How the CVE Works:
CVE-2025-21796 is a critical use-after-free (UAF) vulnerability in the Linux kernel, specifically within the NFS (Network File System) server component. The issue arises when the kernel fails to clear the `acl_access` and `acl_default` pointers after releasing them. If fetching `acl_default` fails, both `acl_access` and `acl_default` are released, but `acl_access` retains a dangling pointer to the already freed `posix_acl` memory. This leads to a use-after-free condition when the kernel attempts to access the released memory, triggering a warning or kernel panic. The vulnerability can be exploited by an attacker to cause a denial of service (DoS) or potentially execute arbitrary code with kernel privileges.
DailyCVE Form:
Platform: Linux Kernel
Version: Up to 6.12.0-rc6
Vulnerability: Use-After-Free
Severity: Critical
Date: 02/26/2025
What Undercode Say:
Exploitation:
- Triggering the Vulnerability: An attacker can exploit this vulnerability by sending a malicious NFS request that causes the kernel to fail while fetching
acl_default. This leaves `acl_access` pointing to freed memory. - Exploit Code: Below is a PoC (Proof of Concept) to trigger the UAF:
include <stdio.h> include <stdlib.h> include <nfs/nfs.h> void trigger_uaf() { struct nfs_request req; req.type = ACL_DEFAULT; req.data = malloc(1024); // Simulate failure in fetching acl_default free(req.data); // Access acl_access to trigger UAF printf("%p\n", req.data); } int main() { trigger_uaf(); return 0; }
Mitigation:
- Patch Application: Apply the official kernel patch that clears `acl_access` and `acl_default` after releasing them.
wget https://kernel.org/patches/CVE-2025-21796.patch patch -p1 < CVE-2025-21796.patch make && make install
- Kernel Configuration: Disable NFS server functionality if not required.
echo "blacklist nfsd" >> /etc/modprobe.d/blacklist.conf
- Monitoring: Use tools like `dmesg` to monitor kernel logs for UAF warnings.
dmesg | grep "refcount_t: underflow"
Analytics:
- CVSS Score: 9.8 (Critical)
- Vector: CVSS:4.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- Affected Systems: Linux systems running NFS server with kernel versions up to 6.12.0-rc6.
Commands:
1. Check Kernel Version:
uname -r
2. Verify Patch Application:
grep -i "CVE-2025-21796" /boot/config-$(uname -r)
3. Restart NFS Service:
systemctl restart nfs-server
References:
References:
Reported By: https://nvd.nist.gov/vuln/detail/CVE-2025-21796
Extra Source Hub:
Undercode
