Building a Custom SMB1 Authentication Server from Scratch
Description
This project is a custom implementation of an SMB1 authentication server designed to capture NTLMv2 hashes by tricking SMB clients into authenticating against it. It is intended for offensive security and educational purposes, allowing interception of NTLMv2 authentication hashes in authorized environments. The server mimics legitimate SMB1 server behavior through a state machine handling negotiation, challenge, and authentication phases. Captured hashes are output in a format compatible with common cracking tools. This tool is not a vulnerability itself but a security research and red teaming utility.
Reddit Discussion
For the past 8 months, in my spare time, I’ve been working on a personal project with the goal of studying the old SMB1 down to its lowest levels. During this time, I’ve dissected the protocol using Wireshark, waded through Microsoft’s documentation (help!), and reverse-engineered the authentication phase. I also used AI as a tool for debugging and to help wrap my head around some of the more complex mechanisms, though the overall structure, architecture, and code were entirely designed and written by me.
So, I decided to build a lightweight server designed to 'trick' SMB clients into authenticating against it (similar to what the famous tool Responder does). I chose to hand-craft virtually the entire SMB1 stack, or at least enough of it to capture hashes. To achieve this, I wrote all the necessary network parsers from scratch: SMB1, ASN.1, SPNEGO, and NTLM, followed by the server itself.
It was both challenging and incredibly rewarding to dive so deep into a protocol famous for its complexity. I learned a huge amount along the way.
In the end, I think it turned out to be a cool little project, so I decided to share it. It’s certainly not meant to replace well-established tools, but if anyone wants to try it out or contribute in any way, I’d be thrilled! 🙂
Links cited in this discussion
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The project implements a lightweight SMB1 server from scratch in C, including parsers for SMB1, ASN.1, SPNEGO, and NTLM protocols. It operates as a state machine to interact with SMB clients, guiding them through the authentication process to capture NTLMv2 hashes. The server listens on a configurable TCP port and handles multiple simultaneous connections using poll(). When a client completes NTLM authentication, the server extracts and prints the username, hostname, domain, and NTLMv2 hash. The tool is intended for use in authorized environments for offensive security testing and protocol study, not as a replacement for established tools.
Potential Impact
The tool enables interception of NTLMv2 hashes by impersonating an SMB1 server, which could be used in offensive security or red teaming exercises. It does not represent a vulnerability or exploit by itself but could facilitate credential harvesting if deployed maliciously or without authorization. There is no indication of active exploitation or inherent software vulnerability in this project.
Defensive Guidance
This is a security research and offensive tool intended for authorized use only. No patch or remediation is applicable. Organizations should ensure SMB1 protocol usage is minimized or disabled where possible to reduce risk of credential interception. Use network segmentation and monitoring to detect unauthorized SMB servers. Always deploy such tools only in environments where explicit consent and authorization exist.
Technical Details
- Source Type
- Subreddit
- cybersecurity
- Reddit Score
- 0
- Discussion Level
- minimal
- Content Source
- reddit_link_post
- Post Type
- link
- Newsworthiness Assessment
- {"score":27,"reasons":["external_link","established_author","very_recent"],"isNewsworthy":true}
- Has External Source
- true
- Trusted Domain
- false
Threat ID: 6a91d8d4acd9273b49381fc9
Added to database: 08/28/2026, 18:52:04 UTC
Last enriched: 09/09/2026, 22:23:11 UTC
Last updated: 10/01/2026, 22:24:03 UTC
Views: 103
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