CVE-2025-21756: How a Tiny Linux Kernel Bug Led to a Full Root Exploit, PoC Releases
Ddos 
In a recently analysis, security researcher Michael Hoefler has exposed the full depth of CVE-2025-21756, a Use-After-Free (UAF) vulnerability affecting the Linux kernel’s vsock subsystem. What began as a simple code adjustment — a few lines modifying the behavior of vsock_remove_sock() — ultimately blocked a sophisticated path to local privilege escalation (LPE) and kernel code execution.
“When an object’s reference counter reaches zero in the kernel, that object is freed to its respective memory manager. Ideally after freeing the vsock object, we will be able to trigger some sort of Use After Free (UAF) to gain a better primitive and escalate privileges,” Hoefler explains.
The vulnerability stemmed from the improper handling of socket binding states during transport reassignment. Specifically, the vsock code incorrectly decreased the reference counter on already unbound sockets, leading to the premature freeing of vsock objects.
“A transport reassignment can trigger vsock_remove_sock, which calls vsock_remove_bound which decreases the reference counter on a vsock object incorrectly,” the patch analysis notes. This introduced a dangerous UAF condition that attackers could leverage for powerful exploitation primitives.
Initial exploitation attempts led to kernel panics, revealing that AppArmor — through Linux Security Module (LSM) hooks — wiped out the sk_security pointer when sockets were freed, thwarting direct function pointer corruption.
“The kernel grants us a dangling pointer to the socket — but AppArmor ensures we crash before we can do anything useful with it,” Hoefler highlights. Thus, attackers had to find kernel functions not protected by AppArmor to continue exploitation.
To bypass Kernel Address Space Layout Randomization (kASLR) without relying on AppArmor-protected functions, Hoefler turned to the vsock_diag_dump() feature, an unprotected netlink diagnostic tool.
By reclaiming the freed vsock socket with crafted pipe buffers and leveraging vsock_diag_dump as a side channel, Hoefler brute-forced the valid skc_net pointer, thus bypassing kASLR.
“Stuck in my tracks, I resorted to the kernelctf community, where @h0mbre suggested brute-forcing the skc_net pointer using vsock_diag_dump as a side channel,” Hoefler recounts. This allowed full control over the dangling socket object with a known memory offset.
To gain code execution, Hoefler exploited the vsock_release() function. Since it wasn’t protected by AppArmor, the attack could hijack the sk->sk_prot->close function pointer and pivot into a ROP (Return-Oriented Programming) chain:
“We are most interested in the call to sk->sk_prot->close(sk, 0). Since we control sk, we need a valid pointer to a pointer to a function,” he explains.
The final ROP chain called commit_creds(init_cred), granting root privileges to the attacker and cleanly returning execution to userspace.
You can find Hoefler’s complete proof-of-concept exploit code here.