CVE-2025-21796: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-21796 is a use-after-free vulnerability discovered in the Linux kernel's Network File System daemon (NFSD) component. The vulnerability was disclosed on February 26, 2025, and affects the ACL (Access Control List) handling functionality. The issue occurs when getting acldefault fails, causing both aclaccess and acldefault to be released simultaneously, while aclaccess retains a pointer to the released posix_acl (NVD).

The vulnerability manifests in the NFSD component when handling ACL operations. If getting acldefault fails, both aclaccess and acldefault are released, but aclaccess maintains a pointer to the released posixacl. This triggers a WARNING in nfs3svcrelease_getacl, leading to a refcount underflow and use-after-free condition. The issue has been assigned a CVSS v3.1 base score of 7.8 (High) with the vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H (NVD).

The vulnerability can trigger a kernel panic when exploited, leading to a system crash. This occurs due to the use-after-free condition in the ACL handling code, which can cause the system to become unstable and potentially result in a denial of service condition (NVD).

The vulnerability has been fixed in the Linux kernel through a patch that clears aclaccess/acldefault pointers after calling posixaclrelease. The fix involves adding NULL assignments to prevent the use-after-free condition from being triggered (Kernel Patch).