dsh-ocsf-forwarder

dsh-ocsf-forwarder

A read-side SIEM forwarder for DeepSeek Harness. It observes the
session event firehose, normalises every event to OCSF 1.9.0 with the native ai_operation
profile, and writes newline-delimited OCSF JSON to a local append-only spool — optionally shipping
it to Splunk HTTP Event Collector or an OTLP/HTTP collector.

📖 Full documentation — including the
complete event → OCSF mapping table for all 48 session event types.

What it does

• Subscribes to session/event, session/created and session/disposed, and sweeps
  ctx.sessions.list() at mount.
• Correlates tool/call ↔ tool/result and approval/asked ↔ approval/decided, emitting
  approval decision latency — the approval-fatigue signal.
• Classifies tool calls by what they do: shell and code execution → Process Activity (1007),
  file tools → File System Activity (1001), web tools → HTTP Activity (4002), approvals and
  sandbox changes → Authorize Session (3003), everything else → API Activity (6003).
• Names the MCP server behind every mcp__<server>__<tool> call.
• Emits a high-severity record when a tool hands the task to an external harness, stating in
  the record that telemetry coverage ends at that boundary.
• Emits a periodic heartbeat carrying counters, live session count and delivery cursor, so a
  host that goes quiet is distinguishable from one that is idle. A spool that has stopped writing
  reports itself there at severity_id: 5, with the count of what it dropped.
• Chains every spooled record with the OCSF record_integrity profile, and ships
  dsh-ocsf-verify to check the chain.
• Replays a resumed or forked session's constructor seed, which never reaches the live firehose.
• Keeps raw values out of the SOC lane: keyed digests, value classifications and lengths instead.

What it does not do

• It never writes to the session log. Session.append() cannot set the envelope's ignorable
  flag, so a plugin-owned event type makes the next resume throw SessionFormatUnsupportedError
  and refuse the entire session. All durable output goes to our own sink, and the plugin registers
  no waterfall listener, so it cannot change a tool call, an approval decision or a model request.
• It is not a containment boundary. It runs in the agent's process at the agent's uid; an agent
  that can run bash can delete or rewrite the spool — and can recompute the hash chain over what
  it wrote, because the algorithm is published. What it buys you is that records leave the host
  promptly and that a gap is visible — the chain's entry numbering, metadata.sequence holes per
  session, and a shipper cursor that stopped advancing.
• It ships no detection content, no alerting and no secret detectors.

The full scope statement →

Install

The profile must already compose a runnable agent — a profile carrying only
@deepseek-ai/dsh-base has no agent loop and this plugin would observe nothing:

dsh plugin --profile <name> add @deepseek-ai/dsh-headless@0.1.1-rc.2
dsh plugin --profile <name> add dsh-ocsf-forwarder
dsh --profile <name> --dump-config      # verify the row is mounted

Pin @deepseek-ai/dsh-headless explicitly — the @deepseek-ai/dsh-* libraries' npm latest tag
still points at 0.0.1-rc.1. Install from the registry or a packed tarball, not from a git
spec: lib/ is a build output git does not carry.

Runs on dsh 0.1.0-rc.6 through the 0.1.1 line; CI runs the end-to-end suite against every line
in that range.

Install in full →

Configure

- id: dsh-ocsf-forwarder
  config:
    spoolPath: /var/log/dsh/ocsf.jsonl      # absolute; created 0640
    splunk:
      endpoint: https://splunk.example:8088
      token: { source: env, variable: SPLUNK_HEC_TOKEN }
    privacy:
      hmacKey: { source: env, variable: DSH_OCSF_KEY }

Every numeric key that is resolved must be a positive finite number, and those counting records or
files must be whole numbers — statsIntervalMs is the one exception, where 0 means "only at
unload". A value outside those ranges fails at load, because the alternative is worse than a
refused mount: batchSize: 0 makes the shipper loop without ever advancing its cursor. A shipper
block with no endpoint configures no shipper and is not resolved, so nothing in it is checked.

The default privacy posture keeps raw values out of the SOC lane — argument values and command
lines are digested, URLs reduced to their host. A second restricted lane carries verbatim
payloads and must be explicitly acknowledged before it will open.

Every configuration key → ·
Record format and the mapping table →

Shipping to a SIEM

Splunk HEC and OTLP/HTTP are both supported; configure exactly one per spool. Delivery is
cursor-based off the spool, so a collector outage costs nothing but disk, and the spool refuses to
delete an un-drained generation rather than silently discarding unacknowledged evidence.

Splunk and OTLP setup → ·
Delivery and failure modes →

Tamper-evidence

Every record carries an OCSF 1.9.0 record_integrity attestation: the SHA-256 fingerprint of the
record, plus the uid and fingerprint of the record before it. Editing, reordering, or deleting a
record from the middle of a spool breaks the chain at that record and at the one after it.

dsh-ocsf-verify /var/log/dsh/ocsf.jsonl                       # 0 intact, 1 broken, 2 unreadable
dsh-ocsf-verify --anchor shipped.jsonl /var/log/dsh/ocsf.jsonl

Deleting from the end breaks nothing — the shorter chain still verifies — so that check needs a
reference the writer cannot reach. Every shipped record is one: --anchor takes records back from
the SIEM and reports a spool that stops short of them. Without anchors the report says no anchor
rather than implying it checked.

The fingerprints are unkeyed, so anyone can recompute them — which is the point, and which also
means the chain does not resist the agent it observes.

The canonicalisation, the threat model, and the cost →

On Linux the complementary move is chattr +a on the spool, which makes truncation and rewriting
fail rather than merely detectable. The plugin tolerates the chmod that attribute refuses — but
it also refuses the rename, so rotation stops permanently and the file's size becomes a manual
job. Harden the live spool file only; hardening its directory breaks the mount.

What chattr +a buys, costs, and breaks →

Running it with dsh-netguard

Both packages emit OCSF into one index and share the correlation_uid scheme
<session>:<callId>, so a Network Activity record from netguard joins to this package's Process
Activity record for the same tool call — answering which tool call opened this connection.

metadata.uid is deliberately not shared: this package's key is <session>:<seq> over the
session log's event sequence, and netguard namespaces its own as <session>:netguard:<seq> so a
SIEM deduplicating on that field cannot mistake one package's records for the other's.

Development

nvm use 22           # Node ^22.19.0 || >=24, and pnpm 11
pnpm install
pnpm run typecheck
pnpm run test:coverage
pnpm run test:e2e    # boots a real dsh against a mock model; no API key

Design decisions and their rationale live in ADR.md. Security policy is in
SECURITY.md.

License

MIT