How the CVE Works:
CVE-2025-21812 is a critical use-after-free (UAF) vulnerability in the Linux kernel’s AX.25 protocol implementation. The issue arises due to improper handling of the `dev->ax25_ptr` pointer in the `ax25_setsockopt()` function. When the AX.25 socket options are modified, the kernel fails to properly protect the `dev->ax25_ptr` pointer with RCU (Read-Copy-Update) mechanisms, leading to a race condition. An attacker can exploit this by triggering a UAF scenario, where the memory referenced by `dev->ax25_ptr` is freed but still accessed. This can result in arbitrary code execution, privilege escalation, or denial of service (DoS). The vulnerability is exacerbated by a circular locking dependency between the `sk_lock-AF_AX25` and `rtnl_mutex` locks, which can cause deadlocks and further destabilize the system.
DailyCVE Form:
Platform: Linux Kernel
Version: Up to 6.13.0-rc3
Vulnerability: Use-After-Free
Severity: Critical
Date: 02/27/2025
What Undercode Say:
Exploitation:
- Trigger UAF: Craft a malicious AX.25 socket request to modify socket options, forcing the kernel to access freed memory.
- Race Condition: Exploit the race between `ax25_setsockopt()` and `ax25_device_event()` to corrupt memory.
- Payload Execution: Use heap spraying techniques to place malicious payloads in the freed memory location.
Protection:
- Patch Application: Apply the latest kernel patch that introduces RCU protection for
dev->ax25_ptr. - Kernel Hardening: Enable kernel hardening features like `CONFIG_SLAB_FREELIST_HARDENED` to mitigate UAF exploits.
- System Monitoring: Use tools like `auditd` to monitor and log suspicious AX.25 socket activities.
Commands:
1. Check Kernel Version:
uname -r
2. Apply Patch:
git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git git cherry-pick <commit-hash>
3. Monitor AX.25 Sockets:
auditctl -a exit,always -F arch=b64 -S socket -F protocol=AX25
Code Snippets:
1. Exploit PoC:
// Hypothetical UAF trigger int sockfd = socket(AF_AX25, SOCK_SEQPACKET, 0); setsockopt(sockfd, SOL_AX25, AX25_OPTION, &malicious_data, sizeof(malicious_data));
2. Kernel Patch:
// Example RCU protection
rcu_read_lock();
ax25_dev ax25_dev = rcu_dereference(dev->ax25_ptr);
if (ax25_dev) {
// Safe access
}
rcu_read_unlock();
Analytics:
1. CVSS 4.0: 9.8 (Critical) – AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- Affected Systems: Linux kernels prior to 6.13.0-rc3 with AX.25 support enabled.
- Exploitability: High due to the lack of RCU protection and race conditions.
- Mitigation Difficulty: Medium, requiring kernel patching and configuration changes.
By following these steps, users can protect their systems from CVE-2025-21812 and mitigate potential exploits.
References:
Reported By: https://nvd.nist.gov/vuln/detail/CVE-2025-21812
Extra Source Hub:
Undercode
