CVE-2023-4458 - Sensitive Data Disclosure in Linux KSMBD Module Due to Flawed Attribute Parsing

Linux is one of the most widely used operating systems across the globe, known for its stability and robust security practices. However, even mature software can have vulnerabilities slip through the cracks. In this article, we’ll break down CVE-2023-4458, a recent and impactful vulnerability in the Linux kernel’s ksmbd module—a server for the SMB3 protocol, which allows file sharing with Windows machines.

Let’s look at what this flaw means, how an exploit could work, and what you need to do to stay safe.

What Is CVE-2023-4458?

CVE-2023-4458 is a vulnerability in Linux’s ksmbd module, which was introduced to provide a fast, in-kernel SMB server implementation. The flaw lies in how ksmbd parses “extended attributes” received from users.

In simple terms:
The code that handles user input (data provided by clients over the network) fails to properly check how big that data is, and ends up reading beyond the boundary of the buffer it’s supposed to stay inside ("out-of-bounds read"). This can let an attacker access extra data held in memory—potentially leaking sensitive information.

Who Is Vulnerable?

Only Linux systems that have ksmbd enabled are vulnerable to this bug. Many Linux systems use samba (user space daemon) for file sharing, which is not affected. The risk only applies to those using the kernel’s new ksmbd module.

CVE-2023-4458: Out-of-bounds read in ksmbd’s attribute parsing.

The problem occurs in code handling Extended Attributes (xattrs). When a user requests attributes, ksmbd does not validate the size of the input thoroughly enough, which means maliciously crafted requests can cause it to read memory that it should not.

This could leak data such as kernel memory addresses, process information, files, or even credentials under some scenarios.

Here’s a simplified version of how the code could look (for demonstration)

int parse_xattr(struct smb_request *req) {
    char buf[128];
    int xattr_size = get_user_xattr_size(req);

    // Vulnerability: No check if xattr_size > sizeof(buf)
    memcpy(buf, req->xattr, xattr_size);

    // Use 'buf', possibly reading beyond it
    process_xattr(buf, xattr_size);
}

In this snippet, the variable xattr_size is controlled by the user. If it’s bigger than the buf array, memcpy writes outside the bounds of buf, or reads too much when processing. Either case leads to the kernel revealing data it should keep private.

Actual Vulnerability Location: See Linux kernel code here.

To exploit the bug, an attacker must

1. Connect to the vulnerable ksmbd server—either from a local network or (if exposed) over the internet.

Send an SMB request with a specially crafted attribute size—the size field is set too large.

3. Trigger ksmbd to respond—when the code reads from memory past where it should, the attacker receives leaked memory as a server response.

Here’s a sample (Python) pseudo-code for sending a crafted request (purely illustrative, not a weaponized exploit):

from smbprotocol.connection import Connection
from smbprotocol.session import Session
from smbprotocol.tree import TreeConnect
from smbprotocol.open import Open, FileAttributes

# Connect to vulnerable server
conn = Connection(uuid.uuid4(), "192.168.1.50", port=445)
conn.connect()
session = Session(conn, "attacker", "password")
session.connect()
tree = TreeConnect(session, r"\\192.168.1.50\share")
tree.connect()

# Try to read an extended attribute with a huge size
file = Open(tree, "testfile.txt", FileAttributes.FILE_ATTRIBUTE_NORMAL)
file.create()
try:
    # Send a xattr request with an intentionally oversized buffer
    file.getxattr(attr_name="user.malicious", size=1024*1024)
except Exception as e:
    print("Trigger sent! Check server response for leaked data.")
file.close()

*(You need the Python smbprotocol package for this; adjust as needed for real testing.)*

Credentials or authentication tokens in rare cases

Luckily, only systems with ksmbd enabled are at risk. If you use the default Samba server in user space, you are safe.

Linux kernel maintainers responded quickly

- Patched in mainline: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a3eaf6e0228a2317e672005830e59516779d8f85

Update your kernel: Make sure you are on a version after the patch above.

- Disable ksmbd if unused: Most distributions allow you to disable the kernel’s ksmbd service if you don’t specifically need it.

You can check if ksmbd is enabled by running

lsmod | grep ksmbd

If it’s loaded and you aren’t using it, disable it

sudo modprobe -r ksmbd

References and Resources

- Official CVE Report for CVE-2023-4458
- Linux Kernel Patch Commit (github.com)
- Extended attributes in Linux
- ksmbd documentation

In Summary

This vulnerability is a good reminder: even new, fast code meant to replace older tools can contain critical bugs if input is not checked strictly. If you’re running Linux file sharing with ksmbd, patch right away or disable the module if you don’t need it.

Stay secure, and always keep your systems updated!


If you found this article useful, share it with your sysadmin friends and help keep the Linux world safe!

Timeline

Published on: 11/14/2024 12:15:17 UTC
Last modified on: 11/15/2024 13:58:08 UTC