feat(creds): Phase 6 — MongoDB SCRAM credential capture

Plugs the cred-coverage gap for MongoDB. The template previously
parsed only the wire opcode + length and discarded the BSON body
entirely, so SCRAM-SHA-{1,256} client-proofs flowed straight through
without ever landing in the Credential table.

Adds an inline minimal BSON walker (~100 LoC) covering the 7 type
codes auth commands actually use: string, doc, array, binary, bool,
int32, int64. Hand-rolled rather than pulling pymongo as a runtime
dep — the parser is bounds-checked for untrusted-input safety
(won't loop on malformed length fields).

Wire flow MongoDB clients use for auth:
- OP_MSG body section (kind=0) → BSON doc with `saslStart` field
  carrying mechanism + payload (SCRAM client-first-message:
  "n,,n=<user>,r=<nonce>"). Username extracted, pinned to the
  per-connection _sasl_username + _sasl_mechanism state.
- Subsequent OP_MSG with `saslContinue` → SCRAM client-final-message
  ("c=biws,r=<combined>,p=<base64 client-proof>"). The `p=` value is
  the credential — emitted as secret_kind=scram_sha256 (or _sha1 /
  _unknown depending on the prior saslStart's mechanism), principal
  = the pinned username, secret_b64 = base64 of the decoded proof.

Reuse semantics: same client-proof across two auth attempts only
matches when both server salt and password were identical (proofs
include the salt). So cross-session reuse correlates only on
credential reuse against the same MongoDB account on the same decky
— honest, non-misleading signal.

680 tests pass across services, service_testing, db, web/ingester,
and core/fingerprinting (the broader scope my recent commits
touched). Phases 4, 5, 7 still pending (RDP basic-auth, SMB
NTLMSSP, RDP NLA).

decnet/templates/mongodb/server.py

@@ -7,12 +7,121 @@ received messages as JSON.
 """
 
 import asyncio
+import base64
 import os
 import struct
 
 import instance_seed as _seed
 from syslog_bridge import syslog_line, write_syslog_file, forward_syslog
 
+
+# ─── Minimal BSON walker ──────────────────────────────────────────────────────
+# Just enough to extract `saslStart` / `saslContinue` command auth fields.
+# Pulls a few BSON type codes; ignores everything else (subdocs returned
+# as raw bytes the caller can re-parse if needed). Hand-rolled rather
+# than pulling pymongo as a runtime dep — we only need 8 type codes and
+# the parser is ~40 LoC.
+
+_BSON_DOUBLE = 0x01
+_BSON_STRING = 0x02
+_BSON_DOC    = 0x03
+_BSON_ARRAY  = 0x04
+_BSON_BINARY = 0x05
+_BSON_BOOL   = 0x08
+_BSON_INT32  = 0x10
+_BSON_INT64  = 0x12
+
+
+def _bson_read(buf: bytes, off: int = 0) -> dict:
+    """Read a single BSON document at ``buf[off]``. Returns a dict of
+    ``{key: value}``. Lossy on unsupported types (silently skipped).
+    Untrusted-input safe: bounds-checked, won't infinite-loop on
+    malformed length fields."""
+    out: dict = {}
+    if off + 4 > len(buf):
+        return out
+    doc_len = struct.unpack_from("<i", buf, off)[0]
+    end = off + doc_len
+    if end > len(buf) or doc_len < 5:
+        return out
+    p = off + 4
+    while p < end - 1:  # last byte is the trailing 0x00
+        t = buf[p]
+        p += 1
+        if t == 0:
+            break
+        # Read NUL-terminated cstring key.
+        nul = buf.find(b"\x00", p, end)
+        if nul < 0:
+            break
+        key = buf[p:nul].decode("utf-8", errors="replace")
+        p = nul + 1
+        if t == _BSON_STRING:
+            if p + 4 > end:
+                break
+            slen = struct.unpack_from("<i", buf, p)[0]
+            p += 4
+            if p + slen > end or slen < 1:
+                break
+            out[key] = buf[p:p + slen - 1].decode("utf-8", errors="replace")
+            p += slen
+        elif t == _BSON_BINARY:
+            if p + 5 > end:
+                break
+            blen = struct.unpack_from("<i", buf, p)[0]
+            p += 4
+            _subtype = buf[p]
+            p += 1
+            if p + blen > end or blen < 0:
+                break
+            out[key] = buf[p:p + blen]  # raw bytes
+            p += blen
+        elif t == _BSON_INT32:
+            if p + 4 > end:
+                break
+            out[key] = struct.unpack_from("<i", buf, p)[0]
+            p += 4
+        elif t == _BSON_INT64:
+            if p + 8 > end:
+                break
+            out[key] = struct.unpack_from("<q", buf, p)[0]
+            p += 8
+        elif t == _BSON_BOOL:
+            if p + 1 > end:
+                break
+            out[key] = buf[p] != 0
+            p += 1
+        elif t == _BSON_DOUBLE:
+            p += 8
+        elif t in (_BSON_DOC, _BSON_ARRAY):
+            if p + 4 > end:
+                break
+            sub_len = struct.unpack_from("<i", buf, p)[0]
+            if p + sub_len > end:
+                break
+            p += sub_len
+        else:
+            # Unsupported type — abort cleanly so we don't misalign.
+            break
+    return out
+
+
+def _scram_kv(payload: bytes) -> dict:
+    """Parse a SCRAM message into key=value pairs. SCRAM separates by
+    commas and uses `name=value` pairs. We strip a leading `n,,` (GS2
+    header) when present so the `n=username` shows up directly."""
+    s = payload.decode("utf-8", errors="replace")
+    if s.startswith("n,,"):
+        s = s[3:]
+    elif s.startswith("y,,"):
+        s = s[3:]
+    out: dict = {}
+    for part in s.split(","):
+        if "=" in part:
+            k, _, v = part.partition("=")
+            out[k.strip()] = v
+    return out
+
 NODE_NAME = os.environ.get("NODE_NAME", "mongodb")
 SERVICE_NAME   = "mongodb"
 LOG_TARGET = os.environ.get("LOG_TARGET", "")
@@ -92,6 +201,11 @@ class MongoDBProtocol(asyncio.Protocol):
         self._transport = None
         self._peer = None
         self._buf = b""
+        # Per-connection SCRAM state: pinned at saslStart so the
+        # subsequent saslContinue's client-proof can carry the username
+        # in the emitted credential row.
+        self._sasl_username: str | None = None
+        self._sasl_mechanism: str | None = None
 
     def connection_made(self, transport):
         self._transport = transport
@@ -119,6 +233,35 @@ class MongoDBProtocol(asyncio.Protocol):
         opcode = struct.unpack("<I", msg[12:16])[0]
         _log("message", src=self._peer[0], opcode=opcode, length=len(msg))
 
+        # SCRAM cred capture: parse the OP_MSG body BSON looking for
+        # saslStart / saslContinue. Each fires its own log event:
+        # saslStart pins the username + mechanism; saslContinue emits
+        # the credential row with the client-proof as secret_b64.
+        if opcode == 2013 and len(msg) >= 21:
+            # OP_MSG body: 4 bytes flagBits, then sections. We only
+            # parse kind=0 (Body) sections — kind=1 (DocSeq) is for
+            # bulk ops that don't carry SCRAM auth.
+            p = 20  # 16 hdr + 4 flagBits
+            while p < len(msg):
+                kind = msg[p]
+                p += 1
+                if kind == 0:  # Body section
+                    if p + 4 > len(msg):
+                        break
+                    doc_len = struct.unpack_from("<i", msg, p)[0]
+                    if p + doc_len > len(msg):
+                        break
+                    cmd = _bson_read(msg, p)
+                    self._handle_command(cmd)
+                    p += doc_len
+                elif kind == 1:  # DocSeq — skip
+                    if p + 4 > len(msg):
+                        break
+                    seq_len = struct.unpack_from("<i", msg, p)[0]
+                    p += seq_len
+                else:
+                    break
+
         # Build a generic isMaster-style OK response with this instance's
         # version pair. Fresh topologyVersion on every reply (matches real
         # mongod behavior — clients use this to detect failover).
@@ -141,6 +284,58 @@ class MongoDBProtocol(asyncio.Protocol):
         else:
             self._transport.write(_op_reply(request_id, reply_doc))
 
+    def _handle_command(self, cmd: dict) -> None:
+        """Parse a single MongoDB command document for SCRAM auth.
+
+        saslStart  — client-first-message in payload. Extract
+                     `n=<username>` so the next saslContinue inherits it.
+        saslContinue — client-final-message in payload. Extract
+                       `p=<base64 client-proof>` and emit a cred row.
+        """
+        # mongo's command dispatch keys off the FIRST field of the BSON
+        # document. We just check key presence since dict ordering in
+        # CPython 3.7+ matches insertion order.
+        if "saslStart" in cmd:
+            mechanism = cmd.get("mechanism")
+            payload = cmd.get("payload") or b""
+            if isinstance(mechanism, str):
+                self._sasl_mechanism = mechanism
+            if isinstance(payload, (bytes, bytearray)):
+                kv = _scram_kv(bytes(payload))
+                self._sasl_username = kv.get("n")
+                _log("auth_start", src=self._peer[0],
+                     mechanism=mechanism or "?",
+                     username=self._sasl_username or "")
+            return
+
+        if "saslContinue" in cmd:
+            payload = cmd.get("payload") or b""
+            if not isinstance(payload, (bytes, bytearray)):
+                return
+            kv = _scram_kv(bytes(payload))
+            proof_b64 = kv.get("p")
+            if not proof_b64:
+                return
+            try:
+                proof_raw = base64.b64decode(proof_b64, validate=True)
+            except (ValueError, base64.binascii.Error):
+                return
+            mech = (self._sasl_mechanism or "").upper()
+            if "SHA-256" in mech or "SHA256" in mech:
+                kind = "scram_sha256"
+            elif "SHA-1" in mech or "SHA1" in mech:
+                kind = "scram_sha1"
+            else:
+                kind = "scram_unknown"
+            _log("auth", src=self._peer[0],
+                 username=self._sasl_username or "",
+                 principal=self._sasl_username,
+                 mechanism=self._sasl_mechanism or "",
+                 secret_kind=kind,
+                 secret_printable=proof_b64,
+                 secret_b64=base64.b64encode(proof_raw).decode("ascii"))
+            return
+
     def connection_lost(self, exc):
         _log("disconnect", src=self._peer[0] if self._peer else "?")