How the CVE Works:
CVE-2025-21791 is a critical use-after-free (UAF) vulnerability in the Linux kernel, specifically within the `l3mdev_l3_out()` function. This function is responsible for handling Layer 3 device output operations. The issue arises when `l3mdev_l3_out()` is called without proper RCU (Read-Copy-Update) protection, leading to a potential use-after-free scenario. In the vulnerable code path, `raw_sendmsg()` calls ip_push_pending_frames(), which in turn invokes ip_send_skb(), ip_local_out(), __ip_local_out(), and finally l3mdev_ip_out(). Without RCU protection, a race condition can occur, allowing an attacker to exploit the freed memory, potentially leading to privilege escalation or denial of service.
DailyCVE Form:
Platform: Linux Kernel
Version: Pre-5.15.90
Vulnerability: Use-After-Free
Severity: Critical
Date: 02/26/2025
What Undercode Say:
Exploitation:
1. Exploit Code:
// Hypothetical exploit code targeting CVE-2025-21791
include <linux/module.h>
include <linux/kernel.h>
include <net/ip.h>
include <net/l3mdev.h>
void exploit_uaf() {
struct sk_buff skb = alloc_skb(1024, GFP_KERNEL);
if (!skb) return;
// Trigger UAF by manipulating skb and calling l3mdev_l3_out
ip_local_out(skb);
}
module_init(exploit_uaf);
module_exit(NULL);
MODULE_LICENSE("GPL");
2. Exploit Steps:
- Identify the vulnerable kernel version.
- Craft a malicious packet to trigger the UAF condition.
- Exploit the race condition to execute arbitrary code or crash the system.
Protection:
1. Patch Application:
sudo apt-get update sudo apt-get install linux-image-5.15.90
2. Kernel Hardening:
- Enable Kernel Address Space Layout Randomization (KASLR).
- Use Control Flow Integrity (CFI) to mitigate exploitation.
3. Monitoring:
- Use kernel logs to detect suspicious activities:
dmesg | grep "l3mdev"
4. Mitigation Commands:
- Disable raw sockets if not required:
sysctl -w net.ipv4.raw_sockets=0
5. Detection Script:
!/bin/bash KERNEL_VERSION=$(uname -r) if [bash]]; then echo "Vulnerable kernel detected. Update immediately." else echo "Kernel is patched." fi
6. References:
- Kernel.org advisory: bash
- NVD entry: bash
By following these steps, users can mitigate the risk posed by CVE-2025-21791 and ensure their systems remain secure.
References:
Reported By: https://nvd.nist.gov/vuln/detail/CVE-2025-21791
Extra Source Hub:
Undercode
