security(smime): fork upstream plugin and fix two audit findings

S-01 audited bulwarkmail/plugins/smime @ 91085a3 (2,935 lines). Nine
findings, two HIGH. No backdoor and no exfiltration path anywhere in the
bundle — the problems are trust-model and input-validation gaps. Full
report in vnc/audits/SMIME-PLUGIN-AUDIT-2026-08-04.md.

Fork is source-only. The upstream smime.zip is a 1.77 MB prebuilt bundle
whose manifest reads 1.0.1 while the source reads 1.0.2, so auditing
src/ would not audit what that zip installs. We build from source.

Finding 1 (HIGH) — certificate substitution. maybeAutoImportSigner gated
on signatureValid alone, but smimeVerify runs checkChain:false, so that
only proves "signed by whoever holds this key", not that the claimed
identity is real. Self-sign a cert asserting victim@example.com, send one
signed message, and it was stored as the encryption target for that
address — the user's next Encrypt to the victim went to the attacker.
Now requires signerEmailMatch === true and !selfSigned. Both values were
already computed and displayed as untrusted in the banner; only the
import path ignored them. Tests for `true` explicitly so an undefined
match (missing From header) fails closed.

Finding 3 (MED-HIGH) — CRLF header injection. Escaping reached only
Subject and attachment filename; display names, raw addresses,
Message-ID, In-Reply-To, References and attachment Content-Type were
emitted verbatim, and formatAddress escapes only backslash and quote.
In-Reply-To/References/display names are copied from inbound mail when
replying or forwarding, so the value is attacker-supplied. Sanitising
inside formatHeader covers all 17 call sites by construction; the three
headers assembled directly get stripCrlf explicitly.

Also adds auth:observe to the manifest. The plugin registers
onAfterLogout/onAccountSwitch — real hooks (lib/plugin-hooks.ts:362-363)
— without declaring the permission, so under B-09 the session-key wipe
would silently stop running.

verify-fixes.mjs carries 19 assertions including source checks that fail
if either guard is removed or a new unsanitised interpolated header
appears. That last one immediately caught the interpolated smime-type
Content-Type header, which manual review had dismissed as static.

Finding 2 (unauthenticated CBC accepted on decrypt) is NOT fixed. This
is not safe for real mail yet — sandbox accounts only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

vnc/plugins/smime/README.md

@@ -0,0 +1,79 @@
+# S/MIME plugin
+
+End-to-end S/MIME (CMS / PKCS#7) for Bulwark Webmail, implemented as a
+**privileged** (same-origin) plugin. All cryptography runs locally in the
+browser using a bundled `pkijs` / `asn1js` / `webcrypto-liner` stack, with no key
+material ever leaves the device.
+
+## What it does
+
+| Capability | How |
+|---|---|
+| **Sign** outgoing mail | `onComposeSend` builds the MIME, wraps it in opaque CMS `SignedData`, and submits via `api.jmap.sendRaw`. |
+| **Encrypt** outgoing mail | `onComposeSend` builds CMS `EnvelopedData` to every recipient (AES-256-GCM by default; AES-128 optional) plus the sender, then submits raw. Sign + Encrypt does proper sign-then-encrypt. |
+| **Verify** incoming signatures | `onRenderEmailBody` fetches the CMS blob (`api.jmap.fetchBlob`), validates the signature cryptographically, checks validity dates, flags self-signed signers and signer≠From mismatches, and renders the inner body. |
+| **Decrypt** incoming mail | `onRenderEmailBody` decrypts `EnvelopedData` with your unlocked key (RSA-OAEP, with an RSAES-PKCS1-v1_5 + 3DES/RC2 legacy fallback for old Outlook/Thunderbird mail). |
+| **Key management** | `settings-section` slot: import PKCS#12 (`.p12`/`.pfx`), unlock/lock, delete, import recipient certificates, set sign/encrypt defaults. |
+| **Status** | `email-banner` slot shows signature / encryption state; `composer-toolbar` slot has per-message Sign / Encrypt toggles. |
+
+## Security model
+
+- **Privileged tier.** Declares `tier: "privileged"` + `crypto:full`. Per
+  `resolvePluginTier`, the same-origin tier is only granted to a **signed,
+  admin-approved (managed)** bundle after high-risk consent. A self-uploaded
+  copy is refused rather than downgraded; sign and ship it through the admin channel.
+- **Keys at rest.** Private keys are imported from PKCS#12 and re-wrapped with
+  AES-256-GCM under a PBKDF2(SHA-256, 600 000) key derived from a passphrase
+  you choose. Stored in IndexedDB; the raw key bytes are never persisted.
+- **Keys in use.** Unlocking imports the key as a **non-extractable**
+  `CryptoKey`. Because the background (hooks) iframe and the visible slot
+  iframes are same-origin, the unlocked handle is shared through a session
+  IndexedDB store. It stays non-extractable and is **wiped on app boot and on
+  logout / account switch** (configurable), mirroring the former native
+  "in-memory, cleared on reload" behaviour.
+- Returned HTML still passes through the host sanitizer.
+
+## Build
+
+```bash
+cd repos/plugins/smime
+npm install          # pulls pkijs / asn1js / pvtsutils / webcrypto-liner + esbuild
+npm run build        # → dist/index.js  (~1.7 MB, under the privileged cap)
+npm run package      # → smime.zip (manifest.json + index.js) for admin upload
+```
+
+The build aliases the Node `crypto` builtin (referenced by a dead
+`typeof process` branch in `asmcrypto.js`) to a browser shim so the bundle is
+self-contained.
+
+## Layout
+
+```
+src/
+  index.js            entry: activate + hooks + slots (React.createElement UI)
+  crypto-engine.js    pkijs CryptoEngine w/ 3DES/RC2 + legacy PKCS#12 PBE
+  certificate-utils.js X.509 parse + metadata + capability classification
+  mime-builder.js     deterministic CRLF MIME builder + CMS RFC822 wrapper
+  mime-parse.js       inner-MIME parser for decrypted/verified content
+  smime-detect.js     detect CMS from Content-Type / bodyStructure / attachments
+  smime-sign.js       CMS SignedData (opaque)
+  smime-encrypt.js    CMS EnvelopedData
+  smime-decrypt.js    CMS decrypt + blob normalisation + recipient matching
+  smime-verify.js     CMS signature verification + signer status
+  pkcs12.js           PKCS#12 import + key wrap/unlock
+  key-storage.js      IndexedDB: key records, recipient certs, session keys
+  util.js             uuid / hex / equality helpers
+  node-crypto-shim.js browser shim for the Node "crypto" builtin
+```
+
+The crypto modules are faithful ports of the host's `lib/smime/*` (the former
+native pipeline), so the plugin produces byte-compatible CMS.
+
+## Note on host wiring
+
+The `onComposeSend` and `onRenderEmailBody` hook buses and the privileged
+`api.jmap` surface exist in the host (see `lib/plugin-hooks.ts`,
+`lib/plugin-sandbox/host-api.ts`). The send/render **takeover** fires once the
+host emits those buses from the composer and viewer (the migration that retires
+the inline native path). The `settings-section`, `composer-toolbar`, and
+`email-banner` slots are active today.