diff --git a/.github/workflows/security-snyk.yml b/.github/workflows/security-snyk.yml
new file mode 100644
index 0000000000..67241ddc34
--- /dev/null
+++ b/.github/workflows/security-snyk.yml
@@ -0,0 +1,96 @@
+name: Snyk security
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+permissions:
+  contents: read
+  actions: read # Required by upload-sarif for private repositories.
+  security-events: write
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
+jobs:
+  snyk:
+    name: Snyk scans
+    runs-on: ubuntu-latest
+    env:
+      SNYK_TOKEN_PRESENT: ${{ secrets.SNYK_TOKEN != '' }}
+    steps:
+      - name: Skip when SNYK_TOKEN is unavailable
+        if: env.SNYK_TOKEN_PRESENT != 'true'
+        run: |
+          echo "SNYK_TOKEN is unavailable; skipping Snyk scans."
+          echo "This is expected for fork PRs, Dependabot PRs, and pre-rollout runs before the secret is added."
+
+      - name: Checkout
+        if: env.SNYK_TOKEN_PRESENT == 'true'
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+
+      - name: Setup Snyk
+        if: env.SNYK_TOKEN_PRESENT == 'true'
+        uses: snyk/actions/setup@8e119fbb6c251787721d34ba683ed48eba792766 # master
+
+      - name: Snyk Code scan
+        if: env.SNYK_TOKEN_PRESENT == 'true'
+        id: snyk-code
+        continue-on-error: true
+        env:
+          SNYK_TOKEN: ${{ secrets.SNYK_TOKEN }}
+        run: |
+          snyk code test . \
+            --severity-threshold=high \
+            --sarif-file-output=snyk-code.sarif
+
+      - name: Warn when Snyk Code SARIF is missing
+        if: env.SNYK_TOKEN_PRESENT == 'true' && hashFiles('snyk-code.sarif') == ''
+        run: |
+          echo "::warning::Snyk Code did not produce snyk-code.sarif; check the scan step logs before relying on SAST findings."
+
+      - name: Upload Snyk Code SARIF
+        if: env.SNYK_TOKEN_PRESENT == 'true' && hashFiles('snyk-code.sarif') != ''
+        uses: github/codeql-action/upload-sarif@03e4368ac7daa2bd82b3e85262f3bf87ee112f57 # v3
+        with:
+          sarif_file: snyk-code.sarif
+          category: snyk-code-swift
+
+      - name: Setup Bun
+        if: env.SNYK_TOKEN_PRESENT == 'true'
+        uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2
+
+      - name: Install web dependencies
+        if: env.SNYK_TOKEN_PRESENT == 'true'
+        working-directory: web
+        run: bun install --frozen-lockfile
+
+      - name: Snyk Open Source scan
+        if: env.SNYK_TOKEN_PRESENT == 'true'
+        id: snyk-oss
+        continue-on-error: true
+        env:
+          SNYK_TOKEN: ${{ secrets.SNYK_TOKEN }}
+        run: |
+          snyk test \
+            --file=web/package.json \
+            --package-manager=npm \
+            --severity-threshold=high \
+            --sarif-file-output=snyk-oss.sarif
+
+      - name: Warn when Snyk Open Source SARIF is missing
+        if: env.SNYK_TOKEN_PRESENT == 'true' && hashFiles('snyk-oss.sarif') == ''
+        run: |
+          echo "::warning::Snyk Open Source did not produce snyk-oss.sarif; check the scan step logs before relying on OSS findings."
+
+      - name: Upload Snyk Open Source SARIF
+        if: env.SNYK_TOKEN_PRESENT == 'true' && hashFiles('snyk-oss.sarif') != ''
+        uses: github/codeql-action/upload-sarif@03e4368ac7daa2bd82b3e85262f3bf87ee112f57 # v3
+        with:
+          sarif_file: snyk-oss.sarif
+          category: snyk-open-source-web-npm
diff --git a/.snyk b/.snyk
new file mode 100644
index 0000000000..808ef82f86
--- /dev/null
+++ b/.snyk
@@ -0,0 +1,33 @@
+# Snyk (https://snyk.io) policy file.
+#
+# CI rollout default: report high and critical findings. The threshold is
+# enforced in .github/workflows/security-snyk.yml with --severity-threshold=high.
+#
+# Code SAST is intentionally scoped to cmux Swift sources for v1. SCA remains
+# scoped by command line to web/package.json.
+version: v1.25.0
+
+exclude:
+  code:
+    - .github/**
+    - cmuxd/**
+    - daemon/**
+    - docs/**
+    - dogfood/**
+    - Examples/**
+    - ghostty/**
+    - homebrew-cmux/**
+    - Prototypes/**
+    - Resources/**
+    - scripts/**
+    - tests/**
+    - tests_v2/**
+    - vendor/**
+    - web/**
+
+# Add temporary ignores with:
+#   snyk ignore --id=<SNYK-ID> --expiry=YYYY-MM-DD --reason="..."
+# Keep every ignore narrow, justified, and expiring.
+ignore: {}
+
+patch: {}
diff --git a/docs/security.md b/docs/security.md
new file mode 100644
index 0000000000..8a5c6b9676
--- /dev/null
+++ b/docs/security.md
@@ -0,0 +1,30 @@
+# Security scanning
+
+cmux uses the `Snyk security` GitHub Actions workflow for rollout-safe security review on pull requests and pushes to `main`.
+
+## What runs
+
+- Snyk Code runs SAST against cmux Swift sources at `high` severity and above.
+- Snyk Open Source runs SCA against the docs site manifest at `web/package.json` at `high` severity and above.
+
+The v1 SCA scope is npm only. The workflow runs `bun install --frozen-lockfile` first, then tells Snyk to treat `web/package.json` as an npm project. Until Snyk supports `bun.lock` directly, Snyk resolves dependencies from the installed `web/node_modules` snapshot rather than parsing the Bun lockfile itself.
+
+Swift Package Manager dependencies are resolved through `GhosttyTabs.xcodeproj/project.pbxproj` instead of a top-level `Package.swift` or `Package.resolved`, so SPM SCA needs a follow-up design before enabling.
+
+## Where findings appear
+
+The workflow writes SARIF for each scanner and uploads it with `github/codeql-action/upload-sarif`. GitHub shows those results in code scanning, the repository Security tab, and inline PR annotations. After the Snyk GitHub App is installed for the `manaflow-ai/cmux` repository, Snyk App comments may also appear on PRs.
+
+## Fork PR behavior
+
+The workflow does not run Snyk scans unless `SNYK_TOKEN` is available. Fork PRs, Dependabot PRs, and the initial rollout period before the secret is added produce a clean skip instead of a red CI result.
+
+## Ignores
+
+Use `.snyk` for reviewed, temporary ignores. Prefer fixing the finding first. If an ignore is necessary, add it with an expiry and a concrete reason:
+
+```bash
+snyk ignore --id=<SNYK-ID> --expiry=YYYY-MM-DD --reason="why this is safe until the expiry"
+```
+
+Keep ignores narrow, expiring, and reviewed in the PR that adds them.