SMB-Server 2.1.1

SMB-Server

A SMB 2/3 server library - written in C# (.NET 9)

This is a library specifically build, for implementing / creating your own SMB Fileserver. The library was created based on the official Microsoft Specifications (MS-ERREF, MS-FSCC, MS-NLMP, MS-SMB2, ...)

What it does

• 📁 File shares with full NTFS semantics (read, write, rename, delete, locking).
• 🔐 Secure by default - signing required, optional per-share encryption, guest/anonymous rejected.
• 🔑 Real login via NTLMv2; the user source (local, LDAP/AD …) is freely pluggable.
• :jigsaw: Modular - plug in your own file backend, auth, lock manager or directory watcher.

Installation

The library is available via download of the source files or as nuget package. URL is coming...

Using the library

You will find an example with different usages under /examples.

More background

The rest of this document is a technical reference for contributors and the curious. You don't need it for normal use of the library.

Architecture

Strict Parse ↔ State ↔ Effect layering - each layer its own project, no cycles:

Foundation and fact-check: SMB2-3_Server_Context.md.

Modular authentication

SESSION_SETUP talks only to ISpnegoNegotiator. New mechanisms = register a new IGssMechanism; new identity source (e.g. LDAP/AD) = a new IIdentityBackend - without touching the protocol or server layer.

ISpnegoNegotiator  ──>  IGssMechanism (NTLM today, Kerberos later)
                                │
                                └──>  IIdentityBackend (local today, LDAP/AD later)

Security defaults & audit

• SMB1 file access off (only the negotiate-upgrade path is provided).
• Signing required by default; 3.1.1 preferred with preauth integrity (SHA-512).
• Cipher preference AES-128-GCM > AES-256-GCM > AES-128-CCM > AES-256-CCM.
• Signing preference AES-GMAC > AES-CMAC > HMAC-SHA256.
• Guest/anonymous rejected by default.
• Per-share encryption enforceable (Share.EncryptData): responses are encrypted, and unencrypted access to an encrypted tree is rejected with RejectUnencryptedAccess (on by default); an encrypted share on a connection without a 3.x cipher → ACCESS_DENIED.
• Crypto exclusively via the .NET BCL (System.Security.Cryptography).

Security audit: status and open items in docs/SECURITY_AUDIT.md (fixed findings are marked in code with [AUDIT-2026-06] and guarded by AuditFixTests). Still open, among others: NTLM MIC verification (O1), 3.1.1 negotiate validation (O3), credit accounting (O6). ⚠️ Cross-check AES-256 key derivation (M3) against a real Windows interop capture before using Kerberos.

Verification

The suite (383 tests) covers, among others:

• Official crypto vectors: AES-CMAC (RFC 4493 §4), MD4 (RFC 1320 A.5), NTOWFv2 (MS-NLMP §4.2 example).
• Wire roundtrips: header (sync/async), NBSS (big-endian length prefix), negotiate contexts (8-byte alignment, offset correctness).
• Crypto: KDF determinism, key derivation (3.0/3.1.1, AES-128/256, ServerIn label with the trailing space), sign/verify + tamper detection (all 3 algorithms), transform roundtrip
  • AEAD tag check, preauth hash chain.
• Server end-to-end: NEGOTIATE → SESSION_SETUP → TREE_CONNECT → ECHO; dialect selection; cipher/signing negotiation; signing enforcement (signed accepted, unsigned rejected); guest rejection; TCP integration over real NBSS.
• Per-share encryption: tree marking + ShareFlags, plaintext request on an encrypted tree rejected (RejectUnencryptedAccess), encrypted-share connect without a cipher rejected, and the host returns the TREE_CONNECT response of an encrypted share as a TRANSFORM frame.
• Oplocks: grant of the requested level on a solo open (Batch/Exclusive), downgrade to Level II + OPLOCK_BREAK notification on a second open of the same file, acknowledgment, release on CLOSE; lease-break acknowledgment (still) → STATUS_NOT_SUPPORTED.
• Audit fixes (AuditFixTests): LOGOFF signing enforcement, MessageId sequence window (replay/out-of-window rejected), MaximalAccess enforcement on CREATE, cap on pending async operations.
• Path sandbox (SymlinkSandboxTests): a symlink/reparse point inside the share that points outside is denied (file and directory); normal in-root access stays allowed (H4).
• QUERY_DIRECTORY paging + stable FileId (QueryDirectoryPagingTests, O2): large directory across multiple pages, single entry, buffer too small → INFO_LENGTH_MISMATCH.
• Share modes / persistent handle (ShareModeManagerTests, LocalFileStoreHandleTests, QueryDirectoryPagingTests, O5): sharing violation on an incompatible second open + release after CLOSE; read/write/DeleteOnClose/rename-while-open over a persistent OS handle; stable FileId.

Roadmap

License note

The underlying Microsoft Open Specifications are covered by the Open Specifications Promise. Structures/constants were reimplemented (no verbatim text was copied).