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Introduction:
Microsoft Remote Procedure Call (MS-RPC) is a powerful inter-process communication mechanism used across Windows environments, but its complexity often hides critical security vulnerabilities. Security researcher Remco van Meer has recently unveiled major updates to his MS-RPC-Fuzzer tool, enabling recursive fuzzing of deeply nested structures while leveraging Event Tracing for Windows (ETW) for real-time monitoring – and demonstrating a reliable path to escalate privileges to NT AUTHORITY\SYSTEM.
Learning Objectives:
- Understand how to set up and execute recursive fuzzing against MS-RPC interfaces
- Implement ETW-based logging to capture crash artifacts and behavioral anomalies
- Reproduce privilege escalation from a low-privileged user to SYSTEM using discovered RPC flaws
You Should Know:
1. Setting Up MS-RPC-Fuzzer for Recursive Fuzzing
The MS-RPC-Fuzzer tool has been updated on GitHub to support recursive traversal of complex RPC structures – think nested pointers, unions, and variable-length arrays. To get started, clone the repository and install dependencies.
Step‑by‑step guide (Windows with Python 3.9+):
git clone https://github.com/remco-/MS-RPC-Fuzzer.git cd MS-RPC-Fuzzer pip install -r requirements.txt
The tool uses Python’s `impacket` and custom MIDL parsing. For Linux (WSL or native), ensure you have `python3-pip` and `gcc-mingw-w64` if cross‑compiling test servers. Basic usage:
python fuzzer.py -t 192.168.1.100 -p 135 -i 12345678-1234-1234-1234-123456789abc
The `-i` flag specifies the RPC interface UUID. Recursive mode triggers automatically when the tool detects complex structures in the IDL.
2. Recursively Fuzzing Complex RPC Structures
Traditional fuzzing often stops at the first level of nested data, missing deep vulnerabilities. The updated recursive engine walks through every branch of a structure, generating mutated inputs for each union case or pointer chain.
How to use recursive fuzzing:
- First, extract the interface definition from a Windows binary using `rpcdump.py` (impacket):
rpcdump.py target/135
- Identify interfaces with `
` attributes or `[bash]` pointers.</li> <li>Run the fuzzer with `--depth 5` to control recursion depth: [bash] python fuzzer.py -t target -i UUID --depth 5 --iterations 10000
- Monitor crashes via ETW (next section). For Linux users, you can run the tool against a Windows target over SMB named pipes or TCP 135.
Example of a generated malformed structure (simplified Python snippet):
def mutate_nested(original, depth): if depth == 0: return b"\x00"len(original) mutated = bytearray(original) mutated[random.randint(0,len(original)-1)] = random.randint(0,255) return mutate_nested(mutated, depth-1)
- Leveraging ETW for Real-Time Logging and Crash Detection
ETW (Event Tracing for Windows) is built into the OS and provides low‑overhead telemetry. The fuzzer now subscribes to Microsoft-Windows-RPC ETW provider, capturing every call, exception, and access violation without touching the target process.
Step‑by‑step guide to enable ETW monitoring:
- On the target Windows machine, open PowerShell as Administrator and start a trace session:
wevtutil qe Microsoft-Windows-RPC/Analytic /f:text /c:10 logman create trace RPC_Fuzz -p Microsoft-Windows-RPC 0xFFFFFFFF 0xFF -ets logman start RPC_Fuzz -ets
- On the attacker side, launch the fuzzer with `–etw` flag, which will parse live ETW events via the Windows API (or remotely using WinRM).
3. To view collected logs in real time:
logman query RPC_Fuzz -ets type C:\PerfLogs\Admin\RPC_Fuzz_000001.etl after stopping
The fuzzer automatically correlates a crash event (e.g., Event ID 5 – RPC_SERVER_UNAVAILABLE) with the exact input that caused it, speeding up root cause analysis.
- Escalating to NT AUTHORITY\SYSTEM Using Discovered RPC Flaws
Remco’s research found that recursively fuzzed structures can trigger a use‑after‑free in the RPC runtime when certain nested pointer loops are incorrectly validated. This flaw allows an authenticated user to overwrite a function pointer in the RPC server’s memory, leading to arbitrary code execution with SYSTEM privileges.
Step‑by‑step exploitation (proof of concept, for authorized testing only):
1. After the fuzzer identifies a crash, replay the crashing input with a custom harness:
crash_replay.py from impacket.dcerpc.v5 import transport trans = transport.DCERPCTransportFactory(r'ncacn_np:target\pipe\spoolss') trans.connect() binding = trans.get_dce_rpc() binding.connect() Send malformed structure from fuzzer crash dump binding.call(0x01, crash_data) assuming opnum 1 is vulnerable
2. Use the Windows API to leak the base address of `rpcrt4.dll` (via NtQuerySystemInformation).
3. Craft a ROP chain that calls `CreateProcess` to spawn `cmd.exe` as SYSTEM.
4. Trigger the flaw again, overwriting the pointer with your ROP address. Successful execution yields:
whoami NT AUTHORITY\SYSTEM
Mitigation: Microsoft has not yet released a patch; monitor `C:\Windows\Logs\RPC\` for anomalous ETW events and consider blocking anonymous RPC access via `sc config RpcSs start= disabled` (not recommended in production – instead use firewall rules to restrict RPC to trusted IPs).
5. Mitigation Strategies and Cloud Hardening for RPC
Given the attack surface, defenders must harden RPC exposure. Apply these measures on Windows servers and cloud‑based VMs:
- Restrict RPC endpoints: Use Windows Firewall to block ports 135, 445, and dynamic RPC ports (49152-65535) except from authorized management subnets.
New-NetFirewallRule -DisplayName "Block RPC from Internet" -Direction Inbound -Protocol TCP -LocalPort 135,445 -Action Block -RemoteAddress Any
- Enable RPC over HTTPS (RPC/HTTPS) in Azure or AWS environments, forcing authentication and TLS encryption.
- Apply Microsoft’s RPC hardening GPO: Computer Configuration → Administrative Templates → System → Remote Procedure Call → “Restrict unauthenticated RPC clients” set to “Authenticated”.
- Monitor ETW logs centrally using Azure Monitor or AWS CloudWatch (forward ETW events via `wevtutil` and a log agent).
- For Linux systems exposing RPC via Samba, update to latest version and disable NTLMv1.
6. Advanced Fuzzing with Custom Python Scripts (Cross‑Platform)
You can integrate MS-RPC-Fuzzer into your own CI/CD pipeline for security testing of internal RPC services. Below is a Linux‑friendly automation script that runs the fuzzer inside a Docker container and stores crash outputs.
!/bin/bash run_fuzzer_linux.sh docker run --rm -v $(pwd)/output:/output python:3.9 bash -c " pip install impacket git clone https://github.com/remco-/MS-RPC-Fuzzer.git cd MS-RPC-Fuzzer python fuzzer.py -t $TARGET_IP -p 135 -i $UUID --depth 3 --etw --output /output/crashes/ " echo "Fuzzing completed. Review /output/crashes/"
Windows users can schedule a PowerShell job:
$job = Start-Job -ScriptBlock { python C:\MS-RPC-Fuzzer\fuzzer.py -t 10.0.0.1 -i $using:uuid }
Receive-Job $job -Wait
For API security, treat any internal RPC endpoint as you would a public REST API: input validation, rate limiting, and least privilege. The same recursive fuzzing technique applies to gRPC and CORBA.
7. API Security Implications and Training Recommendations
The MS-RPC fuzzing approach translates directly to testing other binary protocols (e.g., DCE/RPC, ONC RPC). Security teams should invest in training courses that cover:
- Offensive RPC Security (SANS SEC660, or Zero-Point Security’s RPC module)
- Windows Internals for Fuzzers (Microsoft Learn: “Advanced Windows Debugging”)
- Python Fuzzing Frameworks (e.g., Boofuzz, AFL++)
Key commands to add to your toolkit for API fuzzing on Linux:
Enumerate RPC endpoints from a Windows target using rpclient (Samba) rpclient -U '' -N //target/ipc$ -c getusername Use ndr.py (impacket) to manually craft NDR structures python -c "from impacket.dcerpc.v5 import transport; print(transport.DCERPCTransportFactory(r'ncacn_ip_tcp:target[bash]'))"
What Undercode Say:
- Key Takeaway 1: Recursive fuzzing is no longer just a theoretical concept – with MS-RPC-Fuzzer’s ETW integration, attackers can reliably discover privilege escalation vectors that bypass traditional static analysis.
- Key Takeaway 2: ETW provides defenders with unprecedented visibility into RPC abuse, but it must be actively configured and monitored; default logging is insufficient to catch these deep structure attacks.
- Analysis: Remco’s work highlights a dangerous gap: most enterprise Windows environments expose RPC broadly for domain controllers, file shares, and management tools. The path from a low‑privileged domain user to SYSTEM is often just one malformed RPC call away. Red teams should immediately adopt this fuzzer for internal assessments, while blue teams need to deploy ETW‑based detection rules (e.g., alert on `RPC_SERVER_UNAVAILABLE` with unusual call depths). Moreover, the technique sets a precedent for fuzzing other Microsoft protocols (SMB, DCOM) with recursive structure traversal.
Prediction:
Within the next six months, we will see multiple CVEs directly attributed to recursive RPC fuzzing, forcing Microsoft to overhaul the NDR (Network Data Representation) engine’s pointer validation. This will spark a new wave of “RPC‑aware” EDRs that parse ETW event IDs 5, 11, and 30 in real time. Meanwhile, nation‑state actors will incorporate these methods into their Windows lateral movement toolkits, making RPC hardening (via firewall micro‑segmentation and authenticated only binding) a critical compliance requirement for any organization running Active Directory. The open‑source release of MS-RPC-Fuzzer will also lower the barrier for small security teams to perform advanced protocol fuzzing – democratizing a capability once reserved for well‑funded labs.
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Reported By: Remco Vandermeer – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
