vett3x/smb-linux-cve-2025-37899Technical analysis of CVE-2025-37899: a use-after-free vulnerability in Linux kernel's ksmbd SMB module enabling remote code execution. Includes attack vectors, impact assessment, and patching guidance.
A use-after-free vulnerability has been discovered in the ksmbd module of the Linux kernel, responsible for implementing the SMB (Server Message Block) protocol for network file sharing. This vulnerability, identified as CVE-2025-37899, could allow remote code execution with elevated privileges.
The discovery was made by researcher Sean Heelan using OpenAI's o3 model, demonstrating the potential of artificial intelligence in identifying complex vulnerabilities. The vulnerability resides in the handling of the SMB2/3 logoff command and can be exploited under certain concurrency conditions.
The use-after-free vulnerability occurs when a thread frees the memory associated with the sess->user object during processing of the command. If another thread attempts to access this freed object, a race condition occurs that can result in memory corruption and, potentially, arbitrary code execution.
logoffThe vulnerable code is located in the smb2_session_logoff function within the ksmbd module. The lack of proper synchronization in accessing the sess->user object allows the memory to be freed while it is still being used by another thread.
// Ejemplo simplificado del código vulnerable
void smb2_session_logoff(struct ksmbd_work *work) {
struct smb_session *sess = work->session;
if (sess->user) {
ksmbd_free_user(sess->user); // Liberación de la memoria
sess->user = NULL;
}
}
// Otro hilo podría acceder a sess->user aquí, después de la liberación
A remote attacker could exploit this vulnerability by sending a series of SMB2/3 commands designed to create a race condition in accessing the sess->user object. This could be achieved by establishing multiple SMB connections and manipulating the session logout to coincide with concurrent access to the object.
Successful exploitation of this vulnerability could allow an attacker to:
Patches It is recommended to apply the security patches provided by your Linux distribution as soon as possible. Major distributions are already working on releasing updates.
// Después (arreglado)
void smb2_session_logoff(struct ksmbd_work *work) {
struct smb_session *sess = work->session;
spin_lock(&sess->user_lock); // Lock para proteger el acceso concurrente
if (sess->user) {
ksmbd_free_user(sess->user);
sess->user = NULL;
}
spin_unlock(&sess->user_lock); // Unlock después de la operación
}
And in functions that access sess->user, it must also be protected:
// Ejemplo de acceso seguro en otra función
void smb2_check_user_session(struct smb_session *sess) {
spin_lock(&sess->user_lock);
if (sess->user) {
// Acceso seguro a sess->user
// ...
}
spin_unlock(&sess->user_lock);
}
If patches cannot be applied immediately, it is recommended to:
Thanks to Sean Heelan for the discovery and to the cybersecurity community for their collaboration in mitigating this vulnerability.