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How the CVE Works:
CVE-2025-21764 is a critical use-after-free (UAF) vulnerability in the Linux kernel’s `ndisc_alloc_skb()` function. This function is responsible for allocating socket buffers (skb) for Neighbor Discovery Protocol (NDP) messages. The vulnerability arises when `ndisc_alloc_skb()` is called without proper synchronization mechanisms like RTNL (Routing Netlink) or RCU (Read-Copy-Update) protection. This lack of protection can lead to a scenario where a memory region is freed but still accessed, causing a use-after-free condition. Attackers can exploit this to execute arbitrary code, escalate privileges, or cause a denial-of-service (DoS) by crashing the kernel.
DailyCVE Form:
Platform: Linux Kernel
Version: Up to 5.15.x
Vulnerability: Use-After-Free (UAF)
Severity: Critical
Date: 02/26/2025
What Undercode Say:
Exploitation:
1. Exploit Code Example:
// Hypothetical exploit code targeting CVE-2025-21764 struct sk_buff skb = ndisc_alloc_skb(dev, len); if (!skb) return -ENOMEM; // Trigger UAF by freeing skb while still in use kfree_skb(skb); // Access freed memory to cause crash or execute payload skb->data[bash] = 0xDEADBEEF;
2. Exploit Steps:
- Identify a system running a vulnerable Linux kernel version.
- Craft malicious NDP packets to trigger
ndisc_alloc_skb(). - Exploit the UAF condition to overwrite kernel memory.
Protection:
1. Patch Application:
- Apply the official kernel patch from bash.
- Example patch command:
wget https://kernel.org/patches/CVE-2025-21764.patch patch -p1 < CVE-2025-21764.patch
2. Mitigation:
- Disable NDP if not required:
sysctl -w net.ipv6.conf.all.disable_ipv6=1
- Use kernel hardening tools like Grsecurity or SELinux.
3. Detection:
- Use kernel logs to detect UAF attempts:
dmesg | grep "use-after-free"
- Monitor network traffic for abnormal NDP packets.
4. System Commands:
- Check kernel version:
uname -r
- Update kernel:
apt-get update && apt-get upgrade linux-image-$(uname -r)
5. Code Fix:
- The patched `ndisc_alloc_skb()` now includes RCU protection:
struct sk_buff ndisc_alloc_skb(struct net_device dev, int len) { struct sk_buff skb; rcu_read_lock(); skb = alloc_skb(len, GFP_ATOMIC); rcu_read_unlock(); return skb; }
6. Analytics:
- CVSS Score: 9.8 (Critical)
- Attack Vector: Network
- Impact: Code Execution, Privilege Escalation, DoS
- Exploitability: High
By following these steps, users can mitigate the risk posed by CVE-2025-21764 and secure their systems against potential exploits.
References:
Reported By: https://nvd.nist.gov/vuln/detail/CVE-2025-21764
Extra Source Hub:
Undercode
