CVE-2025-37899 - Deep Dive Into A "ksmbd" Use-After-Free Vulnerability (With Exploit Details & Patches)

The Linux kernel, powering much of the modern internet and many mission-critical systems, is no stranger to security threats. Today, we’ll break down CVE-2025-37899—a freshly patched vulnerability in the ksmbd server—a component that enables SMB3 file sharing natively on Linux.

This post is for those wanting a clear, exclusive look at this bug, how the issue happens in code, the risks involved, and how an attacker could use it in the wild. I’ll also walk you through the patch that closed the hole.

What is "ksmbd"?

ksmbd is a relatively new kernel-level SMB (Server Message Block) server for Linux. SMB allows file sharing across the local network and is widely used in enterprises—think Windows File Sharing. Since ksmbd runs in kernel-space, vulnerabilities here can be especially severe.

Vulnerability Summary

CVE-2025-37899 is a classic "use-after-free" bug. If you’re not familiar, it means a part of the program tries to use memory after it’s been freed. When this happens, attackers can sometimes take control of what happens next.

How The Bug Happens

Imagine two people (clients) are talking to the server at the same time, using the same session. When one of them logs off, the server frees some memory tied to the session. But—at the very same moment—the other person might be setting up that session and using this recently-freed memory! Because kernel code moves fast, these two events can overlap, causing major trouble.

In the code, this "user" object is linked to a session (sess->user). One thread can be freeing it because a user logs off, while another thread (think: a second SMB connection) tries to use it in a session setup.

In Code (Pre-patch Commit)

Here’s a minimal snippet to show what’s going on (note: real kernels are much more complex, but this shows the risky pattern):

// In session logoff handler:
user = sess->user;               // Save pointer to user object
sess->user = NULL;               // Unlink user from session
kref_put(&user->refcount, user_release);  // Free if no other refs

// ...Meanwhile, in another thread:
smb2_sess_setup(sess)
{
    if (sess->user) {
        // <-- Potential use-after-free if user was just freed
        do_something(sess->user);
    }
}

There’s nothing stopping another thread from using sess->user right after it’s freed, leading to undefined behavior, crashes, or even privilege escalation.

A race condition like this is a goldmine for attackers. Here’s what could happen

- Crash the kernel (DoS): If the code dereferences memory that’s been freed and possibly reused for something else.
- Elevate privileges or run code: More complex, if the attacker can control *what* is placed in the just-freed memory and trick the kernel into treating it as a legitimate user object.

On connection A: Send a logoff request, causing the user object to be freed.

3. On connection B: Immediately try to perform a session setup (bind to same session), causing use of the just-freed pointer.

Repeat rapidly: to increase the chance of the race condition occuring (timing is everything).

5. Allocate/spray data: Try to fill system memory with attacker-controlled objects to occupy the space where the user object just was, manipulating what the kernel sees when it dereferences the dangling pointer.

The Patch: How Was It Fixed?

The kernel patch for ksmbd solved this by ensuring the session’s user pointer is not used unsafely and is only freed when truly safe. In technical terms, they make sure the reference count on the user object is increased before any operation that could free it, and only decreased once totally done.

See the commit on LKML

- PATCH: ksmbd: fix use-after-free in session logoff (lkml.org)

Relevant code fix (abridged)

// Instead of freeing user as soon as logoff is called...
user = sess->user;
sess->user = NULL;
if (user)
   kref_put(&user->refcount, user_release);

// They ensure that any thread using sess->user holds its own reference count,
// preventing it from being freed out from under another thread.

Recommendations

- Update your kernel. This is fixed in all modern kernels as of June 2024—check your distribution’s advisory for CVE-2025-37899.
- Red Hat advisory
- Linux kernel upstream commit
- Audit SMB exposures. Don’t expose ksmbd externally unless necessary—keep it internal and behind firewalls.
- Monitor logs and use kernel security tools (like SELinux, AppArmor) to limit damage from exploitation.

Conclusion

CVE-2025-37899 is a classic concurrency failure with serious implications in a network file server. If you run any Linux system with ksmbd support enabled—patch now. Vulnerabilities like this remind us why kernel code must be written with extreme care when it comes to memory and concurrency.

More References

- Linux Kernel Mailing List post
- CVE Record (Mitre)
- ksmbd kernel documentation
- Understanding Use-After-Free bugs


Stay safe! Patch early, patch often. If you have questions about kernel security or want to see more walk-throughs like this, let me know in the comments.

Timeline

Published on: 05/20/2025 16:15:26 UTC
Last modified on: 05/26/2025 05:23:18 UTC