GCE Image SUSE-Multi-Linux-Manager-Server-EC2-llc Image SUSE-Multi-Linux-Manager-Server-EC2-ltd Affected
Container suse/sl-micro/6.0/kvm-os-container:2.1.3-6.12 Container suse/sl-micro/6.1/kvm-os-container:2.2.0-4.20 Affected
Container suse/sl-micro/6.0/rt-os-container:2.1.3-7.15 Container suse/sl-micro/6.1/rt-os-container:2.2.0-4.21 Affected
Image SLES12-SP5-Azure-BYOS Image SLES12-SP5-Azure-HPC-BYOS Image SLES12-SP5-EC2-BYOS Image SLES12-SP5-EC2-ECS-On-Demand Image SLES12-SP5-EC2-On-Demand Image SLES12-SP5-GCE-BYOS Image SLES12-SP5-GCE-On-Demand Affected

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CVE-2025-21796 vulnerability details – vuln.today
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