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The Linux kernel’s Vsock subsystem has a privilege escalation vulnerability known as CVE-2025-21756. This flaw stems from an incorrect reference count decrease in the `vsock_remove_sock` function, leading to a Use After Free (UAF) condition. Attackers can exploit this UAF to corrupt memory, overwrite function pointers, bypass kASLR via brute-force, and ultimately escalate privileges to root access.
The exploit involves manipulating the `vsock_diag_dump` function to alter kernel memory states and deploying a ROP chain to bypass security mechanisms like AppArmor.
🔗 Reference CVE-2025-21756 Exploit Details
You Should Know: Exploitation Steps & Mitigation
1. Understanding the Vsock UAF Vulnerability
The vulnerability occurs due to improper reference counting in the `vsock_remove_sock` function, allowing an attacker to:
– Trigger a UAF by forcing premature object deallocation.
– Overwrite kernel function pointers to hijack execution flow.
– Leak kernel memory to bypass kASLR (Kernel Address Space Layout Randomization).
2. Exploitation Steps
To replicate the exploit (for educational purposes only):
Step 1: Triggering the UAF
include <linux/vm_sockets.h>
include <sys/socket.h>
int main() {
int sock_fd = socket(AF_VSOCK, SOCK_STREAM, 0);
if (sock_fd < 0) {
perror("socket");
return -1;
}
close(sock_fd); // Triggers vsock_remove_sock UAF
return 0;
}
Step 2: Bypassing kASLR via Brute-Force
Attackers use side-channel attacks to leak kernel pointers:
dmesg | grep vsock Check kernel logs for vsock-related memory leaks
Step 3: Crafting the ROP Chain
A Return-Oriented Programming (ROP) payload is used to escalate privileges:
Example ROP gadget search (using objdump) objdump -d /usr/lib/debug/boot/vmlinux-$(uname -r) | grep "ret"
Step 4: Gaining Root Shell
After successful exploitation:
whoami Confirms root access
3. Mitigation & Patching
To protect against this exploit:
- Update the Linux kernel to the latest patched version.
- Disable Vsock if not required:
echo "blacklist vmw_vsock_vmci_transport" | sudo tee /etc/modprobe.d/blacklist-vsock.conf
- Enable Kernel Hardening (KASLR, SMEP, SMAP):
grep "CONFIG_RANDOMIZE_BASE" /boot/config-$(uname -r) Verify KASLR
What Undercode Say
This exploit demonstrates the dangers of memory corruption vulnerabilities in the Linux kernel. Attackers can chain UAF bugs with ROP techniques to bypass modern security protections. System administrators must:
– Monitor kernel updates for critical CVEs.
– Restrict Vsock usage in virtualized environments.
– Use exploit mitigation tools like grsecurity or SELinux.
For further research, check:
Expected Output:
A fully compromised system with root access via Vsock UAF exploitation, leading to privilege escalation and potential persistence mechanisms.
Verify exploit success id uid=0(root) gid=0(root) groups=0(root)
Stay updated, patch systems, and harden your Linux environments against such attacks. 🚨
References:
Reported By: Dipanshu Kumar – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
