In early 2022, Microsoft patched a security issue known as CVE-2022-21993. This vulnerability was found in the Windows Services for NFS (Network File System), specifically within the ONCRPC XDR driver. Although it wasn’t a high-profile code execution flaw, this bug allowed attackers to read sensitive information from system memory—a classic example of an "information disclosure" vulnerability.

Here, we’ll break down what happened, how the bug works, share references for deeper reading, and go through a basic demonstration, all written in straightforward terms. This post is exclusive, aiming to simplify the technical details behind this CVE.

What is Windows Services for NFS and ONCRPC XDR?

Services for NFS is a Windows feature that enables Windows machines to share files with UNIX systems using the NFS protocol. Under the hood, it relies on a technology called ONCRPC (Open Network Computing Remote Procedure Call) and formats its data using XDR (External Data Representation).

The ONCRPC XDR driver (nfs41_driver.sys) handles formatting and parsing these network packets for the NFS service.

What Went Wrong?

When the NFS server receives requests from clients, the ONCRPC XDR driver processes those inputs and prepares responses. CVE-2022-21993 was caused by a flaw in how this driver initialized data structures. In some cases, it failed to properly clear memory buffers before sending responses.

The result? Parts of the server’s memory (potentially with sensitive info like credentials) could be sent back to the attacker as part of the NFS traffic—without any authorization checks.

Send a crafted NFS request to a vulnerable Windows NFS Server.

2. The server would process the request using ONCRPC/XDR logic.

The attacker would analyze the extra data and harvest whatever secrets were exposed.

No authentication was needed, and the attacker only needed network access to the server’s NFS port (normally TCP/UDP port 2049).

Technical Breakdown: Code Snippet Example

Let’s model the bug in simplified C-like code, not the exact proprietary driver code (which is unavailable), but to illustrate the concept.

// Example: unsafe use of local buffer
struct ReplyData {
    char content[256];
};

void handle_nfs_response(Request *req) {
    struct ReplyData reply;
    // ...code processes request...
    
    // Only partially fills content based on req input
    int length = process_request(req, reply.content);
    // Oops! The rest of reply.content may contain old memory values

    send_to_client(reply.content, 256); // sends ALL 256 bytes
}

See the problem? If process_request fills only 100 bytes, the extra 156 bytes could include sensitive info left over from memory—credentials, tokens, or other activity.

How to Test for Exposure: A Minimal PoC

Real-world exploitation requires parsing NFS responses. Here’s a skeleton of how you’d probe from Linux using rpcinfo or custom ONCRPC libraries.

import socket

# Connect to NFS server (change IP and port below)
server_ip = "192.168.1.10"
server_port = 2049

s = socket.socket()
s.connect((server_ip, server_port))

# Send malformed or edge-case NFS request
# (In real attacks, ONCRPC/XDR encoding is needed)
s.sendall(b"\x00" * 120) # Fuzzing payload

# Receive and check for unexpected (extra) data in response
resp = s.recv(512)
print("Raw NFS response:", resp.hex())

*(Note: In a true exploit, you’d use existing ONCRPC tools or a fuzzer, looking for abnormal extra data in the reply.)*

Mitigation & Patch

Microsoft’s advisory:  
- Original reference and updates: MSRC CVE-2022-21993

Patch published in February 2022.

- If your organization uses NFS on Windows, update immediately; otherwise, consider disabling NFS services.

External References

- Microsoft Security Update Guide: CVE-2022-21993
- NFS and ONCRPC documentation (MS Docs)
- XDR Standard (RFC 4506)

Conclusion

CVE-2022-21993 might not let someone take over your system, but it lets attackers peek into memory they never should have seen—often the first step in a bigger attack. Make sure your NFS servers are updated, monitor for unusual NFS activity, and remember that even harmless-looking services can have very real security implications!

Stay safe and keep your systems patched.

If you want to deep dive, check out Microsoft’s advisory and consider monitoring your network for unwanted or strange-looking NFS traffic. Disabling unnecessary services remains one of the simplest and cheapest defenses.

Timeline

Published on: 02/09/2022 17:15:00 UTC
Last modified on: 05/23/2022 17:29:00 UTC